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6/01 c:/ClRC/DateDue.p65-p.15

LOWERING OF THE WATER TABLE, IRRIGATION POLICY CHANGE AND
ITS INFLUENCE ON THE GROUNDWATER ARSENIC CONTAMINATION

PROBLEM IN BANGLADESH

By

Md. Nazrul Islam

A DISSERTATION

Submitted to
Michigan State University
In partial fulﬁllment of the requirements
For the degree of
DOCTOR OF PHILOSOPHY
Biosystem Engineering
Agricultural Engineering Department

2003

ABSTRACT
LOWERING OF THE WATER TABLE, IRRIGATION POLICY CHANGE AND ITs
INFLUENCE ON THE GROUNDWATER ARSENIC CONTAMINATION
PROBLEM IN BANGLADESH
By

Md. Nazrul Islam

This study is based on the hypothesis that shortage of dissolved oxygen at
and below the water table and extraction of groundwater by irrigation wells at a
rate greater than the aquifer recharge rate are the main causes of arsenic
release in the groundwater of Bangladesh. The main purpose of this study was
to identify the processes that produce high arsenic concentrations in the
groundwater system and to formulate a quantitative irrigation development policy
for Bangladesh. This dissertation evaluated the currently existing oxidation and
reduction theories of arsenic release from the context of dissolved oxygen
shortage in recharging groundwater. This study also conducted both numerical
and thermodynamic analyses to demonstrate how the oxidation theory of arsenic
release was not adequate to explain the cause of arsenic release in the
groundwater system and how shortage of dissolved oxygen in recharging
groundwater at and below the water table produces high arsenic concentrations.

This study focused on quantitative analysis of the oxygen diffusion rate in
deeper layers of the arsenic-contaminated aquifer and the variations in redox
potentials over the aquifer’s depths in order to evaluate the oxidation and

reduction theory of arsenic release. This study also computed the amount of

shallow aquifer recharge (SAR) available and the amount of groundwater ﬂow
passing through the deeper layers of the aquifer in order to establish a relation
between lowering of the water table, the tube well extraction rate, and the
likelihood of arsenic migration from the currently contaminated layers to
uncontaminated deeper layers of the aquifer.

Based on the dissertation’s main ﬁndings, it can be concluded that
variable management decisions such as the tube well extraction rate, increased
irrigation system efﬁciencies, shifted and adjusted cropping patterns and
seasons, rainwater harvesting, and avoidance of pumping from the top layers of
the aquifer can improve the control of the system and reduce the risk of arsenic

migration.

ACKNOWLEDGEMENTS

Committee chairman Dr. R.D. von Bemuth deserves special recognition for
generously taking me under his wings and expertly guiding me throughout all
stages of my coursework and dissertation. Dr. Bemuth’s exceptional intelligence,
humanity, patience and calming influence were especially helpful and
appreciated throughout my dissertation period. He is real teacher and I am very
much grateful to him for his major contribution to complete this dissertation.

Committee member Dr. S. Davies deserves special recognition for his
valuable guidance on the complicated geochemical issues and thermodynamic
analysis of this dissertation. Committee member Dr. William Northcott deserves
special recognition for offering me excellent advice on handling the GIS issues.
Committee member Dr. Karen Chow also deserves my special appreciation for
helping me understand the extent of the toxicological issues of arsenic-
contaminated groundwater. I value Dr. Larry J. Segerlind’s and Dr. Shuguang Li’s
help understanding the oxygen diffusion modeling by ﬁnite element solution
techniques and groundwater modeling with the interactive groundwater model
respectively.

The Govemment of the People’s Republic of Bangladesh, the World Bank,
and the Bangladesh Rice Research Institute (BRRl) also deserve my special
appreciation for helping me fund my doctoral course, giving me the opportunity to

study at Michigan State University.

iv

My heartiest gratitude is also deserved by the Surface Water Modeling
Center (SWMC), the Bangladesh Water Development Board (BWDB), and the
British Geological Survey (868). I must acknowledge the contribution of Dr.
Sadiqul Islam Bhuyian and Dr. Habib Chowdhury for their strong support to
keep my sprit high. I also thank Mr. Abdullah, Mr. Monir Hossain, Mr. Nurul
Islam, Dr. M.A. Ghani, Dr. M. A. Sattar, Dr. M.k. Mondal, Mr. H.R. Molla, my
youngest sister Mrs. Shamima Sultana Ruby and her husband , and all of those
who helped collecting ﬁeld data from Bangladesh . I also acknowledge Miss Tina

Urbain’ 3 help during the dissertation formatting process.

DEDICATION

This dissertation is dedicated to my parents, my wife “Khuku, ” my two lovely
daughters (Nitu & Jibu) and those who were brutally killed by the occupied force

and their local associates during the liberation war of Bangladesh in 1971.

vi

TABLE OF CONTENTS

Page No.
LIST OF TABLES xv
LIST OF FIGURES xviii
CHAPTER 1: INTRODUCTION 1
1.1 Statement of the problem 3
1.2 Scale of the groundwater arsenic problem 4
1.3 Rationale 5
1.4 Hypotheses 6
1.5 Objectives 8
CHAPTER 2: LITERATURE REVIEW 9
2.1 Water resources and irrigation water use 9
2.2 History of groundwater irrigation development projects and 12
the arsenic contamination problem in Bangladesh
2.3 Global perspective of arsenic contamination and groundwater 13
use
2.4 Redox potential values, dissolved oxygen content, and 16
arsenic release processes in the groundwater
2.4.1 Dissolved oxygen supply, redox potential values, and 18
their association with the arsenic release process in
groundwater
2.4.2 The sequence of redox process in the groundwater 19
system
2.4.3 Controlling redox potentials in the groundwater 20
system

2.4.4 Importance of dissolved oxygen in the recharging 21

vii

groundwater system

2.5 Existing theories of arsenic release mechanism in the
groundwater of Bangladesh

2.6 Lowering of water table, ability of atmospheric oxygen
access, and redox potentials of the aquifer

2.7 Lowering of the water table, change in thermodynamic
properties, and arsenic release into the groundwater system

2.8 Arsenic transportation in the groundwater system

CHAPTER 3: DESCRIPTION OF THE STUDY AREA

3.1 Geology of arsenic contaminated area in Bangladesh

3.2 Geomorphology of Bangladesh and primary source of arsenic
3.2.1 Land and soil classiﬁcations

3.3 Hydrology and hydrostratigraphy of Bangladesh
3.3.1 Aquifer properties

3.4 Groundwater chemistry in Bangladesh

3.5 History of irrigation development and area coverage in
Bangladesh

3.6 Rice irrigation and rice water requirements in the southwest
zone study area

3.7 Groundwater level in speciﬁc study area in the southwest
(SW) zone

CHAPTER 4: METHODOLOGY

4.1 Research approach and research issues
4.1.1 Data collection strategy
4.2 Analysis of research issues

4.2.1 Statistical analysis of the arsenic distribution pattern
and its relationship with surface geology, depth of the
aquifer, dissolved iron content, and groundwater flow

viii

23

26

29

3O
33
34
34
37
38
40
43

43

45

45

47
47
50
51
52

direction

4.2.2 Computation of probability of arsenic concentrations
exceeding threshold level in Bangladesh’s groundwater

4.2.3 Distribution pattern of arsenic contamination over
hydrologic zones

4.2.4 Correlation between arsenic pollution distribution,
physiography and surface geology of Bangladesh

4.2.5 Analysis of the correlation between arsenic pollution
and intensity of tube well irrigation in Bangladesh

4.2.6 Depth of irrigation well s versus arsenic
concentration over aquifer depth

4.2.7 Correlation between arsenic concentration and
dissolved iron content in Bangladesh' 3 ground water

4.2.8 Correlation between arsenic distribution pattern and
groundwater ﬂow direction

4.3 Diffusive transportation of atmospheric oxygen to the
saturated zones Of groundwater systems in Bangladesh

4.3.1 Diffusive transportation of atmospheric oxygen into
the aquifer

4.3.2 Formulation of governing equation for the oxygen
transportation by diffusion into the groundwater system

4.3.3 Formulation of ﬁnite element technique to solve the
governing equation for oxygen diffusion in the groundwater
system

4.3.4 Analysis of inﬂuence of water table lowering and
oxygen concentration distribution in the deep layers of
groundwater

4.3.5 Inﬂuence of initial boundary conditions on oxygen
concentration

52

55

61

61

62

67

7O

72

72

72

75

82

88

90

4.4 Computation of redox potential (pe) values and stability
analysis of arsenic (III and V) redox couples in Bangladesh’s
groundwater

4.4.1 Measurement of redox potential values

4.4.2 Use of p8 as a variable to measure the iron controlled
(Fe2+/Fe3+) redox potentials in the groundwater

4.4.3 Computation of oxygen-controlled redox potential
values

4.4.4 Stability analysis and functional relationship among

redox potential pe, pH, and the construction of a pe-pH
diagram

4.4.4.a Functional relationship among iron redox
couples (Fe (OH) 3 & Fe”), pe and pH

4.4.4.b Stability limit of iron and arsenic species and
construction of pe-pH diagram

4.4.5 Shifting of thermodynamic equilibrium, lowering of
water table and arsenic release process in groundwater Of
Bangladesh

4.5 Computation of shallow aquifer recharge (SAR) in
groundwater system

4.5.1 Parameter estimation in water balance equation

4.5.2 Computation of volume of shallow aquifer recharge
(SAR)

4.5.3 Amount of shallow aquifer recharge under HYV—rice
irrigated system

4.5.4 Computation of SAR, SUR using soil moisture
balance

CHAPTER 5: MATHEMATICAL MODELING TO ESTIMATE
GROUNDWATER MOVEMENT 8: LIKELIHOOD OF ARSENIC
MIGRATION

5.1 One-dimensional analytical model to evaluate the impact of
the aquifer’s physical properties on arsenic concentration

5.1.1 Conceptual one-dimensional analytical model

5.2 Three-dimensional numerical modeling to understand shallow
aquifer recharge (SAR) rate, ﬂow components, and their impact

X

90
91

94

98

99

100

104

107

107

110

112

113

117

117

119
122

on arsenic transportation
5.2.1 Derivation of 3-D transport equation

5.2.2 Development of conceptual three-dimensional model
for the study area

5.2.3 Computation of head distribution and ﬂow patterns of
uniform 3-D model (M-1)

5.2.4 Computation of mass balance to demonstrate the
impact of extraction rate (Q) and aquifer recharge rate
(SAR) on groundwater movement and ﬂow patterns

5.3 Inﬂuence of extraction or pumping on hydraulic gradient ﬂow
to and from the river

5.3.1 Impact of dispersivity and heterogeneity on arsenic
migration

5.3.2 Inﬂuence on arsenic migration of placing tube wells
in the deeper layers

5.4 Development of site-speciﬁc model and the inﬂuence of
pumping on arsenic transportation

5.4.1 Inﬂuence of pumping on the vertical migration of
arsenic using the models that reﬂected more realistic
complexities in the ﬁeld

5.4.2 Inﬂuence of extraction on arsenic concentration
migration in the site-speciﬁc real model

CHAPTER 6: EVALUATION AND DISCUSSION OF RESULTS
6.1 Research issues and strategic Questions

6.1.1 Why arsenic concentrations in groundwater exhibited
correlations with the hydrologic zones, geomorphology and
surface geology

6.1.2 Explanation of why and how arsenic contamination is
related to the dissolved iron content in the groundwater

6.1.2.a Thermodynamic explanation of the
correlation between high arsenic and dissolved iron
content in the groundwater system

xi

123
126

130

131

138

138

141

146

147

148

150

151

152

155

157

6.2 Interaction between lowering of the water table, dissolved 159
oxygen and arsenic concentration triggering

6.2.1 Why arsenic concentration varies with aquifer depths 160

6.2.2 Why arsenic concentration again decreases over the 162
depth ranging from 100 to 350 m (in Fig. 6.2.b)

6.2.3 What impact the lowering of the water table had on 166
diffusive oxygen transportation to the deeper layers of the
aquifer

6.3 Whether the oxidation or the reduction process mainly 167

controls arsenic mobilization in Bangladesh’s groundwater

6.3.2 What theory is applicable to describe the arsenic 170
release into the groundwater of Bangladesh
6.3.2.a What was the critical difference between the 171
reduction theory and hypothesis offered in this
study?
6.3.3 Why the pyrite oxidation theory was not applicable to 172
explain the arsenic contamination problem in Bangladesh’s
groundwater
6.3.4 Why the hypothesis offered in this study is a valid 175
explanation of arsenic release which should not be
rejected
6.3.5 Interactions between shortage of dissolved oxygen, 179

extraction rate, lowering of the water table position, and
rejection or acceptance of the oxidation theory of arsenic
release in groundwater

6.3.6 Can thermodynamic principles be applied to interpret 184
the relations among the lowering water table, the shortage

of dissolved oxygen in recharging water and arsenic

contamination in the groundwater of Bangladesh

6.4 Inﬂuence of aquifer thickness on the arsenic concentration of 189
water entering a pumping well

6.4.1 What is the inﬂuence of aquitard thickness (b’) on the 181
amount of water entering the well screen and
concentration in the pumping water?

xii

6.4.2 What is the inﬂuence of aquitard conductivity on the 192
amount of water entering into the well screen

6.4.3 How aquifer physical properties (aquitard thickness 194
and conductivity) could be used to describe the validity of
oxidation theory of arsenic release.

6.5 Inﬂuence of extraction at a rate greater than that of aquifer 195
recharge (SAR) on groundwater movements, ﬂow patterns, and
the likelihood of arsenic release

6.5.1 The interaction between the extraction or pumping 197
rate (Q) and the availability of shallow aquifer recharge
(SAR)

6.5.1.a Maximum pumping limit, aquifer recharge 198

rate (SAR) and lowering of the water table

6.5.1.b The lower limit of aquifer recharge (SAR), 199
potential recharge and amount of surplus (SUR)

6.5.1.0 Seasonal ﬂuctuation of the water table and 200
computation of shallow aquifer recharge (SAR)

6.5.2 Inﬂuence of cropping patterns on the amount of 201
shallow aquifer recharge (SAR) available

6.5.2.a Estimation of the amount of surplus volume 201
(SUR) under rain-fed and irrigated HYV-rice
growing conditions in the study area

6.5.3 Impact of irrigation system capacities on the potential 204
recharge or surplus volume (SUR)

6.5.3.a Design capacity of irrigation systems and its 204
inﬂuence on surplus volume (SUR)

6.5.4 Inﬂuence of SAR on groundwater movement, ﬂow 207
patterns, and likelihood of arsenic migration

6.5.4.a Inﬂuence of shallow aquifer recharge (SAR) 208
on groundwater movement and ﬂow patterns

6.5.4.b Inﬂuence of aquifer conductivity and the 211
aquifer recharge rate (SAR) on the downward ﬂow
through deeper layers

xiii

6.5.4.c Groundwater movement variations under 213
natural groundwater ﬂow conditions and extraction
conditions

6.5.4.d Inﬂuence of aquifer pumping and 220
dispersivity on the vertical migration of arsenic

6.6 Placing of tube wells screens in the deeper layers and its 223
inﬂuence on arsenic movement from contaminated layers to the
uncontaminated layers

6.6.1 Impact on the ﬂow components and likelihood of 223
arsenic migration of placing deep tube well screens at
deeper layers

6.7 Administrative and management decision variables to reduce 226
the risk of arsenic concentration in the case of further irrigation
development

6.7.1 How irrigation system capacity reduction and 227
adjustments in cropping patterns or seasons can help
increase the amount of aquifer recharge

6.7.2 How rainwater harvesting and soil-based arsenic- 229
contaminated groundwater treatment systems can be used
to reduce the risk of contamination.

6.7.2.1 How rainwater can be used to reduce the 229
risk of arsenic contamination

6.7.2.2 How soil-based arsenic-contaminated 230
groundwater can be used to use for irrigation to
reduce the risk of further arsenic contamination

235
CHAPTER 7: SUMMARY, CONCLUSIONS AND
RECOMMENDATIONS
7.1 Conclusions 246
7.2 Findings of the study 248
7.3 Recommendations 251
7.3.1 Recommendations for further research 253

xiv

Table No.
2.1

2.1.3

2.2

3.1
3.2

3.3

3.4

3.4.a
3.4..b

3.5

4.2.1

4.2.2

4.2.3

LIST OF TABLES

Title

Comparison of world water use and per capita water use
in 1950 and 1995

Total volume of water available in different forms on this
eanh

Concentration of soluble arsenic species, Mn, and Fe2+
as a function of soil redox-pH condition

General features of Bangladesh

Average monthly discharge and sediment load in the
major river systems in Bangladesh

Lithology and hydraulic properties in the study area of
Bangladesh

Average aquifer properties in different geological
formations and zone aquifer properties

Approximate wet season regional groundwater gradient

Average groundwater chemistry in different zones in
Bangladesh

Total number of deep tube wells (DTW), shallow tube
wells (STW), and low lift pumps (LLP) (irrigation devices)
use for both surface and groundwater irrigation in
Bangladesh from 1994 to 200

Probability of exceedence of arsenic concentration in
different hydrologic zones in Bangladesh

Description of types of hydrologic zone, geomorphology
and surface geology in Bangladesh

Regression analysis between aquifer depth and arsenic
pollution distribution in Bangladesh’s groundwater

XV

Page No.
10

11

22

33
36

38

41

42
42

44

55

56

65

4.2.4

4.3.1

4.3.2

4.3.3

4.4.1

4.4.2

4.4.3

4.5.1

5.1

5.1.b

5.2

5.3

5.4

Statistical relationship between arsenic and iron
concentration at different grid locations
(latitude/longitude) in Bangladesh

Number of elements, number of nodes and material
properties of the aquifer material for the shallow and
deep tube well system in Bangladesh

Diffusive oxygen concentration at pyretic layer L5 before
and after the introduction of well ﬁelds, assuming the
initial 02 concentration to be 0.21 atm for all nodes

Diffusive oxygen concentration at pyretic layer 6 before
and after the introduction of well ﬁelds in Bangladesh,
while considering the initial 02 concentrations of 0.15
atm for nodes in the saturated zone

Iron controlled redox potential values in arsenic
contaminated wells in different zones of Bangladesh

Dissolved oxygen concentration available in groundwater
and computation of redox potential values for natural
water (pH=7)

Chemical and thermodynamic equations for pe-pH
diagram of arsenic (lll & V) and Fe (OH) 3 / Fe2+

Monthly average rainfall and rainfall of 2 year and 5 year
return period in the study area (SW zone of Bangladesh)

Computation of mass balance in model M-1 under
natural ﬂow conditions

Computation of mass balance in the M-1 model under
extraction conditions

Computation of mass balance in the M-3 model under
natural ﬂow condition

Computation of mass balance in the M-3 model under
extraction conditions with average, medium, and high
recharge rates

Mass balance when wells are placed at different layers
with recharge rate (SAR of 381mm/Yr) less than the
Pumping rate Q

xvi

69

85

87

89

94

106

116

134

135

136

137

143

5.5

5.5.a

5.6

6.5.1
6.5.2

6.6.1

Mass balance when wells are placed at different layers
with recharge rate of 635 mm/yr.

Mass balance when the well screen is placed at the third
layer in the M-3 model, which represented the upper
ﬁning sequence of the aquifer

Arsenic concentration at different layers of the site-
speciﬁc model as a function of SAR rate under natural
groundwater ﬂow conditions after 10000 days

Impact of cropping patterns on amount of surplus volume

Percentage of total ﬂow associated with each layer in the
geological cross section of the conceptual model

Computation of mass balance when wells are placed at
different layers with recharge rate Of 635 mm/Y r.

xvii

144

145

149

206
220

225

Figure No.
1

2.1
2.2

3.1
3.2
3.3

3.4

4.2.3

4.2.b

4.3.a
4.3.b

4.3.c

LIST OF FIGURES

Title

Map of Bangladesh indicating wells with arsenic levels
above 50 ppb with red spots. Blue and dark green
spots are wells containing arsenic less than 25 and 10
ppb, respectively.

Sequence of redox potentials in the groundwater system

Oxygen Diffusion Rate (ODR) decreasing over depth at
15, 30 and 60 cm below the ground surface

Surface geology map of Bangladesh
Geomorphology of Bangladesh

Growth Of shallow tube wells from 1994 to 2000 in
Bangladesh

Fluctuation of groundwater level in the speciﬁc study
area in SW zone of Bangladesh

Blue dots on the map (1 and 2) indicate arsenic-
contaminated tube wells with arsenic concentrations
>10 ppb and >25 ppb, respectively. Red dots on the
map (3, 4, and 5) indicate wells with arsenic >50 ppb,
100 ppb, and 250 ppb, respectively.

The probability of arsenic in exceedence of the
threshold level (50 ppb) is low (18 to 39%) in the NW,
NC and SC zones, but in the NE, SW and SE areas it is
higher (53 to 71%).

Geomorphology of Bangladesh

General distribution pattern of arsenic over surface
geology and physiographic unit. (Red spots indicate
arsenic >50 ppb.)

Surface geology (left) and the physiographic map of

xviii

Page No.
5

20
28

35
37
45

46

57

58

59
60

60

4.4

4.5

4.6.3

4.6.b

4.7.a

4.7.b

4.8

4.9

4.10

4.11

4.12

4.13

4.14

Bangladesh (right)

Distribution of groundwater irrigation coverage in
Bangladesh. The darkest areas use more than 40%
groundwater for irrigation, darker areas use 5 to 40%
and white areas use less than 5% groundwater for
ﬁﬁgaﬁon.

Arsenic concentration range over the depth of
Bangladesh’s aquifer

Arsenic content increases linearly with depth from 9 to
30 m

Arsenic content decreases linearly with depth from 100
to 350 m

Relation between arsenic concentration and dissolved
iron content in grid 9, grid 10

Relation between arsenic concentration and dissolved
iron content in grid 12, grid 15

Distribution pattern of arsenic concentration along the
groundwater ﬂow direction (especially considering grids
6, 7, 8, 9, and 10, where the surface elevation gradient
was considered as the hydraulic gradient).

Use of control volume to derive one-dimensional oxygen
diffusion equation

Aquifer model cross section of a 10-meter deep layer
(from L1, L2 to L10, representing ten elements).

Predicted Changes in the diffusive oxygen supply at
arsenic-contaminated pyretic layer L5 and L10 in the
aquifer

Inﬂuence of boundary conditions on oxygen
concentration in case 1 and case 2

Stability diagram of water, iron and arsenic species in
the contaminated groundwater system

Shifting of groundwater systems from oxygenated
conditions to reducing condition due to the lowering Of

xix

63

64

66

66

68

68

71

73

84

86

88

103

105

4.15

4.16.a

4.16.b

4.17

5.1.a

5.2.a

5.2.b

5.2.0
5.3
5.4

5.5

5.5.3

water table in Bangladesh.

Conceptual water balance model: rectangular areas
represent reservoirs for water and arrows represent the
ﬂux.

Cumulative sum of the surplus (SUR), outflow and
annual evaporation

ET, PET and the groundwater level in the study area of
the SW zone

Amount of SUR (blue lines), rainfall (green), and soil
moisture storage in the SW zone study area. Blue lines
(on the top & bottom graphs) are the amount of SUR
(201 mm for irrigated and 716 mm for non-irrigated
condition)

Schematic of a typical cross-section of aquifer in
Bangladesh, where deep tube wells (DTVV) and shallow
tube wells (STW) were installed for irrigation

Conceptual model to investigate the inﬂuence of
aquitard depth (b’) and conductivity (K’) on arsenic
concentration in the water in a pumping well

Inﬂuence of aquitard thickness (b’) and permeability (K’)
on steady state arsenic concentration of water entering

in a pumping well (Q=2448m3/day).

Derivation of three-dimensional contaminant

transportation equations
Cross section of a conceptual model in the study area
Groundwater gradient to the river

Inﬂuence of aquifer recharge (SAR) on the ﬂow patterns
(@ 635 mm/yr. on the left side, @ 1230 mm/yr. on the
right side) .

Increasing the conductivity at L2 drew the stream lines
deeper

Components of ﬂow coming in and out from all
directions as a part of the mass balance during model

XX

108

111

111

115

118

118

122

123

129
130
131

132

133

5.6

5.7

5.8

5.9
5.10

5.11

6.1

6.2.a

6.2.b

6.3.a

6.3.b

6.4

6.5.a

simulation

Flow path of arsenic after 100 and 5000 days, assuming
movement of arsenic particles when DL/DT = 0, Q =
2448 m3lday

Flow path of arsenic movement when DL/DT = 5, Q =
2448 m3/day, after 100 and 5000 days

Inﬂuence of the pumping rate on arsenic movement,
when DL/DT = 5, Q = 2 CFS, and the ﬂow path of arsenic
is given after 3000 and 5000 days

Cross-section of site-speciﬁc model in the study area

Signiﬁcant change in groundwater ﬂow patterns due to
the extraction rate in a site speciﬁc model that reﬂect the
real ﬁeld complexity

Impact of pumping on the arsenic migration under
natural ﬂow condition (left side) and pumping condition
(right side) into the deeper layer after 5000 days without
retardation

The vast catchment area of the rivers Ganges and
Meghna, mostly in India and Nepal. Blue lines are the
ﬂowing river systems

Arsenic content increases linearly with depth from of 9
to 30 m below the aquifer

Arsenic content decreases linearly with depth from 100
to 350 m below the aquifer

Redox potential values decrease over the depth range
from 10 to 30 m below the ground

Redox potential values increased over the depth
ranging from 150 to 350 m in the most contaminated
aquifer

Oxygen concentration decreased with increasing aquifer
depths over time.

Predicted changes in diffusive oxygen supply at arsenic-
contaminated pyretic layers

xxi

139

139

140

146

147

148

155

164

164

165

165

166

182

6.5.b

6.6

6.7

6.8.3

6.8.b

6.9

6.10

6.11

6.12

6.13

6.14

6.15

6.16

Predicted change in dissolved oxygen concentrations
when the boundary conditions were changed

Lowering of the water table and shifting of the
equilibrium conditions

Line 12 shows the equilibrium line of the so;2 and H28
redox couple; all of the p8 values fall below line 12

Inﬂuence of aquitard thickness and permeability on
arsenic concentration in a well being pumped

Evaluation of shallow aquifer recharge (SAR) under ﬁeld
conditions in the SW zone study area of Bangladesh

Amount of SUR, rainfall and soil moisture balance under
a rain-fed system in the SW zone of Bangladesh. The
solid green line is rainfall and the blue line indicates the
volume of SUR (716.26mm/yr).

Amount of SUR and soil moisture under a HYV-rice
irrigation system. The blue line was the amount of SUR
under the HYV-rice irrigated system

Amount Of SUR computed under different system
capacity levels

Doubling and halving the recharge (SAR) in the uniform
(M-1) model had little or no impact on the groundwater
ﬂow pattern.

Streamlines moved deeper in the M-2 model after
increasing the conductivity, showing that the lowest
streamline moved below the line of 315.03 m beneath
the surface.

Contrasting ﬂow patterns between un-pumped and
pumped conditions

Arsenic dispersion after 8400 days with pumping rate of
2448 m3/day (left) and after 4250 days with 4896
m3/day pumping rate (right).

Inﬂuence of dispersivity and abstraction on arsenic

xxii

183

184

187

193

200

202

203

205

209

210

218

221

222

6.17

6.18

6.19

6.20

migration as a particle tracking

By adjusting the cropping season and cropping patterns,
the best use of rainfall can save 500 mm of pumping in

a year

Aquifer delineation for long-term supply of arsenic-free,

safe water on the basis of redox potential values (Eh)

Shallow tube well-based groundwater arsenic treatment

system using locally available materials

Subsurface-Flow constructed wetland system
treating arsenic contaminated groundwater

xxiii

for

228

230

233

234

APPENDICES
Page

Appendix A Computation of surplus volume (SUR) and soil storage 254
capacity for the study area in SW zone of Bangladesh

Appendix B Computation of the amount of shallow aquifer recharge 259
(SAR) for the groundwater of the study area in
Bangladesh

Appendix C Computation of the amount of surplus volume (SUR) 261
under rice irrigated condition of the study area in
Bangladesh

Appendix D Computation of the amount of surplus volume (SUR) 266
under rain -fed rice condition of the study area in
Bangladesh

Appendix E Demonstration of velocity distribution patterns of site 271
speciﬁc non homogeneous model under natural
groundwater ﬂow and pumping condition.

Appendix E-1 Vertical velocity distribution of site speciﬁc model under 272
pumping condition at seven layers

Appendix E-2 Vertical velocity distribution pattern in the conceptual 273
model with and without variation in hydraulic conductivity

Appendix E-3 Vertical velocity distribution in non homogeneous model 274
with variation in hydraulic conductivity under natural
groundwater ﬂow condition.

Appendix E—4 Variation of hydraulic conductivity values in non 275
homogeneous model at layer 1, 2 and 3

References 276

xxiv

1. INTRODUCTION

Bangladesh is in the midst of what the World Health Organization calls
the "largest mass poisoning of a population in history”. Some 35 million of its 126
million people are drinking arsenic—contaminated water, the poison accumulating
within them day by day, sip by sip. High concentrations of naturally occurring
arsenic have already been found in water from thousands of tubewells, the main
source of potable water, in 59 out of Bangladesh's 64 administrative districts. "It
seems like nonsense to people, telling them the water is killing them when it
looks so clean and nice," said Dr.Allan H. Smith, an epidemiologist at the
University of California at Berkeley, and he has called the situation in Bangladesh
"the highest environmental cancer risk ever found," worse than Bhopal or
Chernobyl. However, the exact cause of the arsenic release that is poisoning the
groundwater is still poorly understood.

Moreover, there are divergences in Opinions about the exact cause of
arsenic release and strong disagreements on the source of arsenic in the
groundwater of Bangladesh. The explanations as to the origin of the arsenic
contamination in the ground water can be grouped into two camps. According to
one explanation, the arsenic adsorbed by the iron hydroxide is released and
reduced to its soluble lethal forms from the reduction of arsenic laden iron
hydroxides ﬂushed from Pleistocene-Holocene sediments (Nickson, et al., 1998.
This is known as the reduction theory of arsenic release.

Another explanation is that the arsenic contamination in drinking water is a

recent phenomenon. Expansion of the vadose zone due to irrigation withdrawal
1

caused arseno—pyrite (FeAsS) oxidation and eventually arsenic to be released
into the groundwater system. This is known as the oxidation theory of arsenic
release. However, neither of these two theories is unequivocally accepted by all,
and from the chemical point of view, these two theories are oppositional and
insufﬁcient to explain the exact cause of groundwater arsenic contamination in
Bangladesh. Neither of these theories considers the impact of upstream
withdrawal of river water and shortage of groundwater recharge on the process
of arsenic release.

This study presents an explanation of arsenic release that is based on the
intuitive idea of lack of oxygenated recharging water at and below the water table
after extension of the unsaturated zone due to large-scale irrigation withdrawal
both from ground and surface water resources. Groundwater withdrawal and
upstream water withdrawal from the major river systems have jointly contributed
to the lowering of the water table and the shortage of aquifer recharge water.
Consequently, they have reduced the dissolved oxygen content in recharging
groundwater.

The hypothesis presented in this study completely contradicts the
oxidation theory of arsenic release and partially supports the oxyhydroxide
reduction theory. It is acknowledged that, in the presence of dissolved oxygen in
recharging groundwater, arsenic will be scavenged by iron hydroxide, which is
supportive to the existing reduction theory. However, this study is mainly focused
on the dissoived oxygen shortage in recharging groundwater, the inﬂuence of

groundwater withdrawal at a rate greater than the amount of available recharging
2

water, and the inﬂuence of this on the groundwater flow dynamics in the deeper

layers of the aquifer and on the likelihood of arsenic migration.

1.1 Statement of me problem

Before the introduction of a large number of irrigation wells in Bangladesh
and of upstream water diversion in the major river systems, the water table
elevation was in close proximity to the ground surface. As a result, the dissolved
oxygen content in the recharging groundwater was in equilibrium with the
atmospheric oxygen content (0.21 atm). Therefore, it was assumed that the
redox potential values were controlled by the dissolved oxygen present in the
aquifer system. The abundant supply of oxygenated water was favorable to
oxidation reactions during which iron oxides scavenge arsenic and stay in solid
particulate form. Therefore, before introducing well ﬁelds, the groundwater

ecosystem was oxic, redox potential values were high, and amorphous iron
oxides (Fe203, Fe (OH)3) were precipitated as solid form.

After 1970, because of large-scale irrigation withdrawal and upstream
diversion of river water, the depth to groundwater was increased. The redox
potential values (p°) decrease and were no longer controlled by the dissolved
oxygen content because dissolved oxygen is consumed by the supply of electron

donors (organic carbons) in the system. As a result, the system redox potential

values were decreased to zero or less and a mild reducing condition developed.

Due to the low oxygen concentration in the aquifer, the chemical equilibrium
has shifted from oxic to suboxic states, which eventually favors the dissolution of
iron hydroxides and the release of arsenic into the groundwater. Present
irrigation withdrawal policy (or unlimited abstraction policy) would promote further
lowering of the water table and changing of shallow aquifer recharge (SAR) and
groundwater movements through the deeper layers.

Because reducing the pumping rates and increasing the present irrigation
system efﬁciencies of the lift irrigation projects in Bangladesh could achieve
improvements in controls, a quantitative irrigation withdrawal policy could stop

further degradation of Bangladesh’s groundwater by arsenic contamination.

1.2 Scale of the groundwater arsenic problem

The most serious groundwater arsenic problem appears in much of the
southem and eastern part of Bangladesh (Fig.1.1). Recent ﬁndings of the British
Geological Survey (868) showed that the groundwater of 61 surveyed districts
out of a total of 64 is contaminated with arsenic (868, 1999). Preliminary reports
submitted by the 868 to the government of Bangladesh (608) indicated that out
of 2,022 samples analyzed in 1998, 51% of the samples were contaminated with
arsenic above 0.010 mglL (10 ppb) [the WHO guideline value], 35% were above
0.050 mglL (50 ppb) [the Bangladesh drinking water standard], 25% were above
0.10 mglL (100 ppb), 8.4% were above 0.30 mglL (300 ppb) and 0.1% were

above 1 mglL (1,000 ppb). Long term ingestion of arsenic contaminated drinking

water has been shown to increase the incidence of hyperpigmentation,

hyperkeratosis and cancer of the skin (NRCC, 1978).

 

 

 

 

 

 

 

Fig.1.1: Map of Bangladesh indicating wells with arsenic levels above 50 ppb
with red spots. Green and yellow spots are the tube wells containing arsenic less

than 10 and 25 ppb, respectively.

1.3 Rationale
Understanding the geochemical processes that produce high arsenic (As)
concentrations and the likelihood of arsenic migration in the groundwater system
has a number of important beneﬁts. Firstly, it helps to delineate safe aquifer
layers for irrigating and supplying drinking water on the basis of the available
shallow aquifer recharge (SAR) and redox values. Secondly, it will help in siting
5

new wells for irrigation development by predicting the future trends of arsenic
transportation. Thirdly, of course, it is of great signiﬁcance to formulating a
quantitative irrigation withdrawal policy in order to continue the present pace of
irrigation development.

Irrigation has been the principal factor behind Bangladesh attaining near
self-sufﬁciency in food grain production, and hence behind a great diminution in
the incidence of famine. Since Bangladesh is an agro-based country, any ban on
groundwater irrigation or any unjustiﬁed rules, regulations or requirements would
have severe adverse consequences for food security, rural livelihoods and the

economic future of Bangladesh.

1.4 Hypotheses
Hypothesis 1

The lack of oxygenated recharging water or the shortage of dissolved
oxygen (DO) at and below the water table after the extension of the unsaturated
zone due to large-scale irrigation water withdrawal is the major cause of arsenic

contamination in the groundwater of Bangladesh.

Rationale

Installation of large-scale irrigation wells and upstream diversion of river
water promoted a rapid fall in the water table and, consequently, a shortage of
oxygenated recharging groundwater. As groundwater traveled to deeper layers, a

diffusive oxygen supply from the atmosphere to the aquifer could not
6

compensate for the oxygen consumed by the organic carbon. Therefore, the
amount of dissolved oxygen concentration in the groundwater decreased.

Due to the low concentration of oxygen, the oxygen diffusion rate (ODR) also
decreased. As a result, the redox potential values (p9) decreased to a minimum

and a mild reducing condition developed in the aquifer. Under mild reducing
conditions, amorphous iron hydroxides are dissolved and eventually associated
arsenic is released into the groundwater system. Also, the recharge from wetland
cultivation would lead to a substantial increase in the dissolved organic carbon
(DOC) pool in the groundwater, which would be sufﬁcient to consume the

dissolved oxygen (electron acceptors) present in the system.

Hypothesis 2

Extraction or irrigation withdrawal with a rate greater than that of
aquifer recharge water and the placing of deep tube well screens at deeper
layers might signiﬁcantly inﬂuence the migration of arsenic into deeper layers of

Bangladesh’s groundwater system.

Rationale

Irrigation wells with realistic abstraction rates dramatically change the
amount of surface recharge ﬂowing through the deeper layers of the aquifer, and
eventually change the groundwater ﬂow patterns. Abstraction at a rate greater
than that of aquifer recharge signiﬁcantly changes the flow movements and

increases the risk of arsenic contamination. As a result, arsenic can migrate from
7

currently contaminated layers to the uncontaminated deeper layers as an

advective ﬂow with moving groundwater.

1.5 Objectives
Overall goal
The overall goal of this research is to understand the processes that give

rise to arsenic concentration in the groundwater and to develop a safe and

quantitative groundwater withdrawal policy for Bangladesh.

Speciﬁc aims

e To identify the processes that produce a high concentration of arsenic in the
groundwater of Bangladesh.

0 To formulate a quantitative irrigation policy in order to maintain the present
pace of irrigation development for the country's food production demand.

. To delineate a safe aquifer zone at the top of the summer water level where
arsenic concentration should be minimal so that small-scale dug wells or
shallow wells can be used to supply water for human consumption with

minirnel risk to human life.

2. LITERATURE REVIEW

At the emergence of the 21st century, many of the third world’s countries
face water quality crises because of limited and stressed water resources. The
World Meteorological Organization (Obasi, 1999) predicts that the explosive
growth of urban areas is rapidly depleting our water resources and detoriating its
quality. By the turn of the next decade, world water demand is likely to claim
almost half of the total global water runoff that is available each year. Between
1900 and 1990, the total worldwide water withdrawals increased at twice the rate
of the population increase while, compared with three centuries ago, water use
rose more than 35«fold (David, 1995).

In the past 45 years from 1950 to 1995, the world’s population grew by
about 2.2 times and the annual usage of water increased by about 2.6 times in
the same period (JIID, 2003). The world population is projected to increase from
the current level (near 6 billion) to 8.3 billion in 2025 and to about 10 billion In
2050 (Obasi 1997). In addition, water crises are already evident in the

competition for water for industrial, domestic and agricultural irrigation uses.

2.1 Water resources and irrigation water use

It is well known that mankind, owing to its basic instinct of getting a sure
and secure food supply, invented agriculture around 6500-7000 BC. Evidence of
this has been found in Neolithic sites in Iran, Iraq and Jordan. In the course of
time, irrigated agriculture was invented, the origin of which can be traced to the

Chalcolithic Period (3000-1000 8.0.) (Bagchi, 1995). Irrigated agriculture is by far
9

the biggest consumer of water worldwide. In Asia, it accounts for 86% of total
annual water withdrawal, compared with 49% in North and Central America and

38% in Europe.

Table 2.1 Comparison of world water use and per capita water use in 1950 and

1995

 

Use of 1950 1995 Rate of increase

 

Water (1) Total (2) Per (3) Total (4) Per (3)/(1) (4)/(2)
Volume Capita Volume Capita

 

 

 

 

 

(°/o) (%)
(km3/yr) (Uday) (kmalyr) (Uday)

Agriculture 1,124 1,235 2,504 1,231 223 100
Industry 182 zoo 714 351 392 176
Domestic 53 58 354 174 668 300
Total 1,359 1,493 3,572 1,756 --- --

Population 2.49 5.57 ---- ---

billion billion

 

 

 

 

 

 

 

 

 

Source: I.A. Shiklomanov. Assessment of Water Resources and Water
Availability in the World, 1996 (WMO).

On the other hand, the freshwater available for use by man is only
0.008% of all water on the planet. The annual evaporation from the ocean and
land area is the same as the annual precipitation (Jones, 1992). Less than
0.027% of the total amount of water distributed over the earth is fresh and

immediately available (Franks, 1972). The mean annual water loss as a crop
10

water use by evaporation in Bangladesh is about 142,000 million m3 (MPO,

1987). The total global water available in different forms is described in Table

2.1.a.

Table 2.1.a. The total volume of water available in different forms on this earth

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Types of Water Volume (in Ratio to Total Ratio to Total
1,0003 1,1113) Water (98) , Fresh Water
. (to
* Salt Water l

Sea Water 1,338,000 96.5 -—

Salty Groundwater 12,870 0.94 --
Saltwater in Lakes 65.4 0.006 —-
Total Salt Water 1,350,955 97.5 --

* Fresh Water
I River Water 2.12 0.0002 0.006
Fresh Water in Lakes 91.0 0.007 0.26
I Freshwater in 11.5 0.0008 0.03
Swamps

Subtotal - 104.62 0.0075 0.3

Fresh Groundwater 10,530 0.76 30.1
Glaciers and Snow 24,064 1.74 68.7
Soil Moisture 16.5 0.001 0.05
Underground Ice 300 0.022 0.86
Water in Living Bodies ' 1.12 0.0001 0.003
Atmospheric Water 12.9 0.001 0.04
Total Fresh Water 35,029 2.5 100

Total water j 1,385,984 I 100

 

 

Source: l.A. Shiklomanov. Assessment of Water Resources and Water Availability in the
Wortd, 1996 (WMO).
1 1

2.2 History of groundwater irrigation development projects and the arsenic
contamination problem in Bangladesh

Unlike many other countries, Bangladesh mostly depends on groundwater
resources as a reliable source of irrigation water, despite having large amounts
of surface water. Over-pumping from aquifers presents multidimensional
problems. This is the general case in large areas of India, Pakistan, Bangladesh,
and Vietnam (IRRI, 1994-95). Despite all these risks, irrigation development is
necessary to reduce hunger and poverty in Bangladesh.

Birth of lift km ' on the Indo-Pak-Bgmgladesh subcontinent;

The birth of the lift irrigation system on the lndo-Pak-Bangladesh
subcontinent took place during the period 1926 to 1935. Sir VIﬂIIiam Stamps, an
irrigation engineer renowned in the earlier part of the 1920’s, ﬁrst conceived the
idea of irrigating large dry tracts of land by using deep tube wells (Bagchi, 1995).
Use of hand tube wells LHTWs) for drinking water

In the early 1970’s, most of Bangladesh's rural population obtained its
drinking water from surface ponds. Nearly a quarter of a million children died
each year from water-borne diseases. The provision of about 4 million hand tube
wells for 97% of the rural population has been credited with bringing down the
high incidence of diarrhea diseases and contributing to a halving of the infant
mortality rate (WB, 1998). Paradoxically, the same wells that saved so many

lives now pose a threat due to the unforeseen hazard of arsenic.

12

Use of shallow and dag tube ﬂIIS for drinkigg water and mm ' '
The contribution of groundwater to the total irrigated area increased from

41% in 1982/1983 to 71% in 199611997 with a tendency of increase in each year,
while the contribution of surface water steadily declined from 59% to 29% over
the same period (NMIDP, 1998). So far there have been 0.75 million deep and
shallow tube wells commissioned in Bangladesh. In 2000, the number of shallow
tube wells (STWs) in the country sharply rose to about 757,044 from about 635
units in 1973-74 (NMIDP, 1999; 608, 1999). During the same period, the
number of deep tube wells (DTWs) also increased nearly 20 times. Before 1970,
there were no shallow tube well irrigation projects in Bangladesh.

Between 1964 and 1984, groundwater withdrawal in Bangladesh

increased from about 30 million m3ryr to over 140 million m3lyr (MPO, 1987).

During the same period, the steady state piezometric pressure head declined
from around 0.7 m/year in 1970 to -7.6 m below sea level in 1984 in Dhaka City

(Fig. 1). Increased withdrawal has caused an average decline of one meter of

pressure for every 15 million m3 in groundwater withdrawal (MP0, 1987).

2.3. Global perspective of arsenic contamination and groundwater use
Recent studies in groundwater from major aquifer systems around the

globe have shown a large range of arsenic concentrations depending on regional

hydrogeological conditions. Arsenic concentrations signiﬁcantly higher than

drinking water standards have been found in groundwater from large parts of

13

Taiwan, China, Inner Mongolia, Mexico, Chile, Argentina and the western USA
where a portion of the population has been affected by arsenicosis, like

Bangladesh and the eastern part of India.

Irrigation withdrawal and arsenic cont_a_minﬂo_n

Abstraction from shallow aquifers created arsenic contamination in many
parts of the globe. A UNICEF and EAWAGICEC (Hanoi National University) joint
research study indicated that there is a signiﬁcant arsenic problem in the shallow
tube wells of the aquifers of the large deltas of the Mekong and Red River in
Vietnam. Concentrations of arsenic above 50 ppb have been identiﬁed in
groundwater from alluvial sediment in the southern part of the Great Hungarian

Plain and in part of Romania (Varsanyi et al., 1991).

Arsenic contamination under anoxic or reducing conditions

Many of the worst cases of arsenic contamination arise in anaerobic
reducing aquifer conditions in the groundwater systems. Reducing aquifer
systems are found in Taiwan, Inner Mongolia, China, Mexico, and the USA. The
shallow aquifer of the southern Carson Desert in Nevada (USA) is highly arsenic
contaminated where the upper 50 ft (15 m) ofthe basin is ﬁlled by sediment. The
hydrologic and geochemical characteristics of the Carson Desert aquifer are
similar to those of other areas of the world with high arsenic and uranium
concentrations (Welch, 1998). The Hetao area of China is a district where

arsenic concentrations in drinking water are elevated and about 180,000 people
14

have been affected (Sun et al., 1995). The depth of contaminated drinking wells

ranged from 18 to 25 m, quite similar to Bangladesh

Arsenic contamination by industrial pollutants

Increased arsenic concentrations caused by mining activity have been
identiﬁed in many parts of the USA. In a gold-mining district of Alaska, an arsenic
concentration of about 48,000 ppb was recorded (Welch et al., 1988).

Arsenic in the natural sﬁtem

Arsenic is the twentieth most abundant natural element in the earth's crust
(average 2 mglkg). Pure arsenic is a gray metal-like material that is usually found
in combination with other elements such as oxygen, sulfur and chlorine.
Elemental arsenic itself is not toxic, but arsenic compounds are poisons, affecting
essentially all human organs (Rochette et al., 1998). Only about 125 to 150
milligrams of the chemical, half an aspirin, could kill a person in a single dose.
Arsenic is also extreme hazardous if ingested in drinking water or used in
cooking water in excess of the maximum permissible limit of 0.05 mglL (the
standard set by the World Health Organization) over an extended period of time.
Humans generally absorb 5-15% of ingested arsenicals (NRCC, 1978). It takes
anywhere between 2 to 14 years for arsenicosis to develop, depending on the

amount of exposure.

15

2.4 Redox potential values, dissolved oxygen content, and arsenic release
processes In the groundwater
Oxidation-reduction gential (ORP)

The oxidation-reduction (redox) potential of groundwater is a measure of
electron activity and an indicator of the relative tendency of a solute species to
accept (gain) or transfer (lose) electrons. Oxidation is deﬁned as the loss of an
electron (or electrons) from a species, while reduction is the gain of an electron

(or electrons) from a species.

Thennodynamic redox potential (En)

Natural hydrogeochemical systems are commonly interpreted Mthin the

framework of thermodynamic equilibrium with the use of redox potential values.
The redox potential (Eh) of groundwater is a measure of the relative amounts of
oxidized to reduced species present in a system. In the geoscience literature
prior to the 1970's, E, was used almost exclusively to express the redox
potentials of natural water. Review of the literature also suggested that redox
levels in groundwater are determined essentially by the relative rate of

atmospheric oxygen circulation and consumption of oxygen by aerobic and

anaerobic bacteria in the aquifer system. Generally, at the surface, recharging
groundwater is oxidizing and has a positive Eh value (500 to 800 mv) governed
largely by their dissolved oxygen (DO) content present (up to 10 mglL) (AECL, ‘
1996).

16

Mex intensity or capacity (pf)

Largely for computational and conceptual convenience, p° has come into

 

common use more recently (since 1970). The redox capacity p° (eql L volt) at
any E, is deﬁned as the quantity of strong reductant in the equivalent which must
be added to a liter of sample solution to lower the E, by 1 volt. Nightingale (1958)
ﬁrst deﬁned the redox capacity as P = dC, Id (Eh), where dCr is the change in
concentration of reductant and d (Eh) is the change in redox potential. According

to Stumm & Morgan (1997), the redox capacity or intensity as p° = - log {e} gives

the hypothetical electron activity at equilibrium and measures the relative

tendency of a solution to accept or transfer electrons. The electron activity can be
expressed in p° or in volts.
In a highly reducing solution, the tendency to donate electrons, or the

hypothetical electron pressure or electron activity, is relatively large. For

example, an alkaline solution has a greater tendency to accept the hydrogen ion.

The activity of hypothetical electrons is very low at high p°, just as the activity of

hypothetical hydrogen ions is very low at high pH. Thus a high p° indicates a
relatively high tendency of oxidation. The review of literature suggested that the

oxidation of organic matter (CHZO) is thermodynamically possible in all ranges of
p° in surface and groundwater systems. Water in solubility equilibrium with

atmospheric oxygen has a well-deﬁned p° value, 13.6 (open air produces a

17

pressure of 1 atm, and since air contains 21% oxygen, the oxygen pressure in
the atmosphere, P02 =0.21 atm, Eh: 800 mV at pH =7 and T=25°C).

In contaminated groundwater systems, the redox gradient is often steep
and the fate of redox-sensitive contaminants may be rate dependent or otherwise
unpredictable (Langmuir. D, 1997). In the presence of 02, with p°> 11, aqueous
iron and manganese are stable only as solid oxidized oxide (Stumm & Morgan,

1996). Such a compound, when hydrated arsenate is adsorbed with iron oxides,
is called pittcite. Its hardness is 1.5 to 2 with a low speciﬁc gravity of 2.5. Stable

Eh measurements are only possible in well-balanced systems (Kent et al., 1994).

2.4.1 Dissolved oxygen supply, redox potential values, and their
association with arsenic release processes in groundwater
Dissolved oxygen (DO) is the most thermodynamically favored electron

acceptor. When recharge water is oxidizing, it will have a signiﬁcant redox buffer

capacity at high E, or p° values. The environment is termed oxic when dissolved

oxygen (D0) is greater than 30 p M (Drever, J., 1988). Under oxidation or oxic
conditions the redox potential (Eh) of groundwater is positive; reducing conditions
are characterized by negative readings.

The redox potential of sedimentary groundwater generally ranges from
-400 miIIi-Volts (mv) to +800 mv (AECL, 1996). Therefore, only mixing with
oxygenated water or lacking reductant minerals or organic matter in the system

can maintain the desired high oxic condition or Eh level in the groundwater.
18

However, it is very difﬁcult to generalize the redox level in the groundwater
because the E. or p8 level depends very much on the time of residence of

groundwater, and the time of residence varies with the groundwater velocity and
length or distance of the aquifer system (Drever, J., 1988). The redox potential
varies along the groundwater flow path in response to contact with reduced
mineral species and organic matter (AECL, 1996). Bottomley et al. (1990)

concluded that it is hard to establish a clear correlation between dissolved
oxygen and E, because Eh varies only slightly in the region where dissolved
oxygen is measurable (0.01-10 mglL).

They also found in their study that most of the groundwater E, was not
controlled by the dissolved oxygen concentrations because the E. was less than
the values expected for 02 control (+700 to 800 mv). However, the E, may also

be quite low (<100 mv) for groundwater where dissolved oxygen (02) can be

detected (AECL, 1996).

2.4.2 The sequence of redox process in the groundwater system
As oxygen is the most electronegative element, it has the strongest

potential to accept electrons and to be reduced. Therefore, oxidation of organic

matter (Cl-I20) is generally observed to occur ﬁrst by reduction of 02 [p°

(W)=13.8]. Once the dissolved 02 has been consumed, oxidation of organic

matter will occur in groundwater, but more slowly. The oxidation of organic

19

matter, driven by microbes, will result in reduction of major and minor dissolved
species in the order of N03, Mn (IV), Fe (III), so}: HC03 and then NH4-N2

(Stumm & Morgan, 1997). Certainly this described sequence would be expected
if reactions tended to occur in order of their thermodynamic possibility. These

dissolved species, present in the amount of more than 10"5 mol/ L, are capable of

controlling the redox potential E, of groundwater (Stumm & Morgan, 1996).

 

Well .—.. (is ,
, an .9.- g- -
. I j e a
WT- if L1 2 22
L2
I —WT - —

Org-zenTethit-ﬁ (.5, :1- 3 5.3)- —
No3-N reduction (p° =12 to 1)
Mn(IV) to soluble Mn (II) p° 10 to 1
Red. ofFe(IlI)to Fe(II) p“ =.5to —7 .5

 

 

  

 

92‘1 or 01‘ I°=-. 0-105
Hydrogen reduction zone

Fig. 2.1 Sequence of redox potentials in the groundwater system
2.4.3. Controlling redox potentials In the groundwater system

Grenthe et al. (1992) concluded that a low concentration of dissolved
Fe2+(>10'5 moi/L) was sufﬁcient to control the system’s redox potential values.
Ross and Gascoyne (1993) also found that groundwater containing low

concentrations of dissolved Fez” (typically<0.5 mglL) has stable and reproducible

20

E). values, which broadly indicate control by the Fe (lI)-Fe (III) couple. In another

study of groundwater where E. has been routinely measured, Gascoyne and
Kamineni (1992) found that E. was decreasing with depth to a value close to
those predicted by the Fe (")1 Fe (III) redox couple. The extent to which Eh
decreases as groundwater travels along a ﬂow system is largely determined by
the associated mineralogy of the flow system, the type and concentrations of the
dissolved redox sensitive species in the groundwater and the presence of
microorganisms to catalyze the reactions (AECL, 1996). Recently, the presence
of dissolved oxygen has been observed in many groundwater sampled from
depths up to 450 m in permeable sandstone in northern Saskatchewan (Cramer

and Smellie 1994)

2.4.4 Importance of dissolved oxygen content in the recharging
groundwater system

Dissolved oxygen concentration in groundwater is an important factor in
terms of water quality, especially for the issue of arsenic contamination. In
oxygenated waters, arsenic acid species (H3AsO4, HzAsO4, HAsO 2', AsO43')
are stable. In oxygenated water, adsorption reactions are thermodynamically
favored by both enthalpy and entropy change (Stem & Morgan, 1997). In
oxygenated water, dissolved Fe2+ is converted to Fe (OH)3 (s) and anions of
arsenic acids can be adsorbed onto the surface of the amorphous solid iron

hydroxides.

21

In natural oxic groundwater and aquatic systems, sorption reactions,
which are different for As (V) and As (III), strongly control the mobility of arsenic
(Mok and Wei, 1994). In an oxic environment with slightly alkaline pH values,
typical of most natural waters, sorption of As (V) onto Fe oxyhydroxides can
transfer arsenic from the dissolved phase to the particulate phase (Xu et al.,
1991; Fuller et al., 1993). A study conducted by Masscheleyn et al. (1991) also
demonstrated that at low Eh (20 mv) the As (Ill) and dissolved iron content were
the highest (Table 2.1) values. Therefore, dissolved oxygen in recharging water

is an important factor in controlling arsenic release.

Table 2.2 Concentration of soluble arsenic species, Mn, and Fe2+ as a function of

soil redox -pH condition

 

 

 

 

 

 

 

Fe
Asa") AslV) Mn (mg/kg
”3" p“ E" ("’V’ ("IQ/k9 soil) (mg/kg soil) sell) (231},‘9

0 5.6 340 1.4 5.6 1.9 17.5
3 6.7 100 46.8 3.1 20.8 15.3
6 6.8 20 76.5 6.6 21.2 72
0 5.8 440 0.9 7.9 2.4 27.3
3 6.6 80 34.3 1.2 17.4 29.5
6 7 -100 69.4 3.4 16.2 75.8

 

 

 

 

 

 

 

 

 

Source: Masscheleyn et al., 1991.

22

2.5 Existing theories of arsenic release mechanism in the groundwater of
Bangladesh

There are divergent opinions regarding the arsenic release
mechanism in Bangladesh’s groundwater. Theories offered so far can be broadly
classiﬁed into two groups, the oxidation theory of arsenic release and the

reduction theory of arsenic release.

Oxidation theou of arsenic release

Increased supply of oxygen in groundwater is welcomed by the groups
believing pyrite oxidation is the dominant arsenic release mechanism in
Bangladesh. The cause of an increased supply of oxygen may be the
development of heavy duty tube-wells known to cause extension of the vadose
zone and to supply more oxygen to the groundwater (Mallick et al., 1995).
Lowering the water table by pumping (Kinniburgh et al., 1997) or by nitrate
pollution of the aquifer (Postrna, 1991) may accelerate pyrite oxidation. In the last
two decades, the Green Revolution in Bangladesh has lowered the groundwater
level and, thus, the release of arsenic (Tsushima, 1999).

Arsenic is released by the oxidation of arsenical pyrites in the alluvial
sediments as aquifers’ drawdown permits atmospheric oxygen to invade the
aquifer (Mallick & Rajagopal, 1995; Chowdhuri et.al., 1999). They suggested that

the high volume extraction of groundwater in the region for irrigation has exposed

23

the younger deltaic sediments to air, which through oxidation reactions has

caused the decomposition of iron pyrites to ferrous sulfate (FeSO4), ferric sulfate
(Fez ($04) 3) and sulfuric acid. The process frees up arsenic, which is then

oxidized into arsenite (As203) and arsenate (As205) both of which are soluble in
ground water (Charkraborti et al., 1996).

Chemists belonging to this oxidation theory group have pointed out that
sediments of the younger deltaic depositions (YDD) have some intermittent
arsenic bearing beds or layers of arseno-pyrites and eventually arsenic can be

released in the form of A5203 and A5205 from these sediments when they come

into contact with atmospheric oxygen.

Reduction theory of arsenic release

On the contrary, investigators from the British Geological Survey (368,
1999), working on Bangladesh’s arsenic problem, explained the arsenic release
mechanism in groundwater as a reduction of arseniferous iron oxihydroxides
when anoxic conditions develop during sediment burial (Nickson et al., 1998).

This group also found strong evidence for and correlation between reduction of

ferric oxides and hydroxides and their by-products as dissolved iron (Fey) and

(HCO'3) production in the groundwater. Several other studies have investigated

arsenic’s geochemical behavior and have pointed out the association between

As (v) and Fe-oxyhydroxides by coprecipitation (Masscheleyn et al., 1991)

24

Dissolved iron, preliminarily present as Fe (II) in ground water, plays a
very signiﬁcant role in adsorbing and releasing arsenic and other trace cations

and anions (Khan et al., 2000). In contact with air, Fe (II) is oxidized to Fe (III) and

precipitates as Fe (OH) 3, hydrous ferric oxides (HFO: Fe203, Fe (H003); 2-3

H20). (Scott et al., 1995). It is well known that HFO binds arsenate formed during

the oxidation process (Sullivan et al., 1996). Therefore, arsenic can be removed
by iron species either by compound formation or by adsorption or both under
oxidation conditions. On the contrary, sorbed arsenic can be released under
reducing conditions from the body of HFO as a function of pH (Raven, et al,
1998).

Opinions contradicting pyrite oxidation theoLy
The 368 (1999) investigators strongly oppose the idea of oxidation of

arsenopyrite as an arsenic release mechanism in Bangladesh. The oxidation
theory of West Bengal (India), however, fails to explain the increase in arsenic
level in deeper wells and anoxic conditions in part of Bangladesh (Nickson et al,
2000). Arsenic in Bangladesh’s groundwater cannot derive from the presently

accepted mechanism, whereby water-level drawdown from abstraction wells
allows atmospheric 0; into the aquifer and so allows the oxidation of arsenic-

bearing pyrite, with concomitant release of arsenic to the groundwater (Das et al.,

1995, Chowdhury, R., et al., 1999). Such a mechanism is incompatible with the

25

redox chemistry of the water. Arsenic produced in this way would be adsorbed to
the FeOOH, the product of oxidation (Mok 8: Way, 1994; Thornton, 1996).

Opinions contradicting ogyhydroxide reduction theopy:

In reduction theory, the correlation between iron and arsenic cannot be
considered as a valid relation. Regarding the appropriateness of considering
extractable iron concentration, Bridge and Husain (2000a; 2000b) noted that the
correlation between extractable iron and arsenic does not necessarily imply the
adsorption (oxy-hydroxide reduction hypothesis). Questions were put forward by
Adel (2000) to Nickson et al. (1999) as to why the arsenic concentration was just
detected in the 1990’s if organic carbon has been taking oxygen for thousands of
years. Adel (2000) also mentioned that, to support the reduction hypothesis,
Nickson et al. (1999) showed 6% organic carbon content in the aquifer sediment,

which goes beyond the boundary of scientiﬁc evidence. Scientists belonging to
this group also claimed that the lack of 804'2 ions in groundwater is not a valid

reason to reject the oxidation theory of arsenic release.

2.6 Lowering of the water table, ability .of atrnospherlc oxygen access, and
redox potentials of the aquifer

The oxidation and reduction potential of groundwater mainly depends on
the access of atmospheric oxygen to the saturated aquifer and the amount of
dissolved oxygen present in the recharge groundwater. A large percentage of soil

pores are ﬁlled with water in saturated soil or aquifer systems. The water content
26

in the aquifer systems varies in time and space because of irrigation, ﬂood,
evaporation and inﬁltration. As water is replacing air (or vice versa), the change
in water content generates a vertical ﬂow of air (Refsgard et al., 1991). An
exchange of gases between the soil and the atmosphere is primarily due to the
mechanism of diffusion (Buckingham, 1904; Penman, 1940).

Generally, two main processes operate to effect gaseous transport within
the aquifer: convection and diffusion. The dynamics of the water level are
basically responsible for the convective ﬂow of air to the soil matrix and the soil-
water system. But this convective transport of oxygen in the soil-water system is
at typical pore-water velocities in the unsaturated zone (1 mlyear) and much
more signiﬁcant than diffusive transport in free water (Refsgard et al., 1991).

However, to understand the sub surface aeration system in a more

comprehensive way, Mukhter et al. (1996) mentioned three well known aeration

indices, namely 02 concentration, the oxygen diffusion rate (ODR) and the redox
potential (Eh) of the system. It was revealed from their study that E), varies

linearly with ODR up to a certain range, and after that, the E), remains constant
with increasing ODR. Mulder et al. (1996) also observed that, in general, an
oxygen concentration of nearly 80% is required to maintain adequate ODR
(above 20x10'8 9 cm'2 min") in the deeper layers.

If the oxygen concentration falls below the critical level, then ODR also

decreases. In another set of experiments on depths, Mukhter et al. (1996) found

that ODR varied inversely with depth (Fig. 2.2). At higher depths, oxygen

27

concentrations were below the critical level, so the ODR decreased. As the

oxygen diffusion coefﬁcient in water is nearly 10,000 times less than in the air

(Kalita, 1999), the diffusive supply of oxygen concentration in the deeper

saturated zone is smaller than that at the surface, which eventually reduces the

oxygen diffusion rates (ODR)).

 

 

 

 

  

 

 

 

30 cm depth
y a 0.20! + 2.85
R’: 0.26 I

<9 0° °, ’53.
0 0° '13:}: .‘..:.:‘ ‘

‘008 W80 IQO. e to".

 

  
    

 

 

IScIndeptli
pub-0.84 0
. w _
. m , li’slut 03 33 »_
I m i o :3, 60 ’
. w ’ '31.. 50 i
(I _ O :3. , 40 .
. 20 . 0 .°. ' 10
(time 9% 4. ‘° «ti -' °
. o m n m w 100
0,conoalttntiorll%suunted)
60 cm depth
y = 0.130): + 5.57
R’ a 0.23

 

 

 

683888838

 

20 ‘0 so 50 100
0, Concentrationlttsnuratadl

 

O 20 40

 

O; Concentr-atlon(%8aturated)

Fig. 2.2. Oxygen Diffusion Rate (ODR) decreasing over depth at 15, 30 and 60

cm below the ground surface. (Source: Mukhter et al., 1996.)

28

There is a common misconception that lowering of the water table
provides a greater atmospheric oxygen supply to the groundwater. This may not
be the real case when oxygen is consumed by organic carbon at a constant rate

in the soil matrix.

2.7 Lowering of the water table, change in thermodynamic properties, and
arsenic release into the groundwater system

Chemical reactions in a system take place spontaneously. The
spontaneity of the chemical and physical reactions is changed by entropy. The
concept of entropy is based on the second law of thermodynamics. Review of
the literature indicated that when a spontaneous chemical change occurs in a
system, if the total entropy change for everything were calculated, a positive
value of entropy would always be obtained. The entropy is the change in
enthalpy, or heat change over temperature, and can be deﬁned by the following
equation (Sawyer & McCarty (1991):

 

dS = gq—If‘l— ............. Entropy Transfer
2

VS = s, —S, = id“:
1 T

where S is the entropy of the system and T is the absolute temperature. The
quantity q m is the amount of heat that the system absorbs if a chemical change

is brought about in an inﬁnitely slow, reversible manner. The entropy of a

substance at 0 K is zero.

29

dSsys = dSm + dSsur
where dsys is the entropy transfer to the system from its surroundings, dSim is

internal entropy change (zero when the system is reversible), and the entropy

transfer to the system by the surroundings is dSSm, or dqlT.

If the AS for the whole system were positive, the reaction would occur
spontaneously. This spontaneous process occurs without a change in internal
energy in the container, but with an increase in entropy. In this research, both
energy and entropy factors were investigated in order to determine what
processes were responsible for making arsenic release reactions spontaneous in
the groundwater system. Both energetic and dynamic information enables us to
understand the processes.

2.8 Arsenic transportation in the groundwater system

The literature review has revealed that chemical contaminants in the
natural and groundwater can be transported in particulate, or colloidal, form and
in ionic form. However, it is not clear from the literature whether arsenic
transportation in the deeper layers is of the colloidal form or the ionic form.
When a pressure gradient is imposed on the solution in a clay-water system,
water molecules can move through the pores, but ions cannot (Hanshaw and
Coplen, 1973). Clay being changed behaves as a semi-permeable membrane,
allowing the passage only of water, not of ions (Drever, 1988). This phenomenon
is known as the anion exclusion. The membrane ﬁltration has been suggested to

be a major process affecting the composition of water trapped in the deeply

30

buried sedimentary rocks (Graf, 1982). Perhaps this is the reason why deep

aquifers in Bangladesh are still safe and less arsenic contaminated.

Particles or colloidal transmrtation of arsenic
A colloidal system is deﬁned as a system in which particles, in a ﬁnely

divided state, are dispersed in a continuous medium (Beneﬁeld, 1982). Colloidal
particles are not limited to any particular group or substance but are deﬁned by
size (1 nanometer to 1 micrometer). Electrophoresis phenomena indicate that
colloidal particles carry electric charges. The sign and magnitude of charges
depend on the nature of the colloidal materials. The stability of colloidal
suspension and the ion exchange property of solids are closely related to the
behavior of double layers (Drever, 1988). When a clay particle (smectite) is
suspended in water, some of the cations pass into water, and the water develops
a negatively charged surface and a diffuse cloud of oppositely charged ions,
called a double layer or Gouy layer.

In the solution, the charged particles repel each other and do not settle.
The electric charge on the surface of the particle is the primary factor responsible
for colloidal stability. The electrophoretic mobility of the particle is called the Zeta
potential. The magnitude of the Zeta potential is a rough measurement of the
stability of the colloidal particle. A high Zeta potential suggests a strong force of
separation and a stable colloidal system. Chemicals are transported in colloidal
form in groundwater unless the process of coagulation overcomes the particles’

strong force of separation. However, in the case of arsenic, it is poorly
31

understood whether arsenic moves under colloidal or ionic form in the deeper

aquifer.

Ionic transportation and Retardation Factor

When any contaminants are introduced into the groundwater system, the
subsequent migration is strongly influenced by the cation exchange process.
When the solid adsorbs a cation, it migrates at a slower rate. If the ion exchange
reaction is rapid and reversible, the rate of advance of the front of the cation is
given by the following equation:

V
17,:

A
”13%,,

where v» is the velocity of the contaminant proﬁle, v is the velocity of the
groundwater, p is the bulk density of the sediment, q) is the porosity of the
sediment, and K, is the distribution coefﬁcient.

The expression [1 +(p/(p) * Kd] is called the retardation factor, which can be
used to estimate how rapidly a pollutant may migrate. The value of (pltp) in the
groundwater sediment is typically between 5 and 20. Unless K, is very small, the
retardation factor will have a large effect on contaminant transportation. K, is a

valid representation of partitioning between liquid and solid only if the reactions
that cause the partitioning are fast and reversible and only if the isotherm is linear

(Freeze and Cherry, 1979).

32

3. DESCRIPTION OF STUDY AREA

The study area covers most of the arsenic affected area in the southwest

zone of Bangladesh. The People’s Republic of Bangladesh lies between latitudes

20° 34'and 26° 38' north and between longitudes 88° 01' and 92° 41' east. The

country is bounded by India on the north and the west, by the Bay of Bengal on

the south and by India and Burma on the east (Fig. 1). It is mostly a low and ﬂat

deltaic land, with the exception of some hilly area in the northeast and southeast

and some highlands. The ground elevation is generally less than 100 meters

above sea level.

Bangladesh occupies the major part of the Ganges, Brahmaputra and

Meghna (GBM) river system, bounded in the east by Tripura Hill and in the north

by Shilong Plateau. Fluvial-deltaic sediments, hundreds to thousands of meters

thick, form highly productive, fresh water aquifers in most parts of the country.

Table 3.1 General facts about Bangladesh

Area Pop. Pop. Pop. Life

(34. km) Growth Dens. Expect-
: '? Rate ' ancy '
(Yrs)

148,396 124 2.10% 752 Male:
mil. year per 59.1

:q. Female:
m 58.6

Climate

Sub —
tropical
with 3
seasons
(winter,
summer,
monsoon)

Data Source: 2000 Statistical Yearbook of Bangladesh

33

Rain- Temp.

fall (°C)

(MM)

2320 Min: 7.2
to12.8,
Max:
23.9to
31.1

Area
Irriga—
ted

(Mha)
3.50

3.1 Geology of arsenic contaminated area In Bangladesh

Bangladesh lies within a geologic feature known as the Bengal Basin (Fig.
3.1). The Bengal basin was formed in the Cretaceous period, and sedimentation
continues today, both in Bangladesh’s sub-areas and offshore in the world's
largest submarine fan complex (the Bengal Fan) (Johnson et al., 1990). The
Bengal basin has four phases of deposition and erosion coinciding with four inter-
glacial and glacial periods in the Quaternary time (0 to 1.6 million years ago).
Prior to the recent deposition to the delta and active riverbanks, a deposition took
place sometime between 25,000 and 80,000 years ago (commonly known as the
younger deltaic deposition, YDD). The groundwater arsenic concentration is
mostly located in the YDD.

The sediments were mainly brought by the Ganges (Mallick et al. 1996).
The maior part of the western alluvial delta is called the Older Deltaic Plain
(ODP). developed during the Quaternary time between 80,000 and 1.6 million
years ago. The ODP extends into the sub-surface below the arsenic belts in the
Ganges River delta of Bangladesh. Perhaps that is why perhaps deeper aquifers

under the arsenic belt (YDD) are as yet free from arsenic contamination.

3.2 Geomorphology of Bangladesh and the primary source of arsenic
Bangladesh is formed mainly from the deposit of the Ganges,

Brahmaputra and Meghna (GBM) River systems (Fig. 3.2). These rivers combine

in Bangladesh to produce the largest active delta in the world (Reimann, 1993).

The majority of the catchments of the Ganges-Brahmaputra and Meghna (GBM)
34

river system lie in India, Nepal, Bhutan and China (Fig. 1.1, Fig.6.1). Fig. 3.2
shows that the deltaic and alluvial deposits of the GBM river systems underlie
nearly 85% of Bangladesh. Most of Bangladesh’s arsenic-affected area lies in the
Quaternary alluvial plains of the Bengal Basin. Because of their position close to
where the Ganges River enters Bangladesh, some authors have suggested they

may be the primary source of arsenic in the Bengal alluvium (Khan, 1995).

 

 

Fig. 3.1 Surface geology map of Bangladesh

 

 

 

35

Table 3.2 Average monthly discharge and sediment load in the major river

system in Bangladesh

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Month Ganges Brahmaputra Meghna
(Hardinge Bridge) (Bahadurabad) (Bhairab Bazar)
Flow Sediment” Flow Sediment‘ Flow Sediment‘
(ma/sec) (m3lsec) (m3/sec)
Jan. 3,113 2.0 5,194 4.0 594 0.02
Feb. 2,712 1.4 4,308 2.5 495 0.01
Mar. 2,312 0.9 4,711 3.6 635 0.06
Apr. 2,056 0.8 6,823 9.7 937 0.06
May. 1,971 0.8 15,844 36.4 1934 0.26
Jun. 4,311 3.9 32,488 87.6 3821 1.4
Jul. 17,871 48.7 44,080 121.6 7814 7.5
Aug. 37,546 168.8 45,107 163.3 8279 3.7
Sep. 36,970 156.1 36,295 117.8 8222 4.8
Oct. 17,244 81.6 21,955 42.9 6239 2.28
Nov. 7,109 10.5 10,477 12.1 3078 0.08
Dec. 4,195 3.2 6,737 5.4 990 0.01
Anrliu 137,408 478.9 234019 607.7 43,039 --
a

 

 

 

 

Data source: Coleman, 1969. * Sediment load is in million tons per annum;
Ganges discharge is the pre—Farakka Barrage monthly average, taken from
Rashid, 1991.

36

 

I
I
I
l
I
l
I
I
I
I
I
I
I
l
I
I
I
I
I

Fig. 3.2. Geomorphology of Bangladesh

3.2.1 Land and soil classiﬁcations

The majority of Bangladesh is covered with Holocene and recent ﬂuvial
and deltaic deposits from the Ganges, Brahmanaputra, 'I'Ista, Jamuna and
Meghna Rivers (Fig. 3.2). The descriptions of the lithology are presented in Table

3.3.
37

Table 3.3 Lithology and hydraulic properties in the study area of Bangladesh

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Source: BGS (2001).

 

 

3.3. Hydrology and hydrostratigraphy of Bangladesh

Rainfall

 

Lithology Hydraulic Conductivity (mlday) Speciﬁc yield (%)
Terrace Flood Plains Terrace Flood Plains
Clay - - 0.5 3
Silt - 0.4 4 5
Very Fine 8 - - -
Sand
Fine Sand 13 12 8 16
Fine to 17 26 - -
Medium Sand
Medium to 21 43 - -
Fine Sand
Medium Sand 25 57 20 20
Medium to 34 61 - -
Coarse Sand
Coarse - 38 63 - -
Medium Sand
Coarse Sand 46 95 25 25
Gravel 25 40 30 30

Bangladesh’s thirty-year national mean annual rainfall was 2320 mm, or

as arranged by region in increasing order, from 1765 mm in the Southwest (SW),

38

 

1820 mm in the Northwest (NW), 2410 mm in the South-Center, (SC), and 2690
mm in the Southeast (SE), to 2830 mm in the northeast (NE) region (MPO,

1985). The mean annual volume of rainfall in Bangladesh is 326,000 million his,

but about 81%, 264.06 billion m3, is received during the summer humid period,

from May to October (MPO, 1987). The ﬁve-year maximum two-day rainfall

variestrommorathanSOOmmintheNEtolessthan180mmintheNWend160

mm in the SW region.

Shallow aguifers

Groundwater in Bangladesh is mainly drawn from Quaternary strata. In
parts of the country covered by major DTW projects (Deep Tube Wells), there are
sufﬁcient Iithological data to permit mapping of the main lithology in the
subsurface of Bangladesh. In Sylhet, two groups of aquifers exist. In the basin
area, Pleistocene to Holocene sand forms exploitable aquifers in the upper 150
meters, but they tend to be thinner than those in the rest of the country {BGS &

Mott MacDonald, 1999).

Dee a u' e 3

At present, aquifers deeper than about 150-200 m are exploited only in the
coastal zone, and only for potable or industrial supply. Very deep aquifers are
deﬁned by depths greater than 350 m. In the southeast and south-center regions,
fresh water extends to the coast and sometimes to the Offshore islands, and it

may have artesian ﬂows. In the southwest, the saline front line lies about 20 km
39

north of Mongla port (Fig. 3.1). The very deepest aquifers have been tentatively
identiﬁed from electric logs of oil and gas wells by Jones (1985) at depths of one
to three thousand meters. However, there is no direct evidence of either the

water-producing potential or water quality of these strata.

3.3.1 Aquifer properties
Transmissivity and grmeability

Estimates of permeability and transmissivity were obtained from about 300
aquifer tests (UNDP, 1982; MMI, 1992) and tests on the municipal tube wells.
Aquifer tests provided detailed insight into the mechanism of flow around wells,
whereas because of their greater number, the DTW data showed the
geographical variation of the aquifer parameters. In different regions the alluvial
aquifers display conﬁned, leaky and unconﬁned responses to pumping. The
aquifer beneath the Holocene Flood Plains, especially along the Jamuna Flood
Plain (Physiographic map in Fig. 3.1), has the highest transmissivity (2,000-5000
mzlday) and is generally unconﬁned or semi-conﬁned. Aquifer properties are

presented in Table 3.4. In general, groundwater gradients over the country are
low, typically between 1 in per km (1:1000) in the north of the country to as low
as 0.01 m per km (1:100,000) in the south.

40

Table 3.4 Average aquifer properties in different geological formations and zones

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Hydrolog- Civil District Geologi- Transmissiv- Storage Refer
ical Zone cal Form- ity (mzlday) Coeffi- -ence
ation cient
Southeast Comilla Chandina 1 .200 1.3E—03 1
(SE) DISIII'ICt Formation
Noakhali 617 6
Northeast Sylhet 460 5.6E-04 4
(NE) Floodplain
North- Dhaka Dhamrai 3,480 7
Center Manikgonge Formation 4,211 7
(NC) Tangail 2803 2.9E—03 1
Bogra Recent 2,380 1.1E—03 1
District Alluvium
Northwest Dinajpur 2,755 2.8E—03 1
(NW) Nawabganj 3,172 6.7E-03 9
District
Pabna 4,316 1
District
Rangpur 4,384 2.6E-03 1
District
Southwest Jessore 3,660 1 .9E-03 1
(SW) District
Khustia 3,780 2.0E-03 1
Khulna 3100 1.0E—03 5
District
North- Dhaka City Dupi Tlla 1,333 8.3E-04 8
COMO! Madhuppur Formation 1161 1.7E—03 1,3,4
(NC) Tract
Northeast Sylhet Hills 249 1.3E-02 2
(NE)
Northwest Barind Tract 1835 1.6E-02 9
(NW)
References:
1. UNDP (1982) 2. HTS (1983) 3. MMPIHTS (1983)
4. MMI (1992) 5. Rus (1985) 6. MMP (1993)

7. Barker et al. (1989) 8. MMP (1991) 9. Ahmed (1994)

41

Table 3.4.a Approximate wet season regional groundwater gradient

 

 

 

 

 

 

 

Location Maximum Gradient Minimum Gradient
(m/km) (Tn/km)
North Zone 2 0.5
Central Zone 0.5 0.1
Southern Zone 0.1 0.01

 

Source: BWDB, 1993.

Table 3.4.b Average groundwater chemistry in different zones in Bangladesh

5, is 9‘ o ’8‘ I
it 8 8 F5 “’ S’s ‘1
NC 56.9 25.9 538 6.83
NC 71.2 25.8 285 6.65
SE 18.1 25.3 792 7.39
NE 55.1 25.1 299 6.94
NW 48.0 26.0 73 6.56
NW 42.4 26.42 339 6.86
NW 37.8 26.7 870 6.81
Avg. 47.1 26.3 528 6.90
References:

3‘
E
V

.c
“J

E if!

(mg/L)
(mg/L)

108 0.38 9.0
283 0.23
95 0.26

283 0.56

0.80
4.7
8.5
3.3
1.2
0.8
3.6

0.16
182 0.28

42

HC03
(mg/L)

213
452
198

231
354
279

A 3 A
9"? 8 9’83 :2
t... 5 v
4.3 20 1.3 2
0.6 2 0.3 2
0.4 61 7.3 2
0.4 4 0.3 2
3.0 1 0 3
5.1 9 4.3 1
2.4 96 - 1
3.1 58 2.1 -

1. Ahmed (1994) 2. Davies & Exley (1992) 3. Consociates (1970)

 

3.4 Groundwater chemistry in Bangladesh
Outside the coastal zone, groundwater is generally fresh. Bicarbonate is
the dominant anion in the fresh water. Sulfate is generally absent, which

contributes to the sulfate reduction.

3.5 History of irrigation development and area coverage in Bangladesh

Very little information exists on irrigation during the British colonial period
in Bangladesh (1757-1947). After the recommendation of the Famine
Commission of 1880, people started using farm ponds, tanks, private canals,
streams and channel for irrigation purposes. By the year 1903, the Dhaka and
Triveni canals of Champakaran district of Bihar were constructed. To assess the
future demand of irrigation, a high-power Irrigation Commission was set up for
the ﬁrst time in 1905-1906, during the period of Lord Curzon (1899-1905). As per
its recommendation, 50,000 miles of canals and distributaries were established
to provide irrigation for 21.5 million acres of the whole British lndo-Pak-
Bangladesh subcontinent.

Between 1947 and 1971, Bangladesh directed attention mostly to surface
irrigation and ﬂood control projects. After 1971, Bangladesh put her all efforts into
making the Green Revolution a success. Most of the Deep Tube Well (DTW) and
Shallow Tube Well (STVV) projects were commissioned from 1972 to 1980 by the
Bangladesh Agricultural Development Corporation (BADC). Irrigation coverage

has been increasing steadily with time, as shown in Fig. 3.3.

43

Table 3.5. Total number of Deep Tube Wells (DTW), Shallow Tube Wells (STW),
and Low Liﬂ Pumps (LLP) (irrigation devices) used for both surface and

groundwater Irrigation in Bangladesh from 1994 to 2000.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Techno- Hydrologic zone Total in
'°9Y 8* SC SE NC sw NW NE Bang'a'
Year Zone Zone Zone Zone Zone Zone desh

SHALLOW Tu BE WELLS ( STW)

1994-95 - 13649 109614 93405 271686 515 488869

1995-96 53 14927 128064 115786 316592 826 576248

1996-97 20 16225 144423 126877 339300 1133 627978

1997-98 32 17687 152816 136033 356704 1399 664671

1998—99 29 19926 174951 152185 387101 1858 736050

1999-00 26 20875 181199 162325 390491 2128 757044

DEEP TUBE WELLS (DTW)

1994-95 - 2613 8890 3028 11924 200 26655

1995-96 1 2461 8806 3149 12536 156 27109

1996-97 - 2260 7717 3143 11891 118 25129

1997-98 - 2312 7795 3118 12017 112 25354

1998-99 - 2448 8147 3126 12871 126 26718

1999-00 - 2291 7495 3056 12162 100 25104

Low LIFT PUMPS (LLP)

1994-95 4527 24115 14149 3263 5642 5349 57045

1995—96 5590 24741 14978 3654 5759 5973 60695

1996-97 5193 25208 15005 3995 6031 6962 62394

1997-98 7298 25531 15470 4384 6155 7412 66250

1998-99 8711 27221 16832 5518 6608 8028 72918

1999—00 7995 25375 16605 6243 7295 8057 71570

 

 

 

 

 

 

 

 

Source: National Minor Irrigation Development Projects (NMIDP, 1996/97).
Reference: Main Report of National Minor Irrigation Census (NMIC), 199912000,

Prepared by NMIDP & DAE Dhaka, January 2001.

3.6 Rice irrigation and rice water requirements In the SW zone study area
AS a general rule, a growing rice crop needs 10 millimeters of water a day.
This is equivalent to 100 tons of water per hectare. To provide 1 hectare with
supplementary water, it needs a capacity of 166 liters per minute (44 US gallons
per minute) if operated 10 hours per day or 70 liters per minute (18.6 gallons per

minute) if operated 24 hours per day during a dry spell.

 

 

 

 

 

1834 1005 IS” 183? 1898 IS” zoos

 

 

 

Fig. 3.3 Growth of shallow tube wells irrigation from 1994 to 2000 in Bangladesh

45

3.7 Groundwater level in specific study area in the southwest (SW) zone
The regional hydraulic gradient is very low, reflecting the low topographic
gradient (Table 3.3.a). In the southern coastal areas, the piezometric surface in
the deep aquifer is approximately 1.0-1.5 m above the mean sea level. The
decline in water levels due to extraction for irrigation during the dry season
through the use of shallow and deep tube wells is signiﬁcant in the speciﬁc SW
zone study area (Jessore and Kustia district) (Fig. 3.4). This represents the
groundwater level of the speciﬁc study area in the Ganges delta area, where
intensive irrigation has been practiced. Also, the groundwater is badly affected by
arsenic contamination. In this area, about 77% of the irrigable area was irrigated

using groundwater, of which 70% was obtained from shallow tube wells.

WWW

Sarsa, Jessore, (JE-13‘)

 

 

 

 

 

1 974 1 978 1 982198 9901
Year

 

 

‘Joydepur BAOC} A8841} _ A -
1985 1986 1987 1988 1989 1990 1991

- A 1 - a 1

 

Fig. 3.4 Fluctuation of the groundwater level in the Speciﬁc study area in the SW
zone of Bangladesh. Source: BWDB
46

4. METHODOLOGY

The main hypothesis of this study is based on the idea that less oxygen is
in the groundwater at and below the water table due to the extension of the
unsaturated zone by overpumping of irrigation wells. Pumping at a rate greater
than that of aquifer recharge causes the water table to lower farther and reduces
the rate of oxygen diffusion to deeper layers. Eventually a reducing condition is
developed and arsenic is released. This analysis mainly targets addressing the
relationship between the arsenic concentration in groundwater and Its
interactions with the irrigation pumping rate and the aquifers physical and
chemical properties such as aquifer thickness, conductivity and redox potential
values (P°)‘. The overall goal of this research is to formulate a quantitative

irrigation policy to maintain the present pace of irrigation development.

4d Research approach and research Issues

To accomplish the purpose of this research study, three analytical
approaches are used: statistical, thermodynamic or geochemical, and
mathematical modelings. Three objectives are presented as portions of the goal
of the research. The approaches utilized to achieve these objectives are
delineated below-
ijective 1

To identify the processesthat produce the high arsenic concentration in

Bangladesh’s groundwater.

The approach to be followed under objective 1 will be to investigate the
following three research questions:

1. What are the hydrologic factors that exhibit a pattern or correlation with high
arsenic contamination in the groundwater system of Bangladesh?

2. What geochemical processes (such as pyrite oxidation or the oxyhydroxide
reduction process) mainly control arsenic contamination in Bangladesh’s
groundwater system?

3. What is the interaction between the lowering of the water table position, the
dissolved oxygen content in recharge water and the triggering of the arsenic
contamination problem in Bangladesh’s groundwater?

The ﬁrst research issue will identify the hydrological and geochemical
factors or activities that facilitate the arsenic contamination problem in the
groundwater system. Statistical analysis (probability of exceedence, correlation
analysis, and regression analysis) will be applied. The second research issue will
identify the geochemical processes responsible for arsenic contamination.
Thermodynamic and mathematical analysis will be done to evaluate whether
oxidation or reduction processes are responsible for arsenic mobilization. The
third research question will deal with the issue of the lowering water table and its
inﬂuence on the ability of atmospheric oxygen to diffuse to deeper layers. A ﬁnite

element analysis will address this issue.

48

O iective 2

To formulate a quantitative irrigation policy to maintain the present pace of
groundwater irrigation development.

The following research issues will be investigated in order to achieve
objective 2:

4. What is the inﬂuence of the irrigation withdrawal rate and of cropping systems
on the amount of shallow aquifer recharge (SAR) available?

5. What is the inﬂuence of Shallow aquifer recharge (SAR) on the groundwater
ﬂow patterns and vertical migration of arsenic from the currently contaminated
layers to the uncontaminated deeper layers of the aquifer?

6. What is the inﬂuence of the placement of tube well screens at deeper layers
on arsenic migration to those deeper layers?

The fourth, ﬁlth and sixth research questions address the managerial
issues in a manner that will reduce the risk of any further arsenic contamination
of groundwater being caused by future irrigation development. The fourth
research question is concerned with whether overpumping of groundwater did
provide the conditions favorable for triggering arsenic release in Bangladesh’s
groundwater. The ﬁfth and sixth research issues will address the inﬂuence of
aquifer pumping rates on groundwater ﬂow patterns and the overall arsenic
contamination problem in Bangladesh’s groundwater.

The IGW interactive groundwater modeling software was used to

understand the inﬂuence of the pumping rate and the aquifer recharge rate on

49

the ﬂow patterns and the possibility of arsenic migration. For example, if in order
to avoid arsenic-contaminated layers in the aquifer, the tube well screen is
placed in deeper layers with solid casing above; arsenic can migrate from the
overlain top layers to deeper layers. Extraction of a greater freshwater volume
than the volume of recharge (SAR) may invite salinity intrusion in the aquifer.

Objective 3

 

To delineate a safe aquifer zone on the top of the summer water level where
arsenic concentration would be minimal so that small-scale wells can be installed
for irrigation and drinking purposes.

The approach to be followed under objective 3 will be to investigate the
following research question:

7. What administrative or managerial decision Changes or policy changes can
reduce the risk of arsenic contamination and help the installation of small-scale
drinking wells?

This research issue will address the dimensions and set out the criteria
that separate the aquifer safe layers from the most arsenic-contaminated layers
on the basis of redox potential values. For example, new recharge water at the
top of the summer water level is rich in dissolved oxygen, has higher redox

potential values, and naturally would be arsenic-free and safe for drinking.

50

4.1.1 Data collection strategy

Despite difﬁculties in obtaining data, a strategy was developed that satisﬁed
the statistical and analytical requirements for this study. There are 64
administrative districts and 453 Thana or sub-districts (second-layer
administrative units in Bangladesh). Each Thana was divided into approximately
10 imaginary cells of equal area where at least 5 sample wells were likely to be
available. The arsenic concentration records of those wells were collected from
the reports submitted to the Government of Bangladesh (GOB) by different
organizations working on the arsenic problem in Bangladesh (BGS, DPHE, etc).

In addition to that, collection of information on surface and subsurface
hydrogeology, the growth rate of deep and shallow tube wells, irrigation
coverage, aquifer properties, varieties of crops grown and water management
practices and policies were collected from sources like the MPO (Master Plan
Organization of GOB), the DPHE, and non-governmental organizations like Mott
MacDonald Ltd. (UK), the British Geological Survey (BGS), SOES (School of
Environmental Studies, India), etc. The raw data are appended to this

dissertation.

4.2. Analysis of research issues
In order to address the ﬁrst research question, the hydrologic factors that
exhibit correlation with arsenic concentrations, the following statistical analyses

were conducted.

51

4.2.1 Statistical analysis of the arsenic distribution pattern and its
relationship with surface geology, depth of the aquifer, dissolved Iron
content, and groundwater ﬂow direction
On the basis of water resources and hydrogeological variations,
Bangladesh was divided into six different water resources zones (MPO, 1987):
northeast (NE), north-center (NC), northwest (NW), southeast (SE), south-center
(SC) and southwest (SW) (Fig.4.2.b and Table 4.2.2). The distribution patterns of
arsenic concentration were statistically analyzed within the boundary of each
group. At ﬁrst, the probability of arsenic levels exceeding 10 ppb (WHO
Standard), 25 ppb (Bangladesh's target), 50 ppb (current Bangladesh standard),
100 ppb and 250 ppb was computed for each classiﬁed zone using exponential

and Gumbel distribution methods. The results are presented in Fig. 4.2.a.

4.2.2 Computation of probability of arsenic concentrations exceeding
threshold level In Bangladesh’s groundwater

For computation of the probability of arsenic concentrations exceeding the
threshold level (50 ppb), Gumbel distribution was used because the arsenic
concentration distribution pattern does not satisfy the conditions of normal
distribution. On the other hand, the descending exponential distribution (p = 0*
e'A) is very common in natural and hydrogeologic events such as contamination
distribution and ﬂood frequency distribution. Here, ‘0’ and 'A‘ are positive

parameters and ‘e‘ is exponential. The main advantage of the exponential

52

probability is that when a point on the probability density function decreases to
one half the maximum value. the half value density therefore marks the point at
which 50% of the data are larger and 50% of the data are smaller. For the unit
area, the probability equation (4.1)
P = c * e 4‘" (4.1)
can be expressed as follows in equation 4.2.

a _1 a C
1=C ”A’dx=C—‘A‘) =—-.
le be . A

Unit area requires C = A.
The exponential probability function was calculated by integrating the probability

equation (Eq. 4.1),
P: AIe'A‘dx =1—e‘A“
(t

The Gumbel or Type 1 distribution (Gumbel, 1945) is an exponential distribution
form and has its skewness ﬁxed at +1.14. The probability of exwedence of

threshold arsenic levels was computed from the following Type-1 equation

1— F(x)=1—e‘e-b
where ,
1 (4.3)

b =
0.779 *0'

 

.(x—p + 0.450)

53

x = magnitude of the concentration, it = average concentration, and o = standard
deviation.

The probability of arsenic in exceedence of the threshold level (10, 25, 50 ppb) in
different hydrologic zones was computed and the results are tabulated in Table
4.2.1.

For examme

 

The probability of arsenic exceeding the threshold level (50 ppb) in the
southeast (SE) zone was computed at 71.40% (Table 4.2.1) by using Equation
4.3. The computed values of ‘u’ and ‘6' were 174.13 and 199.14, respectively
(Table 4.2.1). To compute the probability of exceedence of arsenic threshold
level (50 ppb),

b = (1 /0.779 * 199.14) * (50 -174.13 + 0.45 * 199.14 - 0.222) and e’e'b = 0.286

So, the probability of exceedence of 50 ppb arsenic in water is

100 *[1- F (x)] = 100 *(1- 0.286) = 71.40.

54

Table 4.2.1 Probability of exceedence of arsenic concentrations in Bangladesh's

different hydrologic zones

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Hydrologic Total Avg. As St. Probability of exceedence (%)

2°” 82%;; ((35:55 Dev' 10 25 50 100 250
dWeus ppb ppb ppb ppb ppb

Northeast 1039 34.00 67.96 58.73 48.7 34.00 14.90 <1

(NE) (0.5-572)

North- 192 28.64 51.43 53.99 48.08 38.90 24.4 4.98

CGI‘IICI’ (0.5-284)

(NC)

Northwest 1072 12.25 47.17 44.93 32.74 18.15 5.00 0.30

(NW) (0.5-708)

Southeast 295 174.13 199.14 80.13 76.26 71.40 59.55 29.14

(SE) (0.5-1090)

Southwest 474 84.80 145.00 66.26 61.41 53.30 38.74 12.00

(SW) (0.5-1660)

South- 295 38.61 113.18 53.99 48.05 38.95 24.42 4.98

Center (0.5-862)

(SC)

 

4.2.3 Distribution pattern of arsenic contamination over hydrologic zones

In Table 4.2.1, the probabilities of arsenic exceeding 10, 50, 100 and 250

ppb in the upper 150 m of the aquifer system were computed. These values were

superimposed on the hydrologic zone maps of Bangladesh (Fig. 4.2.b) in order to

reveal the distribution patterns of arsenic and its relationship with the hydrologic

zone. The northwest (NW), northwest (NE), southwest (SW), southeast (SE),

north-center (NC) and south-center (SC) zones are described in terms of surface

geology and geomorphology in Table 4.2.2. Arc View spatial analyst software

was used for surface interpolation and statistical data analysis.

55

 

The probability of arsenic in exceedence of the threshold value (>50)
strongly correlates with hydrologic zones. The probability Of arsenic exceeding
the threshold level (50 ppb) was greater than 50% in the SE and SW zones and
less than 50% in the northwest (NW), north-center (NC) and south-center (SC)
regions of Bangladesh (Fig. 4.2.b and Table 4.2.1).

Table 4.2.2 Description of types of hydrologic zone, geomorphology and surface

geology in Bangladesh

 

 

 

 

 

 

Hydrologic Types of Surface Geology Types of
Zone Geomorphology (Fig. 4.3.c ) Physiographic
(Fig. 4.2.b) (Fig. 4.3.a, ) Un'ts (F'9- 4-3-0)
Brahmaputra, Sylhet Old Brahmaputra
Northeast (NE) Surma/ Sylhet depression, Old Flood Plain (Fp),
Basrn Brahmanputra Sylhet depressron
Flood Plain (FP),
Meghalaya
Foothills
North—Center Terrace and Madhupur Tract Ganges River F P,
(NC) Alluvial Sand and Ganges Tidal
FP
Old Himalayan
Fan, Tista Flood
P'a'” (FP)' L°wer Tista Fan, Tista Fan,
Purnabhaba F P, F d
Northwest Karatoya -Bengali Jemima F|°°d 48mm 'loo
- PlaIn, Atral Flood Plain, AtraI Flood
(NW) FP, Meghna River Pl . B . d PI .
FP, Meghna am, am am
Estuarine FP,
Chittagong Coast
and Hills
Meghna River FP,
Southeast (SE) Meghna Alluvial Deposits Tippera Surface
Estuarine F P,
Chittagong Coast
and Hills

 

 

 

 

 

 

 

(4) (5)

 

 

 

Fig. 4.2.a Blue dots on the map (1 and 2) indicate arsenic-contaminated tube wells with
probability of arsenic concentrations >10 ppb and >25 ppb, respectively. Red dots on the

map (3, 4, and 5) indicate wells with arsenic >50 ppb, 100 ppb, and 250 ppb,

respectively.

57

 

 

 

 

 

Fig 4.2 b. The probability of arsenic in exceedence of the threshold level (50 ppb)
is low (18 to 39%) in the NW, NC and SC zones, but in the NE, SW and SE
areas It is higher (53 to 71%).

58

 

Old Me 3%

 

Fig. 4.33 Geomorphology of Bangladesh

59

Ganges Delta

 

Fig. 4.3.b General distribution pattern of arsenic over surface geology and

physiographic unit. (Red spots indicate arsenic >50 ppb.)

 

 

 

 

Fig. 4.3.c Surface geology (left) and the physiographic map of Bangladesh (right)

60

4.2.4 Correlation between arsenic pollution distribution, physIography and
surface geology of Bangladesh

Despite having large variability in spatial distributions, it was found that the most
arsenic contaminated area was mostly conﬁned to the lower ﬂood delta plains of the
Ganges River in southeast (SE) and southwest (SW) region of Bangladesh (Fig.4.2.a).
The dominant surface geomorphology is the Ganges ﬂood plain and the Meghna River
FP (Fig.4.3.a).

Relation between physiography and arsenic concentration in Bangladesh

The physiographic map analysis in Fig. 4.3.b clearly indicated that the highest
arsenic concentration levels (>250 ppb) are mostly in the SE region where the
physiograpic unit is dominated by Tippera surface (Fig.4.3.b & Table.4.2.2). This issue
will be discussed further from the context of hydrology and inﬂuence of river boundary on

the transportation of arsenic in the groundwater system.

4.2.5 Analysis of the correlation between arsenic pollution and intensity
of tube wall irrigation in Bangladesh

In this study, it was an important hypothesis that the lowering of the water
table due to tube well irrigation might have triggered the arsenic contamination in
Bangladesh’s groundwater. Therefore, a relation between the distribution of
arsenic pollution and intensity of tube well irrigation as well as the depth of the
wells was expected. In order to ﬁnd relationships, a groundwater irrigated area
map prepared by the GOB (1999) was considered as a base map on which the

highest percentage of net cultivable area irrigated by groundwater was shown by

61

the darkest coloration (right side of map in Fig. 4.4). Then the arsenic
contamination records with tube-well coordinates were superimposed in order to
establish the relation. The results showed that the most intensively irrigated
areas do not coincide with the most highly arsenic-contaminated areas.

The graphical evidence in Fig. 4.4 is apparently antipathetic and does not
provide any causal links or support to the relationship between arsenic pollution
and tube well irrigation. However, at this stage it cannot simply be said that
intensive irrigation does not contribute to the arsenic problem by observing only
the area irrigated. In order to get a comprehensive idea, it is necessary to include
parameters such as depth to the top of the main aquifer, speciﬁc yield, storage
coefﬁcient, recharge potential, gradients in the head, and geological and

thermodynamic explanations of arsenic release in Bangladesh’s groundwater.

4.2.6 Depth of irrigation wells versus arsenic concentration distribution
over the aquifer’s depth
The general patterns of arsenic concentration distribution over the aquifer

depth were plotted for 3500 sampled wells distributed over Bangladesh (Fig. 4.5).
One would expect higher R2 values if the arsenic concentrations were related to

the depths of the aquifer (O to 50 m and 100 to 350 m of the aquifer). The main
difﬁculty was to relate the depths with the arsenic concentrations because the

concentration records are totally based on stratiﬁed random variables such as

62

the aquifer's depth, hydraulic conductivity, vertical permeability, storage

coefﬁcient, etc.

 

 

>40% Am covered by
men (darkest zone)

 

 

 

 

 

 

 

 

Fig. 4.4 Distribution of groundwater irrigation coverage in Bangladesh. The
darkest areas use more than 40% groundwater for irrigation, darker areas use 5

to 40% and white areas use less than 5% groundwater for irrigation.

63

Arsenic Concentration Vs Depth of the wells

_A
Cl)
C)
C)

As (ppb)

 

L a Q '

0 50 100‘ 150 2007250 3007350 400
WellDepth(M)

Fig. 4.5 Arsenic concentration range over the depth of Bangladesh’s aquifer

To determine the relation between depth of the aquifer and arsenic
concentration for the study area , a band regression analysis was performed.
The average depth and arsenic concentration within the selected band or group
was estimated by the method known as Ensemble Average. The main advantage
of this method is that it carries the value of concentration (C) of the immediately

preceding one in a particular series or group of concentration values.

The average concentration was computed as follows.

n -1 1
C A" n (x ): ———_* C n—l(x) + TC n (x) (4'4)

’7

where C4,.” = average concentration at any particular depth range 'x', and n =

number of layers. An ensemble average value was taken for each band. A

preliminary test of correlation between depth and arsenic concentration was

performed and the results are presented in Table 4.2.3 and in Fig. 4.6.a and

4.6.b.

Table 4.2.3 Regression analysis between aquifer depth and arsenic pollution

distribution in Bangladesh’s groundwater

 

 

 

 

Zone Depth Range Regression R2 Value
(m) Equation (%)
All regions 0 to 10 m Y=5.09 x 58
(NW, NE, SW, SE)
SE and SW regions 9 - 30 Y=0.3428+1.79 log (x) 60
All regions 50 -350 Y= -33.178 Ln (x)+196.29 67
(NW, NE, SW, SE) 100—350 = -52.64 Ln (x)+301.83 85.17

 

65

Regression Plot
Log(As(ppt)) = 0.342776 + 1.79096 logmepthm —- 30 m)
S = 0.243388 R-sq = 59.8% R-sq(arﬁ) = 57.5%

 

 

 

 

 

— Regimen
------ 95% CT
- - - 95%?1‘
r r r r r r
8.0 9.0 10 15 20 30
Depth (9 - 30 m)

Fig. 4.6.a Arsenic content increases ﬁnearly with depth from 9' to 30 m

Regression Plot
Log(As(ppb)) = 76.9101 - 0.249561 Depth(m)
S = 13.9116 R-sq = 72.4% R-sq(adj) = 68.4%

 

 

 

 

 

_ Regression
_ _____ 95%CI‘
_ _ 95% PT
r r r r r r
100 150 200 250 300 .350
DWI")

Fig. 4.6.b Arsenic content decreases ﬁnearly with depth from 100 to 350 m

4.2.7 Correlation between arsenic concentration and dissolved iron content
in Bangladesh’s groundwater

It was hypothesized in this study that the lack of oxygen in the vadose
zone develops a reducing condition and which causes the release of arsenic into
Bangladesh’s groundwater. This release is associated with the amount of
amorphous iron oxides and hydroxides present in the water in dissolved form.
Obviously, one would consider this dissolved iron as geochemical evidence in
order to support the hypothesis. lf arsenic release in the groundwater is

associated with the iron oxides and hydroxides at reducing condition, then one
would expect higher R2 values between arsenic species and the redox potential

values or the dissolved iron in the groundwater system.

In order to reduce the spatial inﬂuence, the whole country was divided into
16 major blocks or square grids based on one-degree intervals of latitude and
longitude. The advantage of a square grid system is that the geochemical
evidences of arsenic released along the ﬂow path of groundwater or along the
hydraulic gradient could easily be monitored. The statistical analysis was
conducted for all grids to determine the relationship between arsenic
concentration and dissolved iron content for each square grid, and the results are
presented in the following Figs.4.7.a and 4.7.b and also in Table 4.2.4. The grid

numbers are shown in Fig. 4.8

67

Arsenic Iron correlation In Grid-9

 

 

0" o
o e
o it".
o .0 ‘6. e ,
' 0 :° “0:
A-1" .s'. .z.‘
3 ' 3"“. ".
. , -'o : v °
5 ° o. H 9' o‘,’.
C I o ‘ . '
i a. ‘ .'°° a
g 2‘ o. o. .00.
. .. ' o z .. ..
3 . o e n.- '. o I? 000
. 0. tee a
3_ l...‘:.o. ' e
o 0 00-300.- » o o .0
I l l r

 

 

LogFe(llIglI)

Arsenic Iron correlation in Grid-10

 

.
M
l

LogAs(Mg/l)

on

 

 

:. 0‘s.:‘.: '
0.. ‘ s ‘ I. u ‘ 0‘ .
-{.o o .. .
l t I
1 0 1

- LogFe(IlgIl)

Fig.4.7.a. Relation between arsenic concentration and dissolved Iron content in the Grid-9,

Grid-10 in Bangladesh

Arsenl Iron relationship in Grid 15

 

 

0—1
0
I' .
'1_ e 0". ‘.
o 00“ .~ ...
a s! L3 .9, ,3
‘ . "x 0....
3 -2 —l . f.
'.
. . ,. . .
. .. .
I.
2 .
41—1 , ,
' I
. . .. .u..- ..
1 1 r

 

 

o
LOQFetllM)

Fig.4.7.b. Relation between arsenic concentration and dissolved Iron content in the

Grid-15 in Bangladesh

 

Table 4.2.4 Statistical relationship between arsenic and iron concentration at

different grid locations (latitude/longitude) in Bangladesh.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Grid Latitude Longi- No. Avg. As (Ill 8 Avg. Iron R2 Value Value
No. (°N) tude of V) (mg/L) (Fe2*) (%) of [31 of
(05) Well Conc. slope
(Mg/L) (32
1 26-26.75 88.2-89 116 0.09 4.5 54 -5.46 0.49
0.816 - .0003 41.8 - 0.02
2 25-26 88.2-89 152 0.002 1.78 65 -2.91 0.33
0.05-0.0005 169-0015
3 24-25 88.2-89 200 0.021 1.06 62 -2.50 0.51
166-0005 172-0005
4 23-24 88.2-89 77 0.091 2.6 60 -1.58 0.52
1.03-0.005 927-006
5 22-23 88.2-89- 13 0.139 3.8 99 -1.52 1
0328-0007 7.6-0.055
6 26-26.75 89-90 34 0.00354 3.88 37 -6.52 0.27
0042-.0005 286-001
7 25-26 89-90 339 0.014 4.57 65 -2.58 0.54
0708-0005 41 .8-0012
8 24-25 89-90 369 0.021 4.26 64.5 -2.40 0.67
.632-0005 482-0005
9 23-24 89-90 328 0.07 3.24 74 -1.83 0.85
.924-0005 206-0005
10 22-23 89-90 243 0.096 3.87 71 -1.90 0.96
.635-0005 304-005
11 24-25 89-90 345 0.025 1.69 76 -2.12 0.86
.573-0005 159-005
12 23-24 90-91 411 .17 3.77 76 -1.62 1.05
1.09-0005 261-002
13 22-23 90-91 284 0.044 1.14 68 -1.99 0.9
.862-0005 127-0005
14 24-25 91 -92 344 0.025 1.69 67 -2.84 0.87
.572-0005 159-0005
15 23-24 91-91.25 411 .17 3.77 74 3.37 1.05
1.09-0005 261-002
16 22-23 91 -92 284 0.044 1.14 63 -2.58 0.9
L .86-0005 127-0005

 

69

 

4.2.8 Correlation between arsenic distribution pattern and groundwater
ﬂow direction

The relation between arsenic distribution patterns and groundwater flow
directions was analyzed by dividing the whole country into a number of square
grids where arsenic concentration records and groundwater ﬂow direction maps
were available. Here a potentiometric surface and elevation map prepared by
MP0 (1987) was used to determine the direction of the hydraulic gradient (left
side map in Fig. 4.8) . Bangladesh’s whole arsenic-contaminated area was
divided into 16 grids. Grids 6, 7, 8, 9, and 10 (right side map in Fig. 4.8) follow
the same hydraulic gradient and direction of ﬂow. The arsenic concentration
records in those ﬁve grids showed a strong correlation between arsenic

concentration and ﬂow direction. Table 4.2.4 states that the value of the slope
factors ([32) in grids 6, 7, 8, 9, and 10 are 0.27, 0.54, 0.67, 0.85, and 0.96,

respectively. The slope factor values are increasing with the grids’ increasing
numbering. This indicates a relationship between arsenic concentration and

groundwater ﬂow direction in Bangladesh.

70

 

 

Arsenic concentration distribution pattern along the
groundwater ﬂow direction In Bangaldesh

y- 1 .0329“ + 0.1621x + 10.099
H“: 0.732

 

 

 

 

Fig. 4.8 Distribution pattern of arsenic concentration along the groundwater ﬂow direction (left
side ) , considering grids 6, 7, 8, 9, and 10(right side) , where the surface elevation gradient
was considered as the hydraulic gradient . .

71

4.3 Diffusive transportation of atmospheric oxygen to the saturated zone in
the groundwater systems of Bangladesh

The statistical analysis (presented in Table 4.2.3, Fig. 4.6.a, Fig. 4.6.b,
and Fig. 4.8) clearly indicates there exists a relation between arsenic pollution
distribution and depth of the aquifer. On the other hand, lowering of the water
table limits the diffusive supply of atmospheric oxygen to the deeper layer. The
third research issue, the interactions between lowering of the water table,
dissolved oxygen content in recharging water and arsenic pollution distribution in

terms of oxygen accessibility in the groundwater systems will be investigated.

4.3.1 Diffusive transportation of atmospheric oxygen into the aquifer

Generally two main processes affect gaseous transportation within the
aquifer: convection and diffusion. The convective transport of oxygen in the soil-
water system is at typical pore-water velocities (pwv) in the unsaturated zone
(Refsgard et al., 1991). Fick's law says that the ﬂux of solute mass crossing a
unit area per unit time in a given direction is proportional to the gradient of solute
concentration in that direction.
4.3.2 Formulation of the governing equation for oxygen transportation by
diffusion into the groundwater system

The concept of control volume was used to demonstrate how conservation

of mass leads to developing a governing equation for oxygen transportation in

the subsurface. A mathematical expression of F ick's law is

72

6C
=—D—. . .
q a: (431)

where q is the solute mass ﬂux, C is the mass concentration of diffusive oxygen,
D is the coefﬁcient of proportionality (L3! T) and the minus sign indicates that

transport is from high to low concentration.

 

 

 

 

 

Flux in = q Flux out = q + (an ax) ’Ax
é a
X Ax X+ Ax

 

 

 

Fig. 4.9 Use of control volume to derive one-dimensional oxygen diffusion

equation

Two parallel surfaces of a unit area are drawn perpendicular to the axis
and separated by a distance Ax. The mass of oxygen ﬂow in q = f(x, t) at area x
is equal to the mass of oxygen ﬂow out [q + (aql ax) *Ax]. Let C (x, t) be the
mass of oxygen concentration per unit volume at the point x at time t. There is
mass C (x, t)* Ax in the line segment bounded by the parallel planes. Since

molecules of oxygen are passing in and out of the volume, deﬁned by the

73

boundary surface, there is a rate of change of mass in the control volume equal

(7

to — Ax.
a:

This rate of change of mass must be equal to the difference in the flux, or

rate of passage of oxygen molecules through each surface, in order to satisfy the

law of conservation of mass. Equating these two gives us

i 7 ,‘
(91] +[i] = 0. (4.3.2)
8x 0!

which yields a relationship between ﬂux q (x, t) and concentration C (x, t). The
governing equation is found by inserting the value of q (x, t) from Eq. 4.3.1 into

Eq. 4.3.2.. In the presence of any source or sink, the above equation can be

 

written as
(5)—C) : D O-C: + Source.0rSink.(a) (4.3.3)
0! 0x“

Finally, the above equation (4.3.3) can be written as the governing equation
describing the non-steady-state oxygen diffusion into the sub-soil aquifer system.

This equation is expressed in the form of a partial differential equation as follows.

3

 

.ﬁ . 3 .
Dx* C C, +(a)—££- =0

(3.1" (.7!
Where, (4.3.4)

a = Decaying. .crms tan I

74

4.3.3 Formulation of ﬁnite element technique to solve the governing
equation for oxygen diffusion in the groundwater system

The above governing equation (4.3.4) can be solved both analytically and
numerically, but the analytical solutions are lengthy processes. There are many
numerical methods available, such as the ﬁnite difference (FD) model, the ﬁnite
element (FE) model, the method of characteristics (MOC) model, and the random
walk (RW) model. In this study, the ﬁnite element formulation technique was
used because of its higher accuracy and adaptability to numerical solutions for
physical processes like diffusion convection, pollution distribution or contaminant
transportation. Many studies have shown that numerical solutions from the ﬁnite
element (FE) model give satisfactory results with a wide range of boundary
conditions with irregular and curved aquifer boundaries. In addition, the FE

solution technique by Galerkin provides simplicity and stability of the solution
(Segerlind, 1996).

lnt_egral formulation

The integral formulation for the governing equation (4.3.4) will lead to a
system of algebraic equations that can be solved for values of the ﬁeld variables
at each node in the mesh to obtain the variation of oxygen concentration C with

time t. The weighted residual method was used in this study.

75

Evaluation of Weighted Residual Integral method

Galerkin's formulation of the weighted residual functions can be
constructed using the shape function Ni and Ni (Segerlind, 1994). Evaluating this

weighted residual integral, which is the result of integrating the product of
diffusion equation (Eq. 4.3.4) and a weighting function, W (x), yields a nodal

equation over the length of the element.

7 v

[R"] = —j[W,.](Da C" +a+/I $de = 0 (4.3.5)

 

,
(3x ‘ (,t

where [Wc]"' = [A1]" is the element weighting function for node I, and is equal to

the shape function or interpolation function. The ﬁnite element formulation is
applied after representing the unknown concentration C by linear approximation

of the form as
C‘“ = N,(',..+ [Ni/(2".

OT
(“"’ = [M N, H: } =[N1{C} (4.3.5.a)

where N and N; are the shape function at nodes i and j respectively, and are the

values of unknown concentration C at nodes i and j.

The conventions { } and [ ] represent vector and matrix quantities, respectively,

and will be represented so throughout the analysis. The shape functions Ni and

Nj in equation 4.3.5.a can be expressed as follows:

76

X —x , —X
N, = ’ .cmd.N’ = r ’
L L

z [N. N, Lanai/W :11}; 1

 

 

In the Galerkin method, W5, the weighted function at the Sth node, consists of the
shape functions associated with the 8th node. Therefore, the transposition of the
matrix of weighting function is equal to the transposition of the shape function,

[WC 1" = [N 1'" . The above equation (4.3.5) can be written for node i &j for each

 

 

 

elementas
[m]: 11[N] 10:: (3”:de +1[N]/iaac.dr]= 0 or
XI N1" 02C XI N 6C
=- ..D ,+a.dx— xt—.dx.=0.
M 1 M- .

After reducing terms for node i and j, equation (4.3.6) can be expressed for node

 

 

iandjas
R m 1— :1N 0.2xfcclx—1Nawdx — :1N. xi —éC—.dx
{Rlvl}”= {Rio-1:1 \.1 5C (4-3-7)
' :1N.D fax—1N0: air—1N A—dx
at

 

 

Equation 4.3.7 can be rewritten as follows

77

The right side of the above equation (4.3.8) can be evaluated for node i, by

breaking it into Part A and Part B and evaluating the time derivative in Part C.

 

 

 

(‘.\" ('.\'-

17 q 2
Part — A =1N,.D.(;Cdx
XI 0A. ~ 8 C
Part — C.. 1 N ..1 .xd
at
Evaluation of the above integrals part by part follows.
PartA
.\'/
Part — 3.. = .1N,..a..dx
PartA of Equation 4.3.8 : 111x10 Liﬁh. = D 1111,”). (:1: (A.

.\’I

Part A cannot be evaluated directly because the continuity of the function (dc/dx)

is unknown. But, according to the product of two-function rule,

 

= (X)

 

 

9 - 3 3,?N .-
((N,(x)dC]N 5C+CCC ,(1)

,
5x 5.x“ 5x 5x

Or by rearranging, Equation 4.3.8.1(Part A) can be rewritten as

.‘DI

The ﬁrst part of RHS of Eq. 4.3.8 (Part A)

 

1DN(x)

 

 

V 5C 5N (x)
(:,[N (X) -——1dx - D1511- (,x dx (4.3.8. Part A)

v-‘7 (1(v
_ —_D.\r1_\( )—
(7X

       

8W

1D%— —:-[N (x.) ———]dr — —-D.A"

 

 

.\’I

because the derivative boundary condition of i atj is equal to zero.

78

The End Qart of RHS of E9. 4.3.8 (Part A)

= D :16CO6Nax (x)dx

 

mause the unknown oxygen concentration function C = N,C,. + N ,C j and

g:— = 3% + o = ‘Tlc, as it is known that the derivative of the shape function

 

 

 

N, = X’ —x..and...§A—l’- = :1
L 6x
Part8
L

‘1 L L S S2
1Nia..dx=1N,.a.ds=1[l———].a.ds= . S———— =a.
x! i 0 0 L 2L
as..it..is..bzown .,. in ...s..coordinate ..system .,s.= % and
NI.=Xj—(XI+S).:L—S:1—i

L L L
N ___ (X, + s)— Xi

 

” L

%-
[N]: [N,. NJ]=[1_:£—

%]

79

Part C:

Evaluation of time derivative

x] '1 I.

[A -[N]" 95m = [/1-[N]"'[N]{C‘"’:-ds-
r: at 0

I. ..

I]! .[A/r]l '[Nr]'61.\"{(,‘”)} : {(v I; it') }{thfl}
0

l,
where, [(3, “"1: l2. [N]"'[N]d.s'
0

CS6), the element capacitance matrix, can be evaluated as

I" N, ’NIN, MN, , 1 0
[(vht]=6[/t.[Ni:l.[NI N,]ds..=/i [iv/N, NINJdSH:AOL[O 1]

('l

 

where N Ni= NJ-Nj = 1 and NiNj=0,

This formulation produces a diagonal capacitance matrix. A diagonal capacitance
matrix is usually referred to as a lumped formulation (Segerlind, 1996). After
collecting of all terms of integral evaluation of Part A, Part B and Part C, the

equation (4.3.8) can be ﬁnally expressed as the matrix-integral form of the

residual equation of (4.3.5) as follows

dc" _ D D a.L
(R‘) D—].X—V, + —L— -—L_ C, _ 3— +£ l 0 (439)
Dd_,_, -2 2 c, 'guz'on '-
dx " L L 2

80

The solution for a general, ordinary differential equation in a matrix-integral form
of residual equation is [R] = {I} + [K] {C}-{F}, where {|}=inter elemental stiffness
matrix due to derivative boundary condition, [K]= conductance matrix due to
material property, and {F} is the force factor due to source and sink. The new

term time derivative in Eq. 4.3.5 x*(dcldt) can be ﬁnally expressed as in matrix

form in equation 4.3.9 as %[:) (1)], and has to be solved by Lumped

formulation method. Using the backward difference form of Lumped formulation

method, the solution of the equation (4.3.9) can be written in matrix form as

(.[A 1+ v r-[K m: L... = [A ]{C }. + v w L... (4.3.10)

where A = element capacitance matrix, K = element conductance matrix,

F = force factor, C. = concentration of oxygen at any time t, and C (wt) =

concentration of oxygen in the element after time step Vt.

[A]=£[1 0]

20]

Equation 4.3.10 gives the nodal value of oxygen concentration {C} H w, in

terms of a set of known initial values { C} t, the vector associated with source or

81

sink {F}, at time t and t+Vt and the ratio of 0. For the backward difference
method 9 =1 to get the value of the oxygen concentration C at each elemental
section of 10 meter length proﬁle with time step Vt, software based on the ﬁnite
element computer program developed by Segerlind (1997) was used to solve a
system of linear equations at each node of each element. The results are
presented in Fig. 4.11 and 4.12 and Table 4.3.2.

In the ﬁnite element formulation, the value of diffusion coefﬁcient Dx =

259.2 cm2 hr'1(4.166*10'4mzs'1) and the oxygen consumption rate a= -0.002125

cmscm’3h'1. The values of Dx and on are taken from the experimental ﬁndings of

Kanwar (1986)and Papendick and Runkles (1965) because, in his experiment,
Kanwar (1986) found a reasonably good agreement between measured and
predicted oxygen concentration by using the techniques of Laplace
transformation to solve the oxygen transport equation (4.3.4), although it is true
that the reaction factor a varies from soil to soil as a result of the variation of
organic carbon.
4.3.4 Analysis of the inﬂuence of water table lowering on oxygen
concentration distribution in the deep layers of groundwater

The above equation (4.3.10) presents an oxygen transportation model with
a constant oxygen consumption rate by organic carbon in the vadose zone of the
aquifer. The ﬁnite element formulation model was applied to solve the equation

(4.3.10) in order to understand the interaction between diffusive oxygen supply,

82

depth of aquifer and their relation with arsenic concentration change. it will be

investigated whether lowering of water table provides a greater oxygen supply to

the hypothetical arsenopyrite layer (L5 and L10) or not (Fig. 4.10).

Descrigtion of the oxygen diffusion model in deemr layers:
The model aquifer was schematically represented in Fig. 4.10. A 10 m

deep aquifer was modeled by using ﬁnite element formulation. The layer

numbers L1, L2, L3 up to L10 in Fig. 4.10 represent the length of each element.

Each layer is considered to be 100 cm deep. Two hypothetical layers, L6 and L10,

(Fig. 4.10) were assumed to contain arsenic-rich pyrite sediment because
arseno—pyrite layers can exist randomly in the aquifer at any depth and at

discontinuous phase. It will be investigated whether there has been any change
in the oxygen concentration of the pyretic layer (L3 & L10) before and after the

introduction of well ﬁelds in Bangladesh.

Case 1 represents the situation before installation of wells when the water
table position (WT -1) was at L2, 2 m below the surface. Case 2 represents the
condition after installation of 0.75 million tube wells when the water table (WT -2)

was lowered to 5 m depth at L5 (Fig. 4.10) as a result of overpumping. In this
study, the change in diffusive supply of oxygen concentration at pyretic layers L6

and L10 under two hypothetical conditions was investigated.

83

Ground Surface (GS) :

L

 

 

 

 

 

 

 

 

 

6
2m L-1 02
0x1 L-2 (WT-1) Before 1970, Case - 1
L-3 5 m
DXz L-4 DX1
. ..... After 1970, Water table
_._._._.L_-5 ________ _ _V_WT-2, Case-2, 5 m below the GS
L-6 Arseno Pyritic Layer
L-7 DXZ
L-8
....... L- 9--._.-.-
L-10 Arseno Pyritic Layer
L ________ ..

Fig. 4.10 Aquifer model cross section of a 10-meter deep layer (from L1, L2 to L10
representing ten elements).

The values of the oxygen diffusion coefﬁcients (0x1 and 0x2), oxygen
consumption or decomposition rate (d) for each element length (L1, L2) were
tabulated in Table 4.3.1. The initial oxygen concentration at the boundary of
each element was 0.21 atm, except for the last one, L10. After inserting all the
values, pre-prepared computer software computed the values of the global
conductance matrix (K), the global capacitance matrix (A), and the source or sink
factor (F) in order to solve equation 4.3.10 for time steps equal to one hour and

to four hours

84

aquifer material for the shallow and deep tube well system in Bangladesh

Table 4.3. 1 Number of elements, number of nodes and material properties of the

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Length Cum. Element n Oxygen Oxygen Diffusion Coeff(Dx)
(cm) L Length No Consumption (cm’lhr)
. (m) Rate (a) Case 1 Case 2
(cm3cm"h") Dx1 Die D‘x ' De
L1=100 1 1, 2 0.5 0.0021 259.2 259.2
_ Lz=100 2 3,4 0.5 0.0021 1 259.2 259.2
‘ L3=100 * 3 7 5,6 0.5 0.0021 if * 129 259.2
L4=100 4 7, 8 0.5 0.0021 129 259.2
‘ L5=100 . 5 9,10 ‘ 0.5 0.0021 ‘ 129 259.2
Lg=100 6 11, 12 0.5 0.0021 129 l * 129
L7=100 7 13, 14 0.5 0.0021 129 129
L5=100 l 8 15, 16 0.5 0.0021 129 129
Lg=100 l 9 17,18 0.5 _ 0.0021 129 129
' Lm=100 r 10 19,20 0.5 , 0.0021 129) r 129 ,_

 

 

 

 

 

 

 

 

* For case 1, the water table was at 2 m depth (L2), so the 0x1 value was used
up to 2 m, and the 0x2 value started from L3. (sz for saturated layers was

considered to be 50% of 0x1.) n=air ﬁll porosity of the aquifer materiat

Finally, the oxygen concentration at each element was computed over the
period of 500 hours. The oxygen concentration records are tabulated in Table

4.10. The graphical presentation in Fig. 4.1 1 indicates that lowering of the water

85

table about 4 m from L2 to L5 did not increase the oxygen concentration at L10

and L6 (Fig. 4.11). The oxygen concentration in case 1 at L5 was found 0.09 atm

(3.64 mglL) after 150 hours. But after lowering of the water table, after the same
interval of time, the oxygen concentration was found to be 0.06 atm (2.39 mglL)
(Table 4.3.2).

 

Dltl'Uslve Sipplyof Owen at Py'ltlc Layer-6
E Whiter Lon-enloaqf-ﬂaterleble

 

  
  

 

 

. .1
. oxygen conc. at A
‘ :°'«¢-.—¢,_€_ Pyritelayer i,3 ,Cgse-i g
l ~ - 0.1 v
! o’
i s
l L-10, ease-2 95>
l ‘b k
i L-10, Cue-1 ”'71 . Q01 g
l L5, Case-2, after lowering ofWT-Z o 001
0 "9 9‘0 "Q «’9 «‘59 13° hﬁ 19° (5'9 “3‘9
11m in er

 

 

 

Fig. 4.11 Predicted changes in the diffusive oxygen supply at arsenic-

contaminated pyretic layer L3 and L10 in the aquifer.

86

Table 4.3.2 Diffusive oxygen concentration at pyretic layer L6 before and after the

introduction of well ﬁelds, assuming the initial 02 concentration to be 0.21 atm for

 

all nodes.
Time Oxygen conc. at layer 6 (pyretic Oxygen conc. at layer 10
(hr) layer at 5 m depth) (sediment layer at 10 m depth)
Case 1 Case 2 Case 1 Case 2
(Before well (After well intro.) (Before well (After well intro.)
intro.) intro.)
(atm) (mg/L) (atm) (mg/L) (am) (mg/L) (atm) (mg/L)
0 0.21 8.37 0.21 8.37 0.21 8.37 0.21 8.37
10 0.193 7.945 0.189 7.545 0.199 7.946 0.189 7.55
20 0.188 7.523 0.171 6.838 0.188 7.522 0.170 6.80
30 0.178 7.121 0.156 6.244 0.178 7.099 0.153 6.103
40 0.169 6.744 0.143 5.732 0.167 6.67 0.136 5.456
50 0.160 6.389 0.132 5.283 0.159 6.255 0.12 4.857
70 0.144 5.739 0.122 4.882 0.136 5.421 0.094 3.778
90 0.129 5.151 0.097 3.873 0.115 4.612 0.070 2.828
110 0.115 4.613 0.083 3.329 0.095 3.825 0.049 1.977
130 0.1031 4.112 0.071 2.836 0.076 3.062 0.030 1.205
150 0.0913 3.642 0.060 2.394 0.058 2.323 0.124 0.0495
170 0.0807 3.216 0.049 1.988 0.040 1.606 0 0
190 0.0690 2.778 0.0406 1.618 0.022 0.914 0 0
210 0.0599 2.391 0.031 1.235 0.006 0.240 0 0
230 0.049 1.958 0.023 0.944 0 0 0 0
250 0.040 1.618 0.0160 0.637 0 0 0 0
270 0.037 1 .474 0.0087 0.346 0 0 0 0
290 0.023 0.916 0.0017 0.067 0 0 0 0
310 0.014 0.558 0 00 0 0 0 0
330 0.0066 0.263 0 0 0 0 0 0
350 0 0 0 0 0 0 0 0

 

 

 

87

4.3.5. Influence of initial boundary conditions on oxygen concentration

The inﬂuence of initial oxygen concentration at each element on the overall

oxygen concentration distribution over the given depth range was also analyzed.

Table 4.3.2 represents the results of oxygen concentration distribution when

initial oxygen concentrations were 0.21 atm at all nodes except the last node (L10

= 0). Table 4.3.3 represents the results when the initial oxygen concentrations

were changed from 0.21 to 0.15 atm for both case 1 and case 2. The inﬂuences

of initial conditions were prominent and are discussed in detail in chapter six.

 

(3838-1 8. 0838-2

, Oxygen concentration change overtime

 

      
 
   

 

.830 '09 (£30 '96 '99 *0 {39 $9

I T 7T T T I l l l

L-IU, Dx1=sz

 

2 1 1 I I l I l I I l I I I I I
i 1’ '19 63 .59 .99
l
5 0.1
‘5
E i} L-6,Dx1=sz
8 L-1I]_Case-2
C
D
o \
E e ‘
g 0.01- /\
5 L-6,Case-2
L—6.Case—1
i
EFL—10, Case-1
0.001
Tlme(hr)

 

 

 

Fig. 4.12 Inﬂuence of boundary conditions on oxygen concentration (atm) before

and after lowering of water table (case-1 and case -2 ).

88

Table 4.3.3 Diffusive oxygen concentration at pyretic layer 6 before and after the

introduction of well ﬁelds in Bangladesh, while considering the initial 02

concentrations of 0.15 atm for nodes in the saturated zone.

 

Time

(hr)

10
20
40
60
80
100
120
140
160
180

460

 

 

Oxygen conc. at layer 6 (pyretic Oxygen conc. at layer 10
’aye’ 9‘ 6 m depth) (sediment layer at 10 m depth)
Case 1 Case 2 Case 1 Case 2
a. (Before) (After) (Before) (After)
(atm) (atm) (atm) (atm)
0.21 0.21 0.21 0.21
0.139 0.141 0.141 0.139
0.129 0.0.133 0.133 0.128
0.108 0.112 0.112 0.107
0.086 0.088 0.088 0.086
0.066 0.0.063 0.063 0.65
0.044 0.036 0.0362 0.044
0.024 0.009 0.009 0.023
0.0027 0 0 0.011
0 0 O 0.001
0 0 0 0
0 0 0 0

 

a. Initial oxygen concentrations at the boundary of each element were changed

to 0.15 atm at all the nodes of the saturated layers.

89

4.4 Computation of redox potential (p‘) values and stability analysis of
arsenic (III 8: V) redox couples in Bangladesh’s groundwater

The stability diagram of iron and arsenic redox species was constructed
and evaluated in order to satisfy the second research issue, whether pyrite
oxidation or reduction of iron oxyhydroxides mainly controls arsenic release in
the groundwater system in Bangladesh. Literature review suggested that a mildly
reducing condition (p°= 0.5 to -3.5) is responsible for arsenic release in to
groundwater. In this analysis, it will be investigated thermodynamically if mildly
reducing condition could be developed after lowering of water table .The redox
potential values will be computed using the activity or concentration of dissolved

iron and arsenic species present in groundwater.

4.4.1 Measurement of redox potential values

Although aqueous solutions do not contain any free protons or electrons, it is
nevertheless possible to deﬁne relative proton and electron activities. The pH (-
log {H*}) of a system measures a relative tendency of a solution to accept or
donate protons. Similarly, the redox intensity (p°= -|og {e}) measures the

tendency to accept or donate electrons in a solution.

Existing methods of measurement involve use of potential sensing inert
metal electrodes or analytical determination of redox indicator species such as
dissolved 02 or Fez. In general, a platinum electrode responds satisfactorily to

very few of the redox couples in natural and groundwater systems. In relatively

90

oxidizing water. E. or p‘ values measured with platinum electrodes can rarelv be
related to a speciﬁc redox pair (Drever.1988). Therefore the activitv or

concentration method is used to measure the Eh or 0" values.

The activity of electrons in a solution (hence its redox level) can be expressed
in units of volts (E..) or in units of electrons activity or p‘. The redox intensity p’ is

related to redox potential (Eh) by thermodynamic equations as follows

, F
1’ =W'E"
or (4.4.1)
p' 16 .9'*E,,,
E" = 0.059 " p'

Where.
R= Gas constant (8.314 "10‘3 K.Jldeg. M60), or R=“ 1.987x 10‘3 kcal/degmot
T- Abédluté Téritpéfafﬂré in K.
F5 Faradays' electric constant (96.42 kJ per vslt gra’rn equivalent). er in eateries,
F: 23.06 kcal per volt gram equivalent, at 25 ° C, 2.3 is the conversion from
natural to base 10 logarithms
4.4.2. Use Of p‘ as a variable f6 measure ﬂié Iron senﬁsnsd (Fs’*IFs’*)
redox potentials in the groundwater
The groundwater sediment contains solid amorphous Fe (OFI) 3, and
assumed that measured potential corresponds t0 an oxidation-reduction potential
of the aquatic environment. At equilibrium Fe (OH) 3 (s) is in equilibrium With Fe”,

the redox reaction being

91

Fe(0H)3(s) + 3H" + e = lie2+ + 311,0.

(4.4.2)
.10th =16.

The standard state redox potential value E°H, can be calculated if the equilibrium

constant value of log K , and standard state free energy VG" value are known.

 

 

 

0
E01. = _VG = RT.ln.K = 2°3'RT.log.K
n.F nF
2.3 * 1.98x10’3(kCal)* 298 .(K ).(volt .gramm..equivalent )
= .(16) = 0.94..V 4 4 3
(KMoIe)! * 23.06..k.CaI ( - - )

or...E,, .. .can..be..exp ressed ..in‘..rerms..of ..redox ..ca‘pacily ...p‘

0
pe° = E " ...=-9424—..=16.05
0.059 0.059

 

From the general redox equation , EH or p‘ for the iron redox reaction (eq. 4.4.2)
can be computed as as follows

3
0.059 F1
0
Eh-(OX-red) =E (Fe(0fl)3.-Fe2+) + n 10% FeZ+ }

.0r..in..terms..of..pe

 

(4.4.4)

+ 3
pe = peo(Fe(0H) Fe“) +log{ ]
3 32+

Taking the value of p°° from eq. 4.4.3 and substituting it in eq. 4.4.4. Assuming ,

[409(W)‘ PH. 409(Fez‘) = pFe", and EM0.059=p’ ],gives the equation 4.4.4

p‘ = 16.05 — 3 * pH -I= pFe2+ (4.4.5)

Using the above functional equation (eq. 4.4.4) and the average concentration of
dissolved Iron (Fe2"), the redox potential values can be calculated under natural
pH levels in the contaminated water, and the results are presented in
Table.4.4.1.

For example:

 

The values of dissolved Iron concentration 0:92.») in (mg/l) of different grids (as
mentioned earlier) were tabulated in Table.4.2.4. The maximum iron
concentration of Grid- 1 in (Table. 4.2.4) was 41.8 mg/l. The units were

converted from mg/l to mol/l. As it is known, 41.8 mg/l =7.484*10“’ mol/ l.

[by dividing iron concentration by its atomic weight 55850 mg.

 

* III
mg "’0’ g’" = 351”— and Log (7.484*104)= -9125].
1* 55.85.gm *1000.mg 55850

Using the redox equation (eq. 4.4.5), the minimum redox intensity at Grid .1 was

computed as (-1.82) and showed in (Table.4.4.1) as follows.
p9: 16.05-3*pH+pFe2+ = 16.05-3*7+3.125 = -1.82

The rest of the results are tabulated in Table (4.4.1.) exactly the same way as

shown above.

93

Table.4.4.1 Iron controlled redox potential values at arsenic contaminated wells
in different zones of Bangladesh

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Location Iron concentration in P° @ natural water No
pFe 2* (pH =7) .of
[-log(Fe2’) in molll ] (Using eq.4.4.5) wells
Zone Grid Max Min Avg Min Max Avg.
No
NW 1 3.12 6.44 4.09 -1.82 1.49 0.85 116
2 3.50 6.57 4.49 -1.43 1.62 0.45 1 52
3 3.51 7.04 4.72 -1.43 2.09 0.22 200
6 3.29 6.74 4.15 -1.65 1.79 0.79 34
7 3.12 6.66 4.08 -1.82 1.71 0.86 339
8 3.06 7.04 4.11 -1.88 2.09 0.83 369
NE 14 3.54 7.04 4.51 -1.40 2.09 0.43 344
SW 4 3.77 6.96 4.33 -1.17 2.01 0.61 77
5 3.86 6.93 4.16 -1.08 1.98 0.78 13
9 3.43 7.04 4.23 -1.516 2.09 0.71 328
10 3.26 7.04 4.15 -1.68 2.09 0.79 248
NC 11 3.54 7.04 4.52 -1.40 2.09 0.42 341
SC 13 3.64 7.04 4.69 -1.30 2.09 0.25 284
SE 15 3.33 6.44 4.17 -1.61 1.49 0.77 411

 

 

 

 

 

 

 

 

 

4.4.3 Computation of oxygen control red ox potential value

In natural condition, both surface and groundwater is stable at a total
pressure of 1 atm At very high p° , the pressure of 02 in equilibrium with liquid
water will exceed 1 atm, and at very low p9 the pressure of hydrogen in

equilibrium with water will exceed 1 atm.

 

211,0 W9" so, + 4H+ + 4e"
The upper stability limit of water is deﬁned above by the water oxidation

reaction, and the lower stability limit of water reduction reaction is deﬁned as

 

1 __ + e ucr'on
j;o,(g)+2e +2H R‘“ ;H20 (4.4.5.3)

The equilibrium constant, K .q, of above water reduction reaction (eq. 4.4.5.a)

can be expressed as shown in below.

K ___ aH20
,, battle-idem}?

Since the activity of liquid water, 9 (H20) =1 (pure water in standard state), after

 

taking log on both side of Kn gives as (-log (ae)=p").

log K W = — ;—. log .P02 + 2 p e + 2 pH (4.4.5.b)
After rearranging the equation 4.4.5.b, the equation of P° can be written as

p‘ = %log.Kq +%.log.P02 - pH (4.4.6)

(The standard state free energy of the reaction, VG°W = VG°(H20) = —237.13JrJ
and the other standard free energy of formation are zero). Now putting the value

of

vc° ...... in thermodynamic equilibrium formula below, the value of log K ... as

 

 

 

10 K —_VGomd _ 237.13.“

g' ‘4 2.3R.T 2.3*8.314*10"(UlK-moll‘293-(K’m0’)
= 237.13 =41.62

5.698

Using the value of log K= 41.61 in equation (4.4.6, the oxygen controlled redox

value in the groundwater system can be computed and results are shown in

95

Table 4.4.2. It revealed from the results that when oxygen controls the redox

potentials (pe) value in any systems, the redox potential values were high.

Example
When the amount of dissolved oxygen is 0.05 atm (referring Table 4.4.2), the

system p‘3 at natural water (pH=7) was computed according to the equation
(4.4.6) is

U 1 , 1 l l q q
p = 5103,19,,+Z.log.1’,,3 —pH =;*4l.62+Z*(—l.3)—7 =13.48

E

Redox values were also calculated for the oxygen concentration level as

0.00001 atm or 0.41-ppm or 410-ppb the results were tabulated in Table.4.4.2.

96

Table 4.4.2 Dissolved oxygen concentration available in groundwater and

computation of redox potential values under natural water pH=7.

 

Activity of Oxygen Dissolved Oxygen (aqueous) Redox intensity p°

 

 

 

 

 

 

 

 

Poz (atm) (mg/L) (0me
0.21 8.66 8660 13.65
0.15 6.18 6180 13.60
0.10 4.12 4120 13.56
0.05 2.06 2060 13.48
0.01 0.41 410 13.31
0.001 0.041 41 13.06
0.0041 4.1 12.81

0.0001

12.31

0.00001 0.00041 0.41

 

97

4.4.4 Stability analysis and functional relationship among redox

potential p°, pH, and construction of p°-pH diagram

The pe-pH diagram is a convenient way of analysis of the stability
relationships where redox reactions are involved. Analysis of the stability
relationships of iron and arsenic redox species will help understand whether
arsenic is released under oxic or reduced environment. Also it helps evaluating
whether arsenic mobilization in groundwater occurred in dissolved form or in

solid form

The upper stability limits of water

 

The upper stability limit of water was expressed in the equation as follows.

RC t/Ht Ilrlll

l
503(9) + 20“ + 211* ———> H,()

H.O————>’ o, + 471* + 40‘ (4.4.7)

The functional relationship between p8 and pH was computed by using the
equation (4.4.6). For oxygen pressure, P02 =0.21, indicates the upper stability
limit for water. Putting log P02 = 0 in equation (4.4.6), we have the functional

relation as

p" = 20.78 - pH (4.4.8)

This plots 3 straight line of slope -1 on the pe-pH diagram (Line-1, in Fig. 4.13).
The lower stability limit of water is conversion into Hydrogen gas, and deﬁned by

the reaction as follows.

98

K = 73 _ (4.4.9)

”I aH + . (re

 

l .
log .K .- = Tlog .P,,‘ + p‘ + pH :0

Log(Keq) of the eq (4.4.9) is zero because, the free energy of reaction involving
Hydrogen is zero. Therefore, G° read = 0, so the functional relationship between

p" and pH is as

p" = —:—|og.[’,,: — pH. = —pH ....... ll'licn....l’,,:. = l.(llm. (4.4.10)

At lower stability limit for water, P (H2) =1 atm, log pH2=0, then we have the

relation of Pe = -pH

4.4.4.a Functional relationship among iron redox couple (Fe (OH) 3 8. Fe”)

, p" and pH

As it is known that the solid amorphous Iron (Fe (OH) 3(9) in the arsenic
contaminated groundwater system is reduced to dissolved Fe?” and released
arsenic. At equilibrium , (Fe (OH) 3(s) is in equilibrium with Fe” . The iron redox
reaction was written in equation (eq 4.4.2) and the p60 value was computed using
eq (4.4.3) as 16.05.

F(’(()H3)(S)+ 3.H+ + c 2 Fe“ + 3H.U...& log K =16.
[)L’”. = l6.05

99

Using above p‘“0 and log K values, the above equation (eq 4.4.2) gives us a
functional relationship between pe and pH and dissolved and solid iron species.

The redox potential can be computed as

.. H.}3
p" = p" (1110;111:034 + log. ,

)F 8‘

or

p" = 16.05 - 3,211 + pro” (4.4.11)

Therefore, the above equation 4.4.11 gives us a functional relationship between
dissolved iron content and the associated redox potential values in the

groundwater systems.

4.4.4.b Stability limit of iron and arsenic species and construction of p°-

pH diagram

In general As (V) predominates under oxidizing conditions and As (Ill)
predominates under reducing conditions. However, the reduction of arsenate (As
(V)) to arsenite (As (ND) is slow, so arsenate can be found in reducing
environments, conversely arsenite can be found in oxidizing environment

(Meriner et al, 1996).

The thermodynamic information obtained from the stability diagram or pe-
pH diagram describes only the systems at equilibrium and indicates the direction

in which a non-equilibrium system will move. We know that arsenic is stable in

100

four oxidation states (+5, +3,0, -3) under EH condition occurring in natural water.
At the high EH values encountered in oxygenated water, arsenic acid species

(H3ASO4, HZASO4', HASO42', and AsO43') become stable (Ferguson, 1971).

Construction of pe-pH diagram

In groundwater system, the stability of iron and arsenic species was
analyzed using pe-pH diagram within the upper and lower boundary of water
stability zone. Fig.4.13 was constructed using the following equations and
indicated the simplest presentation of stability diagram of arsenic species that

can be available in groundwater.
Upper stability limit of water (Line-1 in Fig.4.13)

The equation (4.4.8) was used to construct Line-1 in (Fig.4.13) indicate upper

limit of water oxidation in a p8 -pH diagram.
Lower stability of limit of water (Line-2 , Fig.4.13)

Equation (4.4.10) was used to indicate lower limit of water reduction in stability

or Pe-pH diagram, where, pe=pH
Stability limit of Fe2+ and Fe(OH):, (5) as redox couple (Line-5, Fig.4.13)

The solid amorphous iron hydroxides are reduced to Fe2+ according to the

following equation below;

Fe(()H,)(.s')+ 3.H+ +e = Fe“ + 3H3() ............. & log K =16...
pa”. = 16.05 '

101

The Functional relationship between pe and pH is derived above and is

expressed by the equation (4.4.12).

1)" = 16.05 —3pH + chz“ (4.4.12)

Stabilitylimit of l=e2+ and Fego3 (s) Redox Couple (Line-4, Fig.4.13)
The boundary of Fe2+-Fe203 is calculated from the equation

F0303 + 2.0" +611 * = 2Fe 3‘ +3H,()
(up, 2+ }3 (4.4.13)
{ac ‘ }2 {(1H + }6

The functional relation between pe and pH of the above equation(4.4.13) can be

K :

Ct]

 

expressed by the following equation

, l u
p‘ = 31012.1(,” - logch' —3pH (4.4.14)

Stability limit of arsenic acid species (Line-2L Fig.4.13)
The pa range within which the arsenic species are detected for each speciﬁed pH
condition was shown in Fig. 4.13. The oxidation product of arsenic acid H3As03°

is deﬁned by the following equations

11,.150,“ + 11,1)4—4 11,/1.41); + 311* + 2c"
and. 1110.. .p" — pH. . . relation. .is exp 1'c.s'.s'ed..by

3
"=ll—;* H
l7 7 [7

~

102

Stability Diagarn of Water, Iron & Mc Spades

 

 

 

 

 

30 lntheG'omdmteromegaldedt
Waterstableasgas
20« .
g 1
52% Halo») Fe” (5,
-30. (4)
0 1 2 3 4 5 8 7 p: 9 10 11 12 13 14

Fig.4.13 Stability diagram of water and arsenic species in the contaminated
groundwater system.

103

4.4.5 Shiftingoftl'lennodynamicequilibrium, loweringofwatortabloand
arsenic release in groundwater of Bangladesh

This analysis was performed considering that the dissolved oxygen (DO)
in recharging groundwater was in equilibrium with atmospheric oxygen before
IoweringthewatertableinBangladesh. TheabundanceofDOcreatedthe
hypothetical equilibrium condition (in the top left green circle in F ig.4.14) showing
thatthearsenicspecieswerestableinsolidstateandwereattachedtothe
surface of solid amorphous Iron oxides. The redox potential values of Fez’VFe 3"
redox couple were calculated assuming oxygen concentration at 0.21 atm, pH
was 5 and total Iron (Fe2“ & Fe") concentration was 0.1 (mg/L). When these
values were inserted in p°-PH diagram, it appeared above the iron - equilibrium
line-10 (Fig.4.14). This implies that redox potentials values were similar to
oxygen controlled p" values, because, the range of oxygen controlled redox

values were calculated from 12.31 to 13.65 in Table 4.4.2.

But, after lowering water table, lack of DO helped develop reducing
condition and reduced the p‘ values. The p° values were computed from the
field records of iron concentration and inserted in F 19.4.14 against natural pH=
7. Under neutral pH of water, the p° values of iron and arsenic appeared below
their equilibrium lines (Line-3 and Line-10). it implies that thermodynamically, iron
and arsenic existed as a dissolved species in ground water system of
Bangladesh. The Fig.4.14 showed that the associated p° values were within the

range of mild reducing level (1.5 to -3.5). The green arrow in Fig.4.14 was used

104

to indicate that the hypothetical equilibrium situation had shifted from oxic
to anoxic condition, and the P° values were reduced from oxygen control range
(12 to 15) to the mild reducing range (0.5 to 3.5). It is most likely due to the
lowering of water table and limited access of atmospheric oxygen to the deeper
aquifer.

The lines in Fig. 4.14 were constructed using the thermodynamic relations
as similar to the stability diagram in Fig. 3.13.

Lowering of water table ,shilling of redox equilibrium aid
stabiity range of arsenic and iron species [ As (III&V) 8. Fe(0H),lFe"]

 

  

 

 

 

25
Hypothetical
20 A 1) Equilibrium atoxic condlton
\ ‘ ( When wate noble .was WT=L,_
15 . \ \ \ ‘ k 5“ before 1970 condlton
H3A504 \
(3) (6) 7“ \-
10 ‘ max. \.- -
.Q \ N
In .
3: 5 « 41 ”“503 Q\ 's - - . ‘ HAsO.‘ (7)
9 t . \
,3 0 . _ \ i
E ‘7 Jim ‘ . _ , Fawn), Solid A50.
8 7* - ‘4 s . ‘
s ’5 T \k - _ ‘
8 Nequullibnum \ \ \ *0 .
a: _10_( alterlowenngwatertable “‘5 “
at WT=L2.1998 \ \ - R 5032-
29 ' k‘ x \Q
-15 + F6 Dissolved
lronredox eqwllbnum Line
(12)
-20 ‘ (10)lron conc @10mgll, \
(11) 1 mgll ano(12) 0.1 mgl (11)
0 2 4 6 DH 8 10 12 14 16

Fig. 4.14 Shifting of ground water systems from oxygenated conditions to
reducing condition due to the lowering of water table in Bangladesh.

105

Table 4.4.3: Chemical and thermodynamic equations for p‘-pH diagram of
Arsenic (Ill 8. V) and Fe (OH) 31 Fe2+

 

 

 

 

 

 

 

 

 

 

 

 

Line Type of Chemical Equation Thermodynamic
Numb Equation Equation
er
(1) Water Oxidation
Reaction 4H”+Oz (g)+4e'=2H20 p° = 20.6 - pH
(2) Water Reduction pE = - pH
Reaction 2H20+2e'= H2(g) + 20H
(3) Arsenic (lll) & As pe = 11-(3/2)* pH -
(v) acid species HsAs03°+HzO=H2AsOi
+3H++2e'
(4) Arsenic Acid As(c ) +3H20=H2AsOs‘ pH+(3/4)*pe=3.1
species +4H*+3e‘
(5) Arsenic Acid H3ASO3O = HzAsO4' +H‘ PH=2
species
(6) Arsenic Acid PH=6.96
species HgAsO.{=HAsO42'+H+
(7) Arsenic Acid HAsO.42'=AsO..3 +H‘ PH=1 1.5
species
(8) Arsenic Acid PH=9.2
species H3A503°= HzAsOg‘I-H’
(9) H2ASO3.= H2A803-24'H‘ PH=12.1
(10) Fe(OH)3s/Fe2* Fe(OH)3(s)+3H”+e- Pe=16-log[Fez*]-
=Fe2++3H20 3pH
(11) Fe(OH)3s/Fe2* Fe(OH)3(s)+3H*+e- Pe =16 -log[Fe2‘]-
=Fe2*+3Hzo 3pH
(12) H2$ISO4
H2$°+4H20=SO42'+10H*+8e' pH+(415 )* pE =
4.1

 

 

106

 

4.5 Computation of shallow aquifer recharge (SAR) in groundwater system
Groundwater recharge is an important ﬂux in the water balance model.
The amount of groundwater extraction greatly exceeds natural recharge in many
areas of Bangladesh. As a result, lowering of the water table occurs. In
Bangladesh, groundwater recharge is primarily attributed through direct
inﬁltration and percolation loss from large rainfall and ﬂoodwater. The objective of
this analysis is to estimate the lower limit of the average vertical groundwater ﬂux
or vertical aquifer recharge.
Concept of groundwater recharge model
Fig. 4.15 describes a conceptual mass balance of the water cycle with
precipitation (P), which strikes the ground surface to become surface water, and
will either ﬂow over the surface as runoff or inﬁltrate downward to the soil’s
unsaturated zone. Part of the inﬁltrated water is stored as soil moisture, some is
evaporated back to the atmosphere, and the rest inﬁltrates downward across the
water table as shallow aquifer recharge. Water that is not stored in or pumped
out from the shallow aquifer sediment must ﬂow down through the shallow
aquifer system into the deeper layers of the aquifer as a recharge. Water that
enters the deep aquifer system becomes part of the shallow regional aquifer
system and will eventually ﬂow laterally offsite.
4.5.1 Parameter estimation in water balance equation
In the study area, the precipitation was recorded daily from the year 1963 to

2000 in the catchment area of about 100,000 ha in the southwest zone (SW) of

107

Bangladesh. The potential evapotranspiration (PE) and actual

evapotranspiration (AE) was calculated from the daily pan evaporation records.
The daily records of permitted outﬂow (D0) which is the volume of water
discharge from the catchment area was measured at the main control structure

of the drainage system.

 

 

ATMOSP HE RE
PRECIPITATION (P)

   
 
 
  
 

_.- nuuorr _p. ounutu.

‘
_

yf:§"}‘_|,:.ra.‘.4 19m1_511_.m;'_;’ .
r I

misc: (Q)

 

mm; Imelﬁm

  

 

 

 

Fig. 4.15 Conceptual water balance model: rectangular areas represent

reservoirs for water and arrows represent the ﬂux.

108

Source of precipitation: Generally, rainfall and ﬂoodwater are the primary
sources of groundwater recharge. But The major source of precipitation in
Bangladesh is rainfall

Rainfall Analysis:

The thirty-year (1951-80) national mean annual rainfall is 2, 320 mm
based on the analysis of 277 stations. (MPO, 1985), ranges upward by region
from1, 765 mm in the Southwest, 1820 mm in Northwest, 2410 mm in the South-
central, 2690 mm in the Southeast to 2830 in the Northeast. Bangladesh
receives 80 to 90 percent of its mean annual rainfall 2320-mm during the ﬁve-
month monsoon season from May to September. The inter monsoon period,
October to May, has three seasons.

The precipitation and evaporation records were taken daily in the project area. In
order to compute the amount of surplus (SUR) and shallow aquifer recharge
(SAR), a soil moisture balance was performed for the study area.
Volume of Surglus (SUR)
The part of precipitation (P) available for groundwater recharge and/or runoff in
excess of meeting evaporative and soil moisture demand is termed surplus
(SUR).

SUR= P-AE—DT (4.5.1)
where P is precipitation, AE is the actual evapotranspiration loss, and DT is the
soil moisture recharge. DT is positive when inﬁltration occurs and negative when

soil moisture is being depleted.

109

4.5.2 Computation of volume of shallow aquifer recharge (SAR)
Subtracting surface runoff from the surplus (SUR) will determine the
recharge of the sediments above the conﬁning layer.
Shallow Aquifer Recharge (SAR) = [Surplus (SUR) —- Surface Runoff] (4.5.2)
But it is known from Fig.4.15 that
Outﬂow = Surface Runoff + Pumpage (Q)
Surface Runoff = Outﬂow — Pumpage
Substituting the value of surface runoff into Equation 4.5.2 yields
Shallow Aquifer Recharge = Surplus (SUR) - Outﬂow + Pumpage (Q)
where the ﬂux due to pumpage (Q) is unknown; we can calculate the lower limit
of the shallow aquifer recharge with 33 years’ average rainfall and outﬂow
records of the study area under a rain-fed cropping system.

The lower limit of shallow aquifer recharge (SAR) = SUR — Outﬂow. (4.5.3)

Lower limit of shallow aquifer recharge (SAR) under rain-fecléystem:

A soil moisture balance analysis was conducted under the rain-fed system with
the rainfall records from 1963 to 2000. The amount of SUR is computed in
Appendix .A. The value of the lower limit of SAR was computed as follows.
Under the rain-fed system, the amount of SUR = 716 mm, and the average
outﬂow =435 mm. Therefore, the lower limit of shallow aquifer recharge (SAR) is

SAR = 716-435 mm =281 mm Near

110

Cumulative Proclpltstlon,8urplus volums(SUR),Out flow

 

   
  
   
 
     

6000
(1)= Precipitation
5000 -
'E (2)- Actual Evapotranspiratlon
€- 4000 “ (3): Surplus (SUR)
3
I; 3000 _ (4) Out—Flow
a
.2
E 2000 4
8
1000 -
0 ‘

 

 

Fig. 4.16.a Cumulative sum of the surplus (SUR), outﬂow, and annual evaporation

Water Level, Rainfall , PET and ET: In the study ma( sw Jone)

 

5° 1) =Rainfall(mm)

40 42) ‘-' ETa
(3) -PET

30 r (4): Ref. Water Table Leve

Flux (mm)

 

 

 

 

-20 llllllllllll
a)

.—l~m N
...

llll
‘— C") (3') CD
'— CV ('21 m V V L“)

Fig.4.16.b. ET, PET and the groundwater level in the study area

111

4.5.3 Amount of shallow aquifer recharge(SAR) under HYV-rice irrigated
system

A soil moisture balance analysis was also performed under rice irrigated
system where rice was grown. The amount of SUR available under rice irrigation
system was computed and the result of the shallow aquifer recharge (SAR) is
presented in Appendix. B.

The amount of shallow-aquifer recharges (SAR) for the year 2001 and 2000
were computed as 470.74 and 498.1 mm, respectively.

SAR = SUR - Outﬂow = 964.12 mm - 493.37 mm = 470.74 mm (year 2001)

SAR = SUR - Outﬂow = 876.28 mm - 378.18 mm = 498.1 mm (year 2000)

The calculation of the value of SUR is presented in Appendix .B. The
cumulative volume of precipitation, annual evaporation, SUR and out-ﬂow was
computed from 1999 to 2001 for the study area and the graphs are presented in
Fig. 4.16.a.

Deep Aquifer Recharge = (Shallow aquifer recharge - aquifer storage)

 

AZ "‘ [Water .Table )J] (454)

Aquifer .. Storage = ¢..( AT

where e is the sediment porosity, V (water table) is the elevation of the water

table, and T is the time.

112

4.5.4 Computation of SAR, SUR using soil moisture balance

A spreadsheet analysis for soil moisture balance was created in order to
compute the SUR, SAR, storage opportunity, and rice water requirement for the
study area. The results of SUR and SAR are presented in Appendix. A and B.
For example, the amounts of surplus moisture (SUR) and soil-storage
opportunity were computed as 21.20 and 100 mm, respectively, in Col. 14 and

Col. 16, row marked 52 th day (Appendix .A). The 3-day cumulative amount of
rainfall was 31.54 mm (Col. 6). The effective rainfall (Pe) minus the 3-day actual
evaporation (AE) was calculated in Col. 10 as 25.8.

The SUR was computed according to Equation 4.5.1 (SUR = P9 - AE +

DT}. The amount of excess water was (Pe — AE = 25.88) (Col.10). Soil moisture
in hand on this day was equal to the moisture status on the previous day, plus
the amount of excess moisture available minus the water holding capacity at
AWC (150 mm). Therefore, the volume of SUR is equal to

SUR = 145.4 +258 — 150 = 21.2 mm.

(107.32 mm soil moisture previous day).
The storage opportunity was calculated to be 100 mm (Col. 16).

Storage Opportunity = [VMC (SAT)-VMC (WP)]* 1m - 150 mm = (0.6-

0.35)*1 m -150 mm = 100 mm.

113

where VMC (SAT) and VMC (WP) are volumetric moisture content at saturation
and at wilting point, respectively. The amount of SUR and soil moisture

computed for the study area was presented in Fig. 4.17.

Precipitation (P) or Ra_i_r£a_ﬂ

The rainfall distribution pattern in the study area (in the SW zone of
Bangladesh) has a clear distinction between the monsoon and the non-monsoon
period. Generally, May to October is regarded as the humid summer season. The
rainfall records from 1967 to 2001 calculated the monthly average rainfall, and
the maximum 3- day rainfall value for a two-year and ﬁve-year return period is

presented in Table 4.5.1.

114

 

SUR , rainfall and storage in the study area under non-

irrigated condition
350

300 ~

 

 
     
 
    

Rainfall
250 ~

Soil Moisture Balance
2°° ‘ SUR=716 mm
150 «

100 -

Flux(mm)

rag-(mmlm)

50 «W ... ;L

«cepeeﬁéeetn‘béé‘regsf’6549

Time in 3 Days in a Year

 

 

 

 

Soil moisture deﬁcit. amount of SUR , and storage opportunity under

 

 

300 1 ralnfed 8 rice based system in SW zone of Bangaldesh
250 _ Soil Moisture Status
A 200 -
E
‘5; 150 - v
2
u.
100 _ lrrgat n
SUR=201 mm , j
50 « ‘ ‘ ‘H : Rainfall ‘ l ‘
u . . a k \’
0 1LAH‘L: 24mm“;
“10941.03 ‘eﬁﬁée‘éﬁa‘bsﬁe‘ Mogogéd‘

Time in 3- days in a cropping year

 

 

 

Fig. 4.17 Amount of SUR (blue lines), rainfall (solid green), and soil moisture in
the SW zone study area. Blue lines (on the top & bottom graphs) are the amount

of SUR (201 mm for irrigated and 716 mm for non-irrigated condition) .

115

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:55 , ..

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116

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5. MATHEMATICAL MODELING TO ESTIMATE GROUNDWATER
MOVEMENT 8: LIKELIHOOD OF ARSENIC MIGRATION
The dynamics of groundwater ﬂow movement in a system are a function of
the aquifer’s physical properties, the amount of aquifer recharge, the extraction
rate and the boundary conditions. Under virgin conditions, the groundwater ﬂow
patterns and dynamics is a function of aquifer properties and boundary
conditions, but under extraction, it is a function of pumping with a rate (Q) greater

or smaller than the aquifer recharge (SAR) rate and boundary conditions.

5.1 One-dimensional analytical model to evaluate the impact of the
aquifer’s physical properties on arsenic concentration.

Observation well records around pumping wells in Bangladesh indicate
that most of the water delivered from the water storage comes as a leakage from
the top unconﬁned layer through the aquitard (MP0, 1987), as it is a leaky
aquifer. The length of the unconﬁned top aquifer and the depth and vertical
permeability of the aquitard basically control the ﬂow and movements of
contaminants in the aquifer.

This analysis was designed to predict the mass volume as a function of
aquitard depth and vertical conductivity of the arsenic concentration in water
entering a deep tube well (DTW). Fig. 5.1.a represents a common case of leaky
conﬁned aquifer in Bangladesh in which the screen of the D'lW was installed at a
depth ranging from 70 to 90 m. Fig. 5.1.b investigates how the aquitard depth (b’)

and vertical conductivity (K’) of the impermeable layer (L2) inﬂuence the arsenic

117

concentration of water entering a pumping well and how the results might be

used to justify the theory of arsenic release by oxidation and reduction.

 

 

 

 

0m """" S‘TW """" L1 """"" Unconfined

20 Aquifer
30 * Leakage

4o Aquita rd

50 “scram“:“-’"'k,....'..d 4“ I "I; ,- . ....1—_-'.~:...,_.. u H'.‘
60 L-2 I orw

70 L3

80 _> ‘—

90 m

91 Leaky confined Aquifer

 

 

Fig. 5.1 a Schematic of a typical cross-section of aquifer in Bangladesh, where

deep tube wells (DTW) and shallow tube wells (STW) were installed for irrigation.

 

Ground 02 Pump (Q.) Conc.=C'(t)

 

 

 

v F
iVadose zone Radius of Well R ,
""""" Pyrite Layer L1 Water table
. Q, As conc. at L: C',

 

Vert. cond. K':. L; Aquitard depth = b'

 

Leaky + H DTW Scree* + b
Conf. aquifer L3 . As conc.atL3=C’3

 

 

 

 

 

Fig. 5.1.b Conceptual model to investigate the inﬂuence of aquitard depth (b')

and conductivity (K') on arsenic concentration in the water in a pumping well.

118

.tions
The ﬁrst layer (L1) was considered to be arsenic-contaminated. For the

simplicity of the problem, convective flow of arsenic was assumed, as the
groundwater velocity was higher near the vicinity of the well screen. It is also
assumed that the concentration of water pumped from a well and the

concentration of water entering into a well are equal.

5.1.1 Conceptual one-dimensional analytical model

A three-layered hypothetical aquifer cross section was considered to be

investigated. Layer number three (L3) in Fig. 5.1.b represents a leaky conﬁned
aquifer with relatively low or no arsenic contamination. Layer L2 reﬂects the depth
of the impermeable layer (50 to 150 m) of the aquitard, which protects layer L3
from being contaminated by the overlying layer, L1.

Q = average pumping rate (2448 m3lday) of groundwater from the aquifer.

03 = flow of water from leaky conﬁned aquifer layer (L3) to the well screen.

(Q. - Q3): flow of water from the layer one (L1) aquifer that contributes to Q as a

leakage ﬂux to the well.

Rt = the radius of influence at any time t with a pumping rate Q ..

0'1 = arsenic concentration (100 ppb) in unpumped layer 1 aquifer (L1)
C'3 = arsenic concentration in pumped leaky aquifer layer (L3)

C'. = resultant concentration of water entering the well at any time t,

119

T, n, and b are transmissivity, porosity, and depth of the pumped aquifer,
respectively. Aquitard depth and hydraulic conductivity are U and K'

respectively.
When pumping starts, the concentration of water entering the well, C'. is
equal to G3, reﬂecting the aquifer's initial concentration. As pumping continues,

C" progresses from G3 to C'1, reﬂecting the increasing contribution of leakage to

the pumped water. To determine C", , a mass balance of arsenic arrival to the
well screen from aquifer water was considered. At any time t, the pumping rate
Q; was comprised of water derived from the leaky aquifer (Q3) and water derived
from layer one (L1) as a leakage factor (Q1). From mass balance, Qt =Q3+Q1, so
it can be expressed as Q1= (Qt - Q3) (Fig. 5.1.b).

According to the principle of mass balance (Ref: Fig.5.1.b), the mass of
arsenic in water inﬂow to the well was equal to the mass of arsenic in water
leaving the well. Geode et al. (1993) of the USGS derived the following equation
based on the mass balance approach to a the leaky aquifer:

C't * Q. = C'3* 03 + CH * (Qt - 03), which yields

C":=C.1+(C.3-C'i)-g—3 (5.1)

. 9

Bear (1979) derived the following formula to determine Q3 under vertical leakage:

condition through the impermeable layer.

9: = 27:1).12, 4R] = g§-Kl[%] (5.2)

120

where Q; is the pumping rate at any time t, R, is the radial distance from the well,
B = (T b'IK')"2 is the leakage parameter, K' is the vertical hydraulic conductivity of

the aquitard, K1 is a modiﬁed Bessel function of the second kind and ﬁrst order,

and (dsldr) is the hydraulic gradient. The speciﬁc discharge towards the well, q,
at a radial distance R is q (R), n is the sediment porosity and the water velocity is
v(R) =q (R) I n.

T ds Q R
R =—.——.= ' * .1.
q[ '] b dr 24:1).3 K' B

 

Now putting the value of q (R) and Q3 into equation 5.1, the concentration of

arsenic water entering the well at any time t (C2) during pumping from a well can

be expressed by the following equation:

 

C,‘ = C} + (C, — c',)*£.*K, [5;]
B , B

 

 

 

(5.3)

The concentration in the water entering the well at any time t was (0.), computed
by spreadsheet analysis considering ﬁrst the radius of inﬂuence R. and then
substituting the value into Equation 5.2 to get the initial discharge (Q3). As
pumping continues, the R. increases and ratio of partial discharge Q3/Q.
decreases and C'1IC'3 increases. Then the inﬂuence of aquitard thickness (b’)

and conductivity (K') on the concentration of water entering into the well were

computed by spreadsheet and shown in Fig. 5.2.a.

121

Inﬂuence of aqultard thickness and permeability on steady state arsenic
concentration during pumping of contaminated well

 

120

 

100<

80‘

 
   

I) b'I150 It. K'I 11.8 Illday
b) b'=65 I'l, K'8 1.18 I'tlday
c) b'8100 1!, K'- 1.18 Mday
d) b'I150 1!, K'- 1.18 ﬁlday

Arsenic concentration in the pump (ppm)
8

 

 

0 500 1000 1500 2000 2500 3000 3500
Radius of Influence (ft) over time

Fig. 5.2 a. Inﬂuence of aquitard thickness (b’) and permeability (K’) on steady

state arsenic concentration of water entering in a pumping well (Q=2448m3/day).

5.2 Three-dimensional numerical modeling to estimated shallow aquifer
recharge (SAR) rate, flow components, and their impact on arsenic
transportation

The arsenic transport equation derived here was based on the
conservation of mass and assumed that the ﬂow was steady state and Darcy’s
law was applicable. Under Darcy’s assumption, the ﬂow was described by the
average linear velocity, which carried the solute by advection. The advection
process, in addition to the hydrodynamic mixing process, transports non- reactive

solutes. The rate of advective transport was equal to the average linear velocity

122

v*=(vln), where v is the speciﬁc discharge and n is porosity. if “C” is the
concentration of solute, then the mass of solute per unit volume of porous media
is n C
5.2.1 Derivation of three-dimensional (3-D) transport equation

The mass of solute transport in the x direction (Fig. 5.2.b) by advection is
equal to v"x nC dA. The mass of solute transport in the x direction by dispersion

is equal to n D" (60! 6x) dA, where (60/ ﬁx) is the concentration gradient in x
direction and dA is the unit area. The dispersion coefﬁcient D)( is related to the
dispersivity ax and the diffusion coefﬁcient 0*. v"',( was the average linear velocity

in groundwater

Dx=ax. vx" + D* (5.3.1)

 

Inﬂow in z = F: . F: + (anI 5:) dz (Outﬂow in 2)

Fy = Fy + (aFy I 0y) dy (Outﬂow in Y)

1

I
- — >
Inflow (in X) = Fx I Outflow = Fx + (anIax)dx

> X

 

Y

 

 

 

Fig. 5.2.b. Derivation of three-dimensional contaminant transportation equations

123

If Fx, Fy and F2 are the total mass of solute per unit cross section area (Fig.
5.2.b) transported in the x, y, and z direction per unit time, then
6C

F=v’.n.C-nD— ..
x , 6x (532)
. 6C
Fy = v yn.C — nDy 3:. (5.33)
F, = v‘.n.c -— nD . 3C— (5.3.4)
62

The difference in the total amount of solute entering and the total amount leaving

the system is equal to the amount of solute accumulated or stored in the system.

6C
6!

 

 

 

 

[aF, 6F, 6F,
+ +

dx. .dz =—
6x 6y 62] dy n

dx.dy.dz

The complete three dimensional (3-D) conservation of mass expressions

 

 

 

 

therefore become
6F 5F , 6F 6 C
" + + ‘ = — , ,
6x 6v 62 n 6t (535

Now substituting Eq. 5.3.2, 5.3.3 and 5.3.4 into Eq. 5.3.5, canceling n from both

sides, the 3-D equation for non-reactive transportation 5.3.5 becomes

[1(0, 29) + -a—-[D E] + 3(0, g—C—J] - [34 .xC)+ —a—(v',C)+ 2-(v°..C )] = §£ (5-3-6)
61: 6): 6y ’ 6y 62 62 6x 6y 62 6t

The one-dimensional non-reactive transport equation then becomes

azc .ac _ac

DJI 2 v —
6x 6x 6!

 

(5.3.7)

124

The one-dimensional reactive solute transport equation was described by adding

a reaction term to Eq. 5.3.7:

 

 

62C 6C p 6C 6C
D‘axz—v‘ax—TK‘ar = a:
(5.3.8)
where . 30— K c 6 C .. is .. reaction .. term
n 6!

Rearrangement gives Eq. 5.3.8 as

2
(1+ ELF-C— = D, a C2, — v‘. 6C (5.3.9)
n 6! 6x 6x

 

 

Therefore, the advection dispersion transport equation with reactive solutes can

be expressed as

.azc_.a_c_ ac

 

D x 2 V x = —
6x 6x 6!
- D . v (5.3.10)
where ....D,r = —— = R..(Re tardation ..Factor )...v,r = ——
1+3K. 1+KK,
n n

Solution to advective diffusive transport wtion:

The three-dimensional (3-D) advective diffusion non-reactive and reactive
transport equation (Eq. 5.3.6 and Eq. 5.3.10) could be solved by traditional ﬁnite
element, ﬁnite difference, method of characteristic (M00) and random walk (RW)
methods. But this study used the interactive groundwater modeling software
(IGW), a new and unique method that was both computationally efﬁcient and
numerically accurate in solving the time-dependent, three—dimensional,
advection-diffusion equation for application to transport in heterogeneous porous
media. IGW addresses the limitations of numerical dispersions and overshooting,

as found in traditional ﬁnite element (FE) and ﬁnite difference (F 0) methods, by

125

utilizing a solution method that has no Peclet or Courant conditions, requires no
time-stepping, and was relatively accurate even at large grid spacing.

Peclet Number Pe criteria

 

P6 = Vx * dx / Dx <2, where Vx = particle velocity in x direction, dx = cell size in x
direction (40 m), and 0x: dispersion coefﬁcient.

For stability, the cell size should not be larger than twice the dispersivity
Courant Number Criteria
The time steps minimize the numerical dispersions and maximize the numerical
stability of the solution.
Co = V * dde (was less than 1)
5.2.2. Development of conceptual three-dimensional model for the

study area

In order to determine the inﬂuence of pumping on advective and
dispersive transportation of arsenic, a study area of one square kilometer was
divided into 25 equal grids with sides 40 m in length. An extraction well (DTW)
was installed in the center of the area (Fig. 5.2.c). Initially, a constant head
boundary and no ﬂow boundary conditions were imposed along the boundary of
the study area, and later a river boundary was set in order to reﬂect the
complexity of real ﬁeld conditions.

Description of the aquifer layer (Fig. 5.2c)

a) Upper shallow aquifer (L1): 0 to 40 m below ground level; ﬁne sand/silt

b) Lower shallow aquifer (L2): 40 to 140 m below ground level; medium sand

126

0) Deep aquifer (L3): 140 to 400 m below ground level; silt clay

 

 

round Pump (Qt).
L-1(0-40 m) Fine/Medium Sand
' 3211671265) a. """""" I "

.Cond. K' "“ Medium Course sand

...... 51" WI

L-3 (140-400 m) DTW Fine Sand
<— ——————— L F- ————————— >

 

 

 

Fig. 5.2.0 Cross section of a conceptual model in the study area
Types of conceptual models studied

Three types of conceptual models (M-1, M-2 and M-3) were designed in

order to reﬂect two ﬁeld conditions. Condition one, where the shallow aquifer (L2)
was more conductive than deeper layer L3 and L1 and was reﬂected in the M-2
model. Condition two was incorporated into M-3, in which deeper aquifer L3 was

more conductive than L2 and L2 was more conductive than L1; it implied an upper

ﬁning sequence condition of the aquifer. Comparing the simulation results of
those two models (M-2 and M-3) with those of the uniform model (M-1) would
help understand the differences in ﬂow patterns and likelihood of contaminant
migration to deeper layers where aquifer properties are different. Descriptions of
models are given below.

Model M-1:

 

127

and K3 represent the conductivity values of layers L1, L2 and L3, respectively).

Later, the uniform model was modiﬁed to incorporate a more realistic
representation of the geology as observed in the borehole log of the experimental

area.

Model M-2:

 

This 3-layer basic model is much like M-i but the hydraulic conductivity

values are representative of condition one in which the lower shallow aquifer (L2)
is more conductive than the upper (L1) and deeper (L3) aquifer (K2=50 m/day, K1

= K3 =15 m/day).

Model M-3:

 

A 3-layer model with hydraulic conductivity values representative of condition two

that reﬂected real ﬁeld conditions (upper ﬁning sequence) in the aquifer

sediments. In this model, hydraulic conductivity is set at K1 = 15 m/day, K2 = 50
m/day, and K3 = 100 m/day (Fig. 5.2.c). Model M-3 differs with M-2 in terms of

permeability of the deeper aquifer (L3).

The model assumptions.

 

The system is steady-state, homogeneous, and isotropic.
The river in the study area is assumed to be the only outﬂow.

The aquifer system is 400 m thick.
128

Table 5.1 Comparison of the layers’ properties by different conceptual models

 

 

 

 

 

 

 

 

 

 

Depth Layer Geology Conductivity Aquifer name
(m) (K) (mlday)

0 to 40 L1 Silt, ﬁne to medium sand K1 = 15 Upper shallow
aquifer

40 to 140 L2 Fine to medium sands K2 = 50 Lower shallow
aquifer
140 to L3 Medium to coarse sand K3 = 100 Deeper
400 with gravel aquifer

 

Jusﬁﬁcaﬁons:

 

. The steady-state assumption was made as it ignored the obvious
seasonality in the groundwater levels.
0 The thickness of the aquifer was chosen to be 400 m to allow the bottom
boundary to be deep enough so as not to inﬂuence the ﬂow patterns.
. The hydraulic conductivity was derived from the transmissivity of the study
area and thickness of the aquifer.
. A no-ﬂow boundary was chosen for the base of the aquifer, and the lateral
boundary to represent the limit of the groundwater catchments
.' The net recharge was based on the soil moisture balance and checked by
the observed seasonal variation of the groundwater and speciﬁc yield.
5.2.3 Computation of head distribution and ﬂow patterns of uniform 3-D
model (M-1)

In order to validate the model head gradient with the local hydraulic
gradient, it was necessary to analyze the head distribution of the study area. In

the study area, the actual hydraulic gradient varied from 0.15 to 0.25 m / km. The

129

computed gradient was 0.18 m/km and agreed with the ﬁeld values. The head
distribution of the top layer and the groundwater ﬂow diagrams were presented

below in Fig. 5.3 and Fig. 5.4.

 

 

 

 

 

Fig. 5.3 Groundwater gradient to the river
(NB: Images in this dissertation are presented in color)
5.2.3.a Computation of ﬂow patterns in the 3-D uniform model (M-1)
Doubling or halving the shallow aquifer recharge (SAR) rate had no or
very little impact on the ﬂow pattern as shown in Fig. 5.4. However, increasing

the conductivity drew water to the deeper layers, as shown in Fig. 5.5.

130

 

 

 

 

 

Fig. 5.4 Inﬂuence of aquifer recharge (SAR) on ﬂow patterns (at 635 mm/yr. on

the left side, at 1230 mm/yr. on the right side) on ﬂow patterns

5.2.4 Computation of mass balance to demonstrate the impact of the
extraction rate (Q) and the aquifer recharge rate (SAR) on groundwater
movement and ﬂow patterns

The mass balance computation was performed by the IGW software
based on the law of mass conservation. The total inﬂow (in x, y and z direction)
was equal to the total outﬂow (in x, y and z direction) at any layer (Fig. 5.2.b).
The impact of average (381 mm), medium (635 mm) and high (1000 mm)
shallow aquifer recharge (SAR) rates on downward ﬂow patterns at different

layers was estimated and the results are presented in Table 5.2.

131

 

 

 

 

 

Fig. 5.5 Increasing the conductivity at L2 drew the stream lines deeper.

Computation of total inﬂow and outﬂow components in each layer of the model
(Ref. Table 5.1a and Table 5.2)

Example of mass balance computation:

Table 5.2 contains the mass balance results of the M-3 model with different
aquifer recharge (SAR) rates. Table 5.1a contains mass balance results of the
uniform model M-1. In Table 5.1a, using the M-1 model and the mass balance of
layer one (L1) with the SAR rate of 381 mm/yr., the total inﬂow is equal to the
total outﬂow.

Total inﬂow 1950 maday'1 (Col. 7) = Body in (Col. 4) + Recharge (Col. 5) + Flow

from top to bottom (Col. 6) = Total outﬂow (Col. 10 = Col. 8 + Col. 9).

132

Inﬂow =1950 (Col. 7) = 0 (Col. 4) + 1950 (Col. 5) + 0 (Col. 6) = 1950 (Col. 7)
Col. 4 = Flow coming into the layer from the lateral side as “Body in” (ﬂow from
a river or lake); in the M-1 model, this was zero (Fig. 5.5.a).
Col. 5 = Recharge in mmlday for the area for which mass balance was
computed.
Cross sectional area (1869.6 * 1000 m2) * SAR (381/365) mmlday = 1950 m3lday

Col. 6 = Flow in “from top to bottom “ (Fig. 5.5.a); in the M-1 model was also

79"!)

 

 

 

 

 

 

6R0 SURFACE Re ge
“’3 I, ”‘6' _
Ar ...... ,
Boundary I Layer-1 -_ River! "
l—p : Bodyh Lake 4‘:
_ Bowl: . FromTop». l -4 »
MW ‘9' Batten : '4." BsdyOat -
r .
awn 1| 4‘— 30th
I Ld,~:l 2
' Bodygg) l. + f Bodyou
————— j— — — — — — — — — — — — — — -
. u
—’Doqh J ‘— Bodyh
r
”'BsdyOIt ' BodyOIR
: Layer-3 Fmtom
I I
v V f

 

 

 

Fig. 5.5.a. Components of ﬂow coming in and out from all directions as a part of

the mass balance during model simulation.

133

              

  

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5.3 Inﬂuence of extraction or pumping on the local hydraulic gradient and
ﬂow to and from the river

Groundwater extraction produced the following effects

 

Pumping had a signiﬁcant inﬂuence on the ﬂow through the deeper layers of the
aquifer system. It inﬂuenced the ﬂow through the deeper layers from three main
aspects.
. In general, extraction or pumping conditions provided more ﬂow through
the deeper layers than the natural groundwater ﬂow conditions did.
. When the extraction rate was greater than the aquifer recharge (SAR)
rate, it increased the ﬂow through the deeper layers when the aquifer was
connected to the river system. i
c When the aquifer recharge (SAR) rate was greater than the pumping rate,
it reduced the ﬂow through the deeper layers.
5.3.1 Impact of dispersivity and heterogeneity on arsenic migration

Adding dispersion and heterogeneity to the model reﬂected real ﬁeld
conditions. In order to see the inﬂuence of the well pumping rate on the
dispersions of arsenic, a particle-tracking approach was used to trace the
movement of arsenic from the surface to the well. This conceptual model was

built in order to understand the inﬂuence of the ratio of the longitudinal
dispersivity (0.) to the transverse dispersivity (DT), and the inﬂuence of the

pumping rate on arsenic migration as tracer particles.

138

 

 

 

 

 

Fig. 5.6 Flow path of arsenic after 100 and 5000 days, assuming movement of

arsenic particles when DLIDT = 0, Q = 2448 m3lday

 

 

 

 

 

 

Fig. 5.7 Flow path of arsenic movement when DL/DT = 5, Q = 2448 m3lday, after

100 and 5000 days

139

Impact of diagram ratio (0.! /Dr) on arsenic movement when pumping (Q)
remains constant

Fig. 5.6 and 5.7 clearly demonstrate the effect of the dispersivity ratio on
arsenic movement when Q remains constant. The arsenic migration tendency
after 5000 days was found to be greater when a higher dispersive ratio was
added (Fig. 5.7, DL/DT= 5).
Impact of pumping or extraction on arsenic migration when the disgrsiviy ratio
mm

Fig. 5.7 and Fig. 5.8 indicate that increasing the pumping rate had a
greater inﬂuence on the arsenic transportation than reduced the pumping rate (2
CFS vs. 1 CFS) when the dispersivity ratio remained constant. Initially, no
retardation of the contaminants was considered, and therefore, K, = 0. A no-ﬂow

boundary was assumed around the model area.

 

 

 

 

 

Fig. 5.8. Inﬂuence of the pumping rate on arsenic movement, when DLIDT - 5, Q
= 4896 m3/day (2 CF S), and the ﬂow path of arsenic is given after 3000 and

5000 days
140

By comparing the ﬁgures (Fig. 5.7 and Fig. 5.8), it is apparent that the pumping
rate has a signiﬁcant inﬂuence on the arsenic movement as a particle form in the

groundwater system.

5.3.2 Inﬂuence of placing tube well screens in to the deeper layers on
arsenic migration

It was a great concern that currently uncontaminated layers may be
contaminated with arsenic as a result of groundwater movement. Placing tube
well screens at deeper layers has been suggested to avoid the problem, but this
practice could cause avoidably bad outcomes. This issue was investigated from
the concept of mass balance results obtained by using the IGW software. The
ﬂow patterns of groundwater and local gradients to the river was used to address
the issue of arsenic migration in the aquifer under the inﬂuence of placing wells
at different layers.

The mass balance results of the M-2 model presented in Tables 5.4 and

5.5 under average recharge (SAR = 381 mmlyr.) and medium recharge (635

mmlyr.) conditions while placing the well screens at different aquifer layers, L1, L2

and L3, was compared. It was found that pumping at a rate greater than the
recharge rate (SAR) was drawing water from the river to the well as ”Body in.”

gamete
(Table 5.4 and Table 5.5)

The component of ﬂow as “body in" from the river was 1210 malday (Col.

4, Table 5.4) when a well-screen was placed at the ﬁrst layer (-20 to -26 m), with
141

the SAR rate at 381 mmlyr. Here, the pumping rate (Q) was greater than the
recharge rate (SAR). When the SAR was increased from 381 to 635 in the M-2

model, the “body in“ component from the river was reduced to 400 m3/day from
1210 m3lday (Table 5.5). Also, both the L2 and the L3 components of ﬂow were

reduced to 1520 and 660 m3lday (Table 5.5) from 1900 and 690 m3/day (Col. 4,
Table 5.4) respectively.
Summagy

It is evident from the above mass balance computation results in Table
5.4 and Table 5.5 that pumping at a rate greater than the recharge rate draws
water from the river system to the well. Installation of wells at deeper layers when
aquifer sediments are more transmissive results in signiﬁcant amounts of water
being drawn from the river, and ﬂow through the vertical layer is increased.
Therefore, it is very likely that the placing of a tube well screen at a deeper layer
could result in arsenic migration with the moving groundwater induced by

advective ﬂow-

142

 

 

 

 

 

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5.4 Development of a site-specific model to predict the inﬂuence of
pumping on arsenic transportation.

A site-speciﬁc conceptual model was built up in order to reﬂect more
Iithologicel parameters in the actual geology of the study area. The actual model
cross section is shown in Fig. 5.9. It is a seven layers model study and was
analyzed to predict the downward ﬂow of arsenic through the deeper layers as a
result of over extraction. This site-speciﬁc real model incorporated more realistic
ﬁeld complexities into the model in order to make the model results

representative.

 

 

 

 

 

Map source: GOB(1999).

Fig. 5.9 Geological cross-section of site-speciﬁc model in the study area. Up to 400 m

depth

146

5.4.1 Inﬂuence of pumping on vertical migration of arsenic in the models
that reﬂected more realistic complexities in the ﬁeld

Flow patterns were captured for both natural ﬂow conditions (left side, Fig.
5.10) and extraction or pumping conditions (right side, Fig. 5.10) while simulating
the site-speciﬁc model. There was signiﬁcant change in the ﬂow pattern because
of extraction (Fig. 5.10). The velocity distribution was examined for each layer,
and the change in velocity distribution graphs were shown in Appendix E. Site-
speciﬁc model simulation showed that placing well screens at deeper layers
increased the downward ﬂow rate through the deeper layers. Therefore. it is most
likely that groundwater arsenic concentration would be increased by extraction

from deeper layers.

 

   

Flow pattern under natural ﬂow

 

 

 

Fig. 5.10. Signiﬁcant change in groundwater ﬂow patterns due to the extraction

rate in a site-speciﬁc model that reﬂects the real ﬁeld complexity.

147

5.4.2 Inﬂuence of extraction on arsenic concentration migration in the site-
speclﬂc real model

In order to estimate the likelihood of arsenic migration in the groundwater
system, three cases of mass balance computation of arsenic concentration were
designed:

1. Arsenic concentration at different layers of the site-speciﬁc model as a function
of the shallow aquifer recharge (SAR) rate.

2. Arsenic concentration at different layers of the site-speciﬁc model as a function
of the pumping rate (Q)

3. Arsenic concentration at different layers of the site-speciﬁc model as a function
of placing well screens at L1, L2 and L3.

The model results for the three cases are shown in Table 5.6.

 

I!“ It. ‘1‘. m" It” m ”4.. 1m 1‘... llllll

Fig.5.11. lmpact of pumping on the arsenic migration under natural ﬂow condition
(left side) and pumping condition (right side) in to the deeper layer after 5000

day 3 without retardation.

Table 5.6. Arsenic concentration at different layers of the site-speciﬁc model as a

function of SAR rate under natural groundwater ﬂow conditions after 10000 days

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Aquifer Depth Condu- SAR SAR SAR SAR
Layer (m) ctivity 381 mm 762 mm 381 mm 762 mm
-1 Cond. Cond. (Not Cond. Cond.
(m d ) (Not Pumped (Pumped) (Pumped)
Pumped) (ppb) (ppb) (ppb)
(ppb)
L1 -10 to — 15 1500 1500 1500 1500
40 (55)
L2 -40 to — 35 897 1049 986 728
120
(80)
L3 -120 to 5 399 515 435 234
—155
(65)
L4 -155 to 30 124 176 162 52.2
—245
. (60)
L5 -245 to 5 33.5 51.5 42 10.66
-270
L6 -270 to 100 5.52 9.7 7 1.26
350
L7 —350 to 100 0.11 0.2 0.14 0.18
-400

 

It appears from the Table 5.6 that pumping conditions helps more arsenic to

migrate in to the deeper layers when the available shallow aquifer recharge rate

(SAR) rate is 381 mm while comparing with the SAR at the rate 762 mm/Yr.

This issue will be further discussed in section 6.6.

149

 

6. EVALUATION AND DISCUSSION OF RESULTS

Chapter six documents the results and applies results to address the
research issues presented in chapter four. The results of analysis will be
discussed in light of the conceptual basis or hypothesis of this study. The
hypothesis is that layers at or below the water table receive less oxygenated
water as a result of pumping. This study mainly investigated seven broad-based
issues to achieve the overall goal and objectives. The issues are as follows:
. The geohydrologic factors that produce high arsenic in the groundwater
system
. Interactions among lowering of the water table, dissolved oxygen (DO)
content in the recharging water (SAR) and the likelihood of arsenic release
. Evaluation of the existing arsenic release hypotheses and establishment of
logic to support the hypothesis presented in this study
. Impact of the irrigation withdrawal rate (Q) and the cropping pattern on the
amount of shallow aquifer recharge (SAR)
. Inﬂuence of extraction (Q) at a rate greater than that of aquifer recharge
(SAR) on groundwater movements, ﬂow patterns and likelihood of arsenic
release
. Placing of well-screens in the deeper layers of the aquifer and the likelihood
of arsenic release
. Administrative and management decision variables to reduce the risk of

arsenic contamination in case of further irrigation development.

150

Finally, recommendations are made based on the ﬁndings. The overall
goal of this study was to formulate a quantitative irrigation development policy
that would reduce the risk of arsenic contamination. The evaluation of this
analysis is based mainly on the research issues presented in chapter four, and

each research issue is discussed along with a few strategic questions.

6.1 Research issues and strategic questions

The arsenic concentration distribution analysis for different hydrologic
zones was plotted in Fig. 4.2.b. Also, analyses presented in Table 4.2.1 and Figs.
4.2.a, 4.5, 4.6.a, 4.6.b, 4.7.a, and 4.7.b indicated that there were patterns or
relationships between arsenic contamination and regional hydrology,
geomorphology, surface geology, physiographic units, depth of the aquifer, and

dissolved iron content in Bangladesh’s groundwater system.

Research issues

 

. Hydrologic factors that correlate with high arsenic concentrations in

Bangladesh’s groundwater system.

Strategic questions

 

1. Why did the arsenic concentration exhibit patterns in hydrologic zones as
shown in Figs. 4.2.b and 4.3.a, varying with the geomorphology, surface geology

and physiographic units of Bangladesh?

151

2. Why and how is the arsenic contamination problem related to the dissolved
iron content of Bangladesh’s groundwater?

The fact is that the cause of arsenic contamination in the groundwater of
Bangladesh is still poorly understood and a strongly debatable issue. The above
strategic questions were investigated and explained by application of
hydrogeochemical and thermodynamic properties. Also, the ﬁeld evidence

available so far justiﬁes the explanations offered.

6.1.1 Why arsenic concentrations in groundwater exhibited correlations
with the hydrologic zones, geomorphology and surface geology

Arsenic concentration analysis exhibited a correlation with the hydrologic
zones as shown in Figs. 4.2.b and 4.3.a. The results will be discussed from the
context of probability of arsenic exceedence in that area to explain the

correlations.

Probability of arsenic concentration exceedingthreshold levels was found to be

 

highest in the southeast (SE) and southwest (SW) zones of Bangladesh

 

The probability of arsenic exceeding threshold levels (50 ppb) was
analyzed, presented in Table 4.2.1, and pictured in Fig. 4.2.b. It appeared from
the results that the southeast (SE) and southwest (SW) zones were the most
arsenic-affected areas, in which the probability of arsenic exceeding threshold

levels (50 ppb and 250 ppb) was found to be 71.4% and 59.55%, respectively

152

(Table 4.2.1), in the SE zone of Bangladesh. It was apparent from Fig. 4.2.b that
there was a distinct regional pattern of arsenic contamination, with the greatest
contamination in the southeast (SE) and southwest (SW) zones of the country.
On the contrary, the northwest (NW), north-center (NC) and south-center (SC)
zones had probabilities of arsenic exceeding threshold levels in the range of 18
to 39% (Table 4.2.1).

Alluvial deposit (in the SE) and deltaic deposit (in the SW) types of surface

 

geology correlated with high arsenic concentration

Despite having large variability in the spatial distribution of arsenic
contamination, it was found that the most highly arsenic-contaminated area was
mostly conﬁned to the lower delta ﬂood plains of the Ganges River, which
comprises the southwest (SW) region, and the Meghna River Flood Plain (FP)
which built up the southeast (SE) region of Bangladesh (Table 4.2.2, Fig. 4.2.a,
and Fig. 4.3.a). The young (Holocene) alluvial and deltaic deposits in the
southeast (SE) and southwest (SW) zones of Bangladesh were the most
affected, whereas the older alluvial sediments in the northwest (NW) and north-
center (NC) regions were not (Fig. 4.3.a) as contaminated.

The least arsenic-contaminated zones were the northwest (NW) and north-
center (NC) zones (Fig. 4.2.b) where the geomorphology was comprised of Old
Himalayan Fan, Tista-Jamuna ﬂoodplains, and Madhupur Tract (Table 4.2.2).
The geomorphologic and surface geological differences among the hydrologic

zones were presented in detail in Table 4.2.2.

153

Physiggramy of the SE rggion (dominated by 'liggra surface) strongly
correlated with high arsenic concentration

 

The physiographic map analysis in Fig. 4.3.b clearly indicated that the
highest arsenic concentration levels (>250 to 1665 ppb) were mostly conﬁned to
the SE region where the physiographic unit was dominated by Tippera surface
(Fig. 4.3.b and Table 4.2.2). The red spots in Fig.4.3.b indicated the area

affected by arsenic levels above 50 ppb.

Geomorpholggy of Ganges & Meghna River Flood Plains produced the highest

sedime load and was correlated hi evel arscenintanatroni

     

Arsenic contamination was found mostly distributed over the Ganges -
Meghna River Flood Plain (FP) (Figs. 6.1 and 4.3.a). The Ganges and Meghna
River FP received sediments from the vast catchments in India and Nepal (Fig.
6.1 & Table 3.2). The combined delta of the Ganges and Meghna River FP
experiences a high rate of sediment ﬂow, about 478.9 million tons per annum
(Table 3.2). it produced a greater sediment load in the SW and SE regions than
in the NW and NC regions of Bangladesh.

Arsenic concentrations in the SW and SE zones were found to be greater
than those in the NW and NC regions because arsenic was inherited in the

sediments of that region through the development of different geomorphology.

154

Therefore, it seemed that the arsenic distribution pattern in the SW and
SE zones highly correlated with the geomorphology of the SE and SW regions of

Bangladesh.

 

 

    
 

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-. 4 .
\ x _ E ‘ .
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.- i ’.

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. I '1' ‘. '-.
"I \
-- (that. r.s~"*'\~

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Fig. 6.1 The vast catchment area of the rivers Ganges and Meghna and
Brahmaputra river basin, mostly in India and Nepal. Blue lines are the ﬂowing

river systems.

6.1.2 Explanation of why and how arsenic contamination is related to the
dissolved iron content in the groundwater

The results analysis shown in Fig. 4.7.a and Fig. 4.7.b suggested that the
distribution pattern of arsenic concentration in the most arsenic-contaminated
aquifers offered a good correlation with the dissolved Iron (Fe2+) concentration

(Table 4.2.4).

155

Why arsenic concentration and dissolved iron content are related

In order to ﬁnd a relationship between arsenic and iron, the whole country
was divided into 16 grids latitudinally from 22.00 to 26.75°N and longitudinally
from 88.2 to 92°E. The number of grids, respective latitudes and longitudes, and
average iron and arsenic content were presented in Table 4.2.4. Grids 5, 9, 10,

12, and 15 had the highest arsenic concentration rates of these grids and also

the largest regression coefﬁcient (R2) values, ranging from 71% to 99% (Table

4.2.4). The conﬁdence interval associated with these R2 values was 95%.

All of the most highly arsenic contaminated areas belong to the SW and
SE zones. The reason for the high iron content in their soil is that the SE and SW
regions of Bangladesh were mainly built up by the deltas’ sediment deposition,
and deltaic sediment contains more iron oxides as associated minerals than
other types of soil (GOB, 1999).

It was found from the literature review that in sediment-water solution
interactions, arsenic is strongly sorbed by the many metal oxides attached to the
sediments, especially iron oxides and hydrous ferric oxides (HFO). The HFOs
had a surface area an order of magnitude greater than that of the most crystalline
iron minerals. The greater the surface area, the more the sorption (Dzombak
and Morel, 1990). As the sediments of the SE & SW zones are rich in dissolved
iron or hydrous iron content, their surface area is more inclined to adsorb arsenic.
Therefore, naturally, the arsenic content in those sediments would be rich. This is

how dissolved iron showed a strong correlation with arsenic in the SE and SW

156

regions of Bangladesh. High dissolved iron content reactions are generally

microbial mediated (Lovely and Chappel, 1995).

6.1.2.a Thermodynamic explanation of the correlation between high arsenic
and dissolved iron content in the groundwater system

A thermodynamic analysis was done in order to correlate the arsenic with
the dissolved iron. Many reactions in nature go 100% to completion; however,
the incomplete reactions are reversible, and products colloid with each other and
produce reactants. When the net change in reactants and products is zero, the
reaction is said to have reached equilibrium. The precipitation and dissolution
reactions of ferric hydroxides are reversible reactions in the environment (Stumm
and Morgan, 1997).

In this study, it was investigated how a lack of dissolved oxygen (DO)
could inﬂuence iron reactions to release arsenic by shifting the chemical
equilibrium condition.

lron precipitation Exothermic
22>

4Fe2++02+10H20 <:~'> 4Fe(OH)3(s) +8H+ (6.1)
<:==

Iron dissolution Endothermic

157

The above reversible reaction (6.1) is considered to be in equilibrium only
when the rate of forward reaction is equal to the rate of reverse reaction. If the
free energy of formation of products is equal to the free energy of formation of
reactants, then the reaction is at equilibrium. Using the value of the reaction
quotient (Q) and the equilibrium constant K, it is possible to understand whether

the reaction in the system was in equilibrium or not. For the iron precipitation
dissolution reaction (Eq. 6.1), the value of equilibrium constant K = 6.2'105 =
105.8
Reaction quotient (o) = Solid [Fe (orilal‘ lift“
2+ 4 10
[Fe 1 P1021 [H201
According to the thermodynamic rules, if the term Q I K equals 1, then the
free energy of the reaction, V G, equals zero and the system is said to be at
equilibrium. If QIK <1, V G is negative and the reaction is spontaneous. in
Equation 6.1, the value of the reaction quotient, Q, was calculated by considering

a hypothetical system of pH = 5 and dissolved iron concentration Fe2+ was 0.1

mglL (10‘5'5 mollL), and the dissolved oxygen content was at equilibrium with the

-3.54

atmosphere [P 02 = 0.21 atm = 10 molIL]. The reaction quotient, Q, was

0’31. As the equilibrium constant K equals 10“, therefore, QIK is

found to equal 1
<1, (1 I 1036's), implying that the equation (6.1) would favor the product and that

the dissolved iron would be precipitated in solid form in the presence of dissolved

158

 

I It]! .1

oxygen.
But when the oxygen concentration was reduced, an increased value of
Q/K was found. It implies that the fonlvard reaction rate was slowed down and the

reverse reaction was powerful. For example, if the oxygen concentration
decreases to 0.000002 atm (10'358 mol/L), then Q = 10'26'42and Q/K = 010322
Therefore, the decreasing amount of oxygen would shift the equilibrium in the
reverse direction. As a result, the dissolution of iron oxides will occur. Eventually,
arsenic will be released from iron oxides. The greater the shortage of oxygen is,
the more reducing conditions will prevail and the more Fe2+ will be dissolved and

the associated arsenic will be released into the water. This is how iron and

arsenic release into groundwater systems are related to each other.

6.2. Interaction between lowering of the water table, dissolved oxygen and
arsenic concentration triggering

Research issue

 

- The second research issue was to understand the interaction between the
lowering of the water table position, the dissolved oxygen content in recharging
water and the triggering of the arsenic contamination problem in Bangladesh’s

groundwater.

159

Strategic questions

 

1. Why does the arsenic concentration vary with depth?

2. What is the role of the lowering water table on dissolved oxygen content in
groundwater, redox potential values and the rise in arsenic concentration?

3. What impact did the lowering of the water table have on the dissolved oxygen

concentrations of the deeper layer of the aquifer?

6.2.1 Why arsenic concentration varies with aquifer depths

With data from the real heterogenic world, one would not expect a high
correlation between aquifer depths and arsenic concentration. However, an R2
value of about 58 to 72% at a regional scale (Table 4.2.3) and about 32 to 99% in

the square grid system of analysis (Table 4.2.4) was obtained.

Arsenic concentration increased from 9 to 30 m

 

Why arsenic concentrations vary with aquifer depth will be investigated in
this section. Figs. 62a and 4.6.3 showed a positive correlation between arsenic

concentration and aquifer depth over the depth ranging from 9 to 30 m below the

ground, and the corresponding R2 value of that layer ranged from 0.58 to 0.72.

160

Arsenic concentration decreased from 100 to 350 m

In contrast, a negative correlation was found between arsenic and
dissolved iron from 100 to 350 m below the ground (R2 = 0.72) (Fig. 6.2.b and

Fig. 4.6.b). From 0 to 9 m and from 50 to 100 m, the correlation was poor. The
above sequence of spatial variations in arsenic concentrations over the depths
was also addressed from the context of thermodynamic principles. The actual

redox stratiﬁcation records from the ﬁeld may explain the situation better.

Variation of redox values over aquifer deLth

In this study, the redox potential values at different aquifer depths were
computed by using the band regression method, and the analysis showed
variations in redox potential values over the aquifer depths (Figs. 6.3.a and
6.3.b). Fig. 6.3.a indicated that the redox potential values decrease with
increasing aquifer depth linearly from 10 to 30 m below the ground. On the
contrary, the redox potential values increase with increasing aquifer depth
ranging from 150 to 350 m (Fig. 6.3.b).

Analysis of the oxygen diffusion model suggests that oxygen
concentration decreases with increasing aquifer depths (Figs. 4.11, 4.12, and
6.4). Comparing all these ﬁgures together (Figs. 6.3.a, 6.3.b, and 6.4), it is
graphically explained that arsenic concentrations increase with decreasing redox
potential values as a result of a low oxygen supply over the depths ranging from

9 to 30 m below the ground.

161

Why the arsenic concentration increased with deith from 9 to 30 m in Fig. 6.23

 

Arsenic concentration increases with decreasing redox potential (pe)

values (Fig. 6.3.a) and increasing aquifer depths when ranging from 9 to 30 m
(Fig. 6.2a). This is due to the fact that the oxygen concentration is decreasing
as a function of depth (Fig. 6.4) and reducing conditions are simultaneously
developing in the deeper layers (Fig. 6.3.a and 6.3.b). This analysis does support
the working hypothesis of this study. However, it was important to ﬁnd out why
the arsenic concentration again decreases from 100 to 350 m below the ground

(Fig. 6.2.b).

6.2.2 Why arsenic concentration again decreases over the depth ranging
from 100 to 350 m (in Fig. 6.2.b)

In case of phreatic or leaky aquifer in coastal or sedimentary areas like
Bangladesh, it is not always expected that the dissolved oxygen content in the
deeper layers would be low, especially at the depth of 100 m below the ground.
The reason is that the water below 100 m in depth may come as the regional
throughﬂow. The regional throughﬂow generally is rich in dissolved oxygen
content; therefore, it may cause the redox potential values to increase. Also, in
deeper layers, the oxygen-consuming minerals are generally less available than

in the upper layers (AECL, 1997). That is why, over the aquifer depth ranging

162

from 150 to 350 m, the water may be oxic and the p6 values at that depth (150 to
350 m) are found to be higher.

It is clearly understood that dissolved oxygen content is an important
factor that controls the arsenic concentration of the groundwater system.
However, arsenic could also be released in groundwater due to the oxidation of
pyrite. The next issue to be addressed is whether lowering the water table can

increase dissolved oxygen content in the aquifer.

163

Regression Plot

Log(As(ppt)) = 0.342776 + 1.79096103(Depth(9 — 30 m)
s = 0.243388 R-sq = 59.8% R-sq(adj) = 57.5%

 

 

 

 

 

 

8.0 9.0 10 15 20 30
Depth (9 — 30 m)

Fig. 6.2.a Arsenic content increases linearly with depth from 9 to 30 m

Regression Plot
Log(As(ppb)) = 76.9101 - 0.249561 Depth(m)
S = 13.9116 R-sq = 72.4% R-sq(adj) = 68.4%
100

 

 

 

 

 

__ Regression
..... 95% CT
95% PT
I I I 1 I I
100 150 200 250 300 350
Depthtm)

Fig. 6.2. b Arsenic content decreases linearly with depth from 100 to 350 m below the
ground

164

Change In flold redox values over dumb (0-20 m)
In grid 1 2 (Lot 23-24‘N . Long OO-Ot'E)

 

. VII-03108810191189

0 -( R'sooass
5 -2 a N p. @pH=
& O

 

 

 

7
-3 . R-eq was °
'4 I U
0 1 0 20 30

Fig.6.3.a. Redox potential values decreases over aquifer depth ranged
from 10 to 30 m belowthe ground.

Depth venue Rue. Potential at 150 to so I

 

1.6-l

i .11
i

 

 

 

0 8; 100 160 2:10 260 310 38,0 “10
Fig.6.3.b. Redox potential values increased over depth ranging from 150

to 350 min the most contaminated aquifer

165

Rate of Change of 6, Cone. Over Time

0.25

 

Surface l

. l
0—2 Depth ass
3 0.15 q Depth d-

0-1 ' Depth d-15

 

 

 

 

0.05 .

Oxygen Concentration

 

.l...___.___

0717117177ﬁrTﬁrTY1j
‘®@§e°.§.(t9.¢9\e°

Time In Hours

 

Fig.6.4. Oxygen concentration decreased with increasing aquifer depths

over time.

6.2.3 What impact the lowering of the water table had on diffusive oxygen

transportation tothedeeperlayersofthe aquifer
This issue was analyzed by using the ﬁnite element model described in
section 4.3.4. and Fig. 4.10. It was assumed in that model that, because of

extraction, the water table was dropped about 4 m (from L2 to L3). The results of

theloweringwatertableandtheassociateddissolvedoxygenconcentraﬁon
values were presented in Tables 4.3.2 and 4.3.3 and graphed in Figs. 4.11 and

4.12. The results showed that a 4-meter lowering of the water table (from L2 to

L5) did not increase the dissolved oxygen concentration at L3 and L10 (Fig. 4.10).

166

The analysis also demonstrated that by changing the parameters’ values
and initial and boundary conditions, the dissolved oxygen concentration at
deeper layers was increased, but these conditions are not practical in ﬁeld
conditions. Therefore, it could be concluded that the lowering of the water table

did not increase oxygen concentration at the deeper layers.

6.3 Whether the oxidation or the reduction process mainly controls
arsenic mobilization in Bangladesh’s groundwater

The third research issue presented in chapter four relates to one crucial
issue: whether oxidation or reduction processes caused arsenic contamination in

the groundwater.

Research issues

 

o What geochemical processes (pyrite oxidation or oxyhydroxide reduction
process) mainly control arsenic contamination in Bangladesh’s groundwater

system?

Strategic questions

 

1. Was pyrite oxidation or reduction of oxyhydroxides responsible for arsenic
release?
2. What exact theory was applicable to describe the arsenic release into the

groundwater of Bangladesh?

167

3. Why could pyrite oxidation not be considered as a principle mechanism of
arsenic release, or what are the inadequacies of this theory?

4. Why is the hypothesis offered in this study a valid explanation of arsenic
release that should not be rejected?

5. What are the interactions between the extraction rate, the lowering the of
water table position, and the rejecting or accepting of the oxidation theory of
arsenic release in groundwater?

6. Can thermodynamic principles be applied to interpret the relations among the
lowering water table, the shortage of dissolved oxygen in recharging water

and arsenic contamination in Bangladesh’s groundwater?

6.3.1 Whether pyrite oxidation or reduction of oxyhydroxides was mainly
responsible for arsenic release in the groundwater

It is still a debatable issue whether oxidation of pyrites or reduction of iron
oxides controls the arsenic release mechanism in Bangladesh’s groundwater.
There have been divergences of views among researchers and considerable
difﬁculties in working on the arsenic problem in Bangladesh. It was not the
purpose of this analysis to focus on this debate, but discussions were had to
evaluate both oxidation and reduction theory in the context of the lowering of the

water table, in order to support the hypothesis offered in this study.

168

Opinions based on the oxidation of arseno-pyrites theory

 

The proponents of the oxidation theory welcome an increase in the supply
of oxygen in the groundwater. There are scientists and chemists who believe
pyrite oxidation was the dominant arsenic release mechanism in Bangladesh.
The cause of increased oxygen supply may be due to the development of heavy
duty tube—wells known to cause extension of the vadose zone and to supply more
oxygen to the groundwater (Mallick et al., 1995). Oxygen-oxidized arseno-pyrite
(FeAsS), and eventually arsenic, was released into the water. Pyrite oxidation
may be accelerated by lowering of the water table due to overpumping
(Kinniburgh et al., 1994) or to nitrate pollution of the aquifer (Postma, 1991).
During the last two decades, the Green Revolution in Bangladesh has lowered
the groundwater level and inadvertently caused the release of arsenic

(Tsushima, 1999).

Opinions based on the reduction theory of arsenic release
The proponents of the oxyhydroxide reduction theory have rejected the
explanations of the oxidation group. According to the reduction hypothesis, the
origin of arsenic in groundwater is a natural process, which implies that has been
present in the groundwater for thousands of years without being ﬂushed from the
delta.
The investigators in this group suggest that arsenic release was due to

the reduction of arseniferous iron oxyhydroxides when anoxic conditions

169

developed during sediment burial (Nickson et al, 1998; Nickson & McArthur et al.,
2000). Arsenic in Bangladesh’s groundwater cannot drive from the presently

accepted mechanism, whereby water-level draw-down from extraction wells
allows atmospheric oxygen (Oz) into the aquifer and so allows the oxidation of

arsenic-bearing pyrites with concomitant release of arsenic to the groundwater
(Das et al., 1995; Roy Chowdhury et al., 1998). Such a mechanism is
incompatible with the redox chemistry of the water. Arsenic produced in this way
would be adsorbed to FeOOH, the product of oxidation (Mok & Way, 1994).
6.3.2 What theory is applicable to describe the arsenic release into the
groundwater of Bangladesh

It appears from the above statements that the two theories are completely
opposite to each other from the chemical point of view. However, in this study, it
was hypothesized that the lack of dissolved oxygen content in recharging
groundwater and the shortage of recharge due to upstream withdrawal of river
water were important factors that controlled the arsenic release process in the
groundwater. The hypothesis presented in this study completely opposes the
pyrite oxidation theory but partially supports the reduction theory. The logic and
supporting evidence to explain why the oxidation theory is inappropriate to
explain the arsenic contamination problem in the groundwater of Bangladesh will

be established. In order to do so, the following questions must be addressed.

170

6.3.2.a What was the critical difference between the reduction theory and
hypothesis offered in this study?

- Like the reduction theory of arsenic release, this study also assumed
that arsenic was released due to the development of reducing conditions wherein
iron hydroxides scavenged arsenic during transportation. This study shows that
this iron precipitation—dissolution reaction (Eq. 6.1) shifts the equilibrium due to a
shortage of oxygenated recharging water in the groundwater, rather than ﬁtting
the usual explanations of microbial activity removing oxygen.

. The reduction group totally ignores surface water’s contribution to the
shallow aquifer recharge (SAR) and its impact on groundwater ﬂow dynamics.
This study completely analyzed the impact of SAR reduction on the groundwater

flow and the likelihood of arsenic migration.

Why the hypothesis offered in this study is partially supportive of the reduction

 

theorLof arsenic release

 

This study also admitted that development of a reducing condition helps to
release arsenic from the surface of iron oxides. But the development of a
reducing condition was not found to be due to microbial activity because ﬁeld
studies do not support the idea that 6% organic carbon can be present at the
deeper layers of the aquifer. And if organic carbon was the major cause of
arsenic release, why was arsenic contamination not found in the earlier days,

and why is there not any prior report of arsenic toxicity in Bangladesh?

171

6.3.3 Why the pyrite oxidation theory was not applicable to explain the
arsenic contamination problem in Bangladesh’s groundwater

The oxidation theory does not provide the geochemical evidence in the
ﬂow path of groundwater needed to support the oxidation theory. For example, if
arseno-pyrite were the major cause of arsenic release, then the SO.;'2 ions would
be dominating the groundwater ﬂow-path. The most important oxidation reaction
of arsenopyrite is as follows”
Fe As s +13 Fe 3* + 8 H20 =>14 Fe 2” + 3042‘ + 13 H+ + H3 As 04 (6.3)
But there is no ﬁeld evidence of increasing numbers of SO4'2 ions along the ﬂow-
path. Rather, 3500 water quality data records (collected by BGS) indicate no
relationship between arsenic and SO4'2.

If the oxidation principle is the major cause of arsenic release into the
groundwater of Bangladesh, the following points must be met in the ﬁeld

conditions.

Point 1: Sulfatgsoﬂ 2’) contamination should be maximal only at the top of the

water table if pyrite oxidation is the real cause

If arseno-pyrite is believed to be the major cause of arsenic contamination
in groundwater of Bangladesh, according to the reaction in Eq. 6.3, arsenic
concentrations always should be maximal at the top of the aquifer. The reason is

that the dissolved oxygen (DO) should always be maximal in the upper layer of

172

the aquifer because of its close proximity to the surface. Arsenic must be found
to be correlated with ions of 8042' and there will be accumulation of 8042' as a
Terminal Electron Accepting (TEA) process along the ﬂow path of groundwater.
But in reality, the ﬁeld evidence does not show that the top most layers were
always arsenic contaminated. Rather, the top most layers were arsenic free (0 to

10 m) in almost 99% of the samples. Field evidence also did not suggest any

relationship between arsenic and ions of 8042'. In addition to that, release of

SO42" ion would also would have to be associated with increased ions of HCOg'1

in the water, but water quality samples records analyses did not show any strong

correlation between arsenic and bicarbonate.

Point 2: There must be a concentration gradient while pumping a deep well if the

 

oxidation theory is valid

 

This study analyzed a conceptual one-dimensional model in section 5.1,
and the results were presented in Fig. 5.2.a. where it was found that aquifer
thickness and conductivity regulated the concentration and leakage ﬂow of the
water entering the well. If oxidation were the major cause of arsenic release,
while pumping from a deep tube well, the arsenic concentration would have to
vary with time because the screens of tube wells are generally placed at least 10
m below the top of the aquifer in the case of shallow tube wells (STW) and 50 to

100 m below the surface in the case of deep tube wells (DTW) (Fig. 5.1.a).

173

According to the pyrite oxidation theory, the top-most layers would always
be more contaminated with arsenic than the deeper layers, where tube well
screens are placed. It should take a long time for contaminated water to reach
the well screen from the arseno-pyrite top layer. Therefore, there must be an
arsenic concentration gradient while pumping water from a well. But no
evidences of arsenic concentration variation were found while pumping wells in

the ﬁeld.

Point 3: Arsenic-contaminated water samples must be oxic and rich in redox

 

potential values if pyrite oxidation is the real source of arsenic

 

If the oxidation theory is correct, the groundwater must be rich in dissolved
oxygen. Most of the water samples analyzed by Nickson et al. (2000) found the
arsenic-contaminated water to be anoxic or anaerobic. in addition to that,
according to oxidation theory, eventually in the course of time there should not be
any arsenic contamination problem in the groundwater because ultimately, the

arseno-pyrite attached to the sediment will be ﬂushed out by dissolved oxygen.

Point 4: There must be a positive correlation between the cumulative arsenic

concentration and the cumulative water table decline if oxidation is the actual

cause Of arsenic release.

174

Point 5: There should be positive correlation between the maximum fall of the
water table and the arsenic concentration if oxidation is the actual cause of

arsenic release.

Point 6: There should be positive correlation between the most arsenic-
contaminated area and the area receiving the greatest amount of recharge if

oxidation is the cause of arsenic release.

Point 7: If the arseno-pyrite oxidation theory is the cause of arsenic release, the
dug well waters should have been those most contaminated by arsenic

On the contrary, a study conducted by the researchers of SOES (2000) on
dug wells in Bangladesh observed that out of 500 dug well samples, none of
them was found with more than 50 ppb arsenic, and 86% of the water samples
have arsenic levels below 10 ppb. That is why the oxidation theory is inadequate

to explain the groundwater arsenic contamination problem in Bangladesh.

6.3.4 Why the hypothesis offered in this study is a valid explanation of
arsenic release which should not be rejected

In order to test the hypothesis that less oxygenated recharging water at
and below the water table, after extension of the vadose zone due to extraction,
is the cause of arsenic release, in section 4.3.2 a conceptual oxygen diffusion

model was analyzed using ﬁnite element techniques. The results indicate that the

175

oxygen concentration decreases with increasing aquifer depth and a 4 m
lowering of the water table did not increase the diffusive oxygen supply to the
deeper layers of the aquifer (Table 4.3.2 and Fig. 4.11). These results clearly
demonstrate that the dissolved oxygen concentration decreases with increasing
depth, supporting to the hypothesis in this study and going against oxidation
theory.

A thermodynamic model was also studied in sections 4.4.5 and 6.3.6, and
the results indicated that arsenic contamination release from iron oxides was
caused by a shortage of dissolved oxygen. As a result, the equilibrium of the iron
precipitation reaction (Eq. 6.1) had shifted to dissolution from precipitation (Fig.
6.6). That is why arsenic- contaminated groundwater is rich in dissolved iron.
This analysis was completely supportive to the hypothesis offered in this study.

Unlike the other two hypotheses (oxidation and reduction), this study does
not ignore the contribution of surface water recharge (SAR) to the issue of
arsenic release. Three-dimensional conceptual models were analyzed in section
5.3.1 to understand the contribution of surface water recharge to groundwater
movements, ﬂow patterns, and the likelihood of arsenic migration. The model
simulation results demonstrated that extraction (Q) with a rate greater than that
of aquifer recharge (SAR) signiﬁcantly increases downward ﬂow through deeper
layers and, thus, the risk of arsenic migration. The simulation results are

supportive to the hypothesis.

176

Dug well samples are relatively arsenic-free, supporting the hypothesis present

 

in this study

 

According to the hypothesis of this study, it is believed that groundwater in

touch with atmospheric oxygen is always oxic and rich in redox potential values
(p8 or Eh). As a result of oxic conditions, the dissolved arsenic contaminants in

the dug well waters were adsorbed onto the solid surfaces of iron oxides. That
led to groundwater having arsenic content less than 10 ppb, because dug wells
are placed in close proximity to the surface and can maintain equilibrium with the
atmospheric oxygen level (0.21 atm).

Moreover, the analysis of aquifer depth versus arsenic concentration (in
Figs. 4.6.a and 4.6.b), aquifer depths versus oxygen concentration (Fig. 6.4), and
depth versus redox potential (Figs. 6.3.a and 6.3.b) clearly demonstrates that
arsenic release is due to a shortage of dissolved oxygen over the aquifer depths,
which decreases the redox potential values with increasing depth. Also, a

thermodynamic model was studied in order to validate the hypothesis.

Geochemical explanation of the high arsenic contamination in Bangladesh’s

 

gLoundwater based on the hypothesis present in this study

 

The sediments in the SW and SE regions contained levels of higher
organic matter (Khan, 1991) than NW and NC regions; therefore, iron-reducing
microbial activity enhanced the oxygen consumption rate in the sediment of the

Ganges and Meghna river delta ﬂood plains (Nickson, 2000). On contrary, the

177

sediment in the NW zone was coarser than that in the SW and SE zones and
there was most likely less microbial activity, which is why the NW region is
relatively arsenic-free.

The above explanations of the higher arsenic levels in the SE and SW
regions were based on the hypothesis offered by the reduction groups, and
particularly by Nickson et al. (1999), who believe that organic carbon takes
oxygen from the iron hydroxides and eventually causes arsenic to be released
into the groundwater. The oxidation group did not accept this geochemical
explanation of arsenic release. Adel (2000) put a very logical question to the
authors (Nickson et al., 1999): Why was arsenic contamination detected only in
1990 in Bangladesh if organic carbon has been taking oxygen for thousands of
years? To support the reduction hypothesis, he also mentioned that Nickson et al
(1999) showed 6% organic carbon in the deeper aquifer layers, which goes
beyond the boundary of scientiﬁc evidence. So far, no reasonable answer has
been given to his question.

This study presents the hypothesis that development of reducing
conditions or low redox potential values (pe) was responsible for the release of
arsenic from the iron hydroxides. However, the development of a reducing
condition was caused by the shortage of dissolved oxygen in recharging water,
not by the microbial consumption of oxygen. The iron precipitation—dissolution

equation (Eq. 6.1) uses thermodynamic values to ﬁnd that so long as any

detectable oxygen (0.41 ppb) is present in the system, the redox potential (pe)

178

values do not fall below 12.31 (Table 4.4.2). This indicates that the shortage of
dissolved oxygen in the recharging groundwater was mainly responsible for

arsenic release under reducing conditions.

6.3.5. interactions between shortage of dissolved oxygen, extraction rate,
lowering of the water table position, and rejection or acceptance of the
oxidation theory of arsenic release in groundwater

A conceptual model of oxygen diffusion was analyzed in Fig. 4.10 in order

to validate the theory of arsenic release by oxidation. The predicted oxygen

concentrations at hypothetical arseno-pyrite layers L5 and L10 were tabulated in

Table 4.3.2.

The assumptions behind the diffusion model

 

The water table in the model aquifer was 2 m below the ground surface
(WT-1) before the introduction of wells in Bangladesh (Fig. 4.10). Dissolved
oxygen in groundwater was also in equilibrium with the atmospheric oxygen
concentration (0.21 atm). This was considered to be Case 1. After the
introduction of well-ﬁelds into the aquifer, the water table traveled 5 m down from

WT-1 to WT-2. The water table (WT-1) lowered from L2 (2 m below ground) to L5,

which was 5 m below the ground (Fig. 4.3.1). This is Case 2.

179

Findings from the oxygen diffusion model

Oxygen concentration decreased with increasing water table lowering depth
It was shown in Fig 6.5.a and Fig.4.11 (derived from Table 4.3.2) that after

the lowering of the water table from its original position (WT -1), the predicted
oxygen concentrations at pyrite layers (L6 and L10 in Fig. 4.10) were less than
before the water table’s lowering. Lowering of the water table did not help
increase the oxygen supply rate to the pyretic. layers (L6 and L10); rather, the

oxygen supply rates decreased over aquifer depth and time (Fig. 6.5.a). The
oxygen concentrations in the deeper layers were found to be lower than

predicted (Table 4.3.2).

Why the oxygen supply rate decreased in deegr layers
The literature review also suggested that the oxygen diffusion rate (ODR)

decreases with increasing depth and with the moisture content of the layer. it

should be mentioned here that a minimum 80% oxygen concentration was
required to maintain an adequate ODR level (20x10‘8 gmlcrnzlmin) (Mukter et al.,
1996). in order to predict the oxygen concentration trend, default values of the ,

oxygen diffusion coefﬁcient for unsaturated soil (Dx=259.2 cmzlhr or 4.1sex10“
mzlsec) were used. The oxygen consumption rate (a) in the governing equation
(4.3.4) was 0002125 cm3/cm3/hr, which is the 1“t order kinetics of oxygen

consumption. In fact, the value or depends on the soil’s organic matter. Using this

180

default value, it was found from the model that after lowering of the water table,
the predicted oxygen concentrations of deeper layers were less than the
concentrations of the top, over-lain layers. in another set of experiments on
depth, Mukhter et al. (1996) found that ODR varied inversely with depth (Fig.
2.2).

Chapgipg initi_a1 contﬂtjons gig not increase t_l1_e oxygen concentration in m

layers
it was evident in Fig. 6.5.b that the oxygen concentration of deeper layer

L10 could not be made higher than that of L5 even when the follovw'ng two
conditions were satisﬁed: (1) The value of the diffusion coefﬁcient of oxygen in
unsaturated soil (0x1) was taken to be equal to the diffusive coefﬁcient of oxygen
in water (sz), and (2) the initial oxygen concentration at L10 was always

maintained in equilibrium pressure (0.21 atm) (Fig. 6.5.b, curve a and b). in
reality, these two conditions can never be maintained because the oxygen

diffusion rate through the vadose zone, Dx1, is always greater than the oxygen

diffusion rate through water, 0X2 (Table 4.3.1).

181

Region of oxidation theogy

it can be summarized from the above discussions that lowering of the
water table cannot or does not provide a greater atmospheric oxygen supply to
the deeper layers of the aquifer. The oxygen diffusion rate (ODR) decreases with

increasing aquifer depth. if the oxygen concentration falls below the critical

concentration level, then the ODR also decreases in deeper layers.

Finally, it can be concluded from this study that the oxidation theory of
arsenic release is not adequate to explain the arsenic release mechanism in
Bangladesh‘s groundwater. it does not explain why the arsenic concentration is

greater in the deeper layers than at the top, while the oxygen concentration does

not increase with depth.

Diffusive Supply of Oxygen at Pyritic Layer-6
before a. after Lowering of Water Table

 

Oxygen Conc at
Pyrite Layer L. ,Case-i ,before
lowering WT-1

   

 

    

l L-10, case-1

Pyrite Layer Oxygen Cone. y/‘x,
i L5, Case-2, after lowering of T-2o

l
ﬂ
ul

1
O
.a

O
0

0
d

Oxygen Cone. (Atm)

 

0.001

 

f T l' 1 Y I ‘l ‘I' 1 V U I ‘l U T V 1 ‘l T I i

°t909x°¢°39§9°4°5~°$

Time in Hour

F ig.6.5.a Predicted change in the diffusive oxygen supply at arsenic

contaminated pyretic layers

182

1

 

teeeeeeyyeeddwddﬁdﬁﬁfﬁﬁﬁd

Oxygen Concentration(atm)

 

 

Fig.6.5.b Predicted change in dissolved oxygen concentration when the boundary

conditions are changed

183

6.3.6 Can thennodynamlc principles be applied to interpret the relations
among the lowering water table, the shortage of dissolved oxygen in
recharging water and arsenic contamination in the groundwater of
Bangladesh

it was assumed in the hypothesis of this study that before introduction of
well ﬁelds, the water table was in close proximity to the ground surface and
dissolved oxygen (DO) content in groundwater was in equilibrium with
atmospheric oxygen (0.21 atm). In this section, it will be discussed how

thermodynamic principles support the working hypothesis presented in this study.

Lavalirlgdvmrmlejlﬂmofredmteqilbnunmd
stabiitymgoolalsoricmdlron medallist-swarms")

 

Hypothetical
20 list-1‘1) Equ‘Ibrium a mic oondton
\ ‘ “i , . WienwaterlmlawasWth

154

10<

5a

04

-5.

 

 

Redox poterxlel vanes p'

 

 

 

-10. “Wigwam 7* \ '
at maltose . \ ~ ‘ so};

- , _ s L

45 , Fo" olssowed \ \ \-

rmredoxeqiilibriun Line \\ (12)
'20 ‘ (101m mmw. (1°) \
, (11) i mglland(12) 0.1mg.l (11)
‘5 I I t I I 1‘ 1
0 2 4 0 pH 8 ‘ to 12 14 16

Fig. 6.6 Lowering of the water table and shifting of the equilibrium conditions

184

The pe—pH diagram basically determines the stability range of the oxidized
and reduced species with respect to their equilibrium lines under natural water
pH levels. in Fig. 6.6, stability diagrams or pe-pH diagrams for arsenite [As (lll)]
and arsenate [As (V)] redox couples (Line 3, Fig. 6.6) were drawn along with Fe
(ii) and Fe (III) redox equilibrium lines (Line 10 and Line 11, Fig. 6.6).

initially, a hypothetical situation was considered before the installation of
wells when the water table level was close to the ground surface and the
dissolved oxygen content was in equilibrium (0.21 atm) with atmospheric oxygen.
Equation 6.1 was used to determine the hypothetical equilibrium situation,

marked in Fig. 6.6 at the top left with a green circle.

The p8 or redox values of the hypothetical system were computed using
field maximum and minimum concentration of dissolved Fe2+ content in arsenic-
contaminated water. Different DO content levels were assumed and those redox
(pe) values were plotted in the pe -pH diagram in Fig. 6.6. All the p'3 values under

the hypothetical equilibrium condition appeared above Line 3 (the arsenic

equilibrium line) and Line 10 (the iron equilibrium line) under the green circle. The
computed ﬁeld redox values (pe) marked by the green circle on the top left (Fig.
6.6) ranged from 12.31 to 13.65, which implied that all pe values were oxygen-

controlied redox values. This was termed the hypothetical equilibrium condition of
dissolved oxygen with atmospheric oxygen when the water table was close to the

surface.

185

As a result of overextraction, the water table lowered and the oxygen
supply to the deeper layers decreased. The maximum and minimum pe-values
were computed from the dissolved arsenic concentration in the ﬁelds and plotted

in Fig. 6.6. The associated pe values of dissolved Fe2+ in groundwater were also

computed and plotted in Fig. 6.6. All the p8 values appeared below equilibrium

line 3 and were marked with a red circle (in the middle of Fig. 6.6).
This shows that the redox potential values have decreased and the
equilibrium condition has shifted from the precipitated solid iron phase (green

circle) to the dissolved iron phase (red circle). The hypothetical equilibrium has
shifted from an oxic to a suboxic environment In which p‘3 values were reduced

from 1.5 to -3.5, which is a mildly reducing condition. The redox values are now
controlled by dissolved iron, not by dissolved oxygen. The lowering of the water
table decreased the oxygen supply rate to the deeper layers and developed the
mildly reducing condition that was strongly favorable for arsenic to be released

into the groundwater.
Figure 6.7 was drawn for the equilibrium redox couple 804'2 and H28 in
order to demonstrate whether arsenic was released due to the oxidation of pyrite

(FeAsS) to 804‘2 or the reduction of Fe3+ to Fe2+. lf pyrite were the cause of
arsenic release, then Fe AsS H28 would be oxidized to SO42. The water would
have been rich in dissolved 804'2 ion, and the p8 values would have fallen below

the equilibrium line L-12 in Fig. 6.7. However, none of the ﬁeld pe values went

186

arsenic release, then Fe Ass H28 would be oxidized to SO4'2. The water would
have been rich in dissolved 804'2 ion, and the p° values would have fallen below

the equilibrium line L-12 in Fig. 6.7. However, none of the ﬁeld p° values went

below line-12. This proves thermodynamically that pyrite oxidation was not the
process of arsenic release in Bangladesh’s groundwater system because the

redox potential values were not within the range of sulfate reduction.

 

-z
nnp.-ptlanpr-nwiltt s04 Jasmdortoouplo

 

Eh(mv)

 

 

 

 

 

 

 

Fig. 6.7 Line 12 shows the equilibrium line of the so;2 and H28 redox

couple; all of the p° values fall below line 12

187

Summagy:

it can be summarized from the above discussions that the lowering of the
water table and the water’s dissolved oxygen (DO) content played important
roles in releasing arsenic into Bangladesh’s groundwater. A smaller oxygen
supply was caused by the increased depth to the water table Therefore, it
appears from the thermodynamic model that the DO content was an important
factor controlling arsenic release and arsenic speciation in Bangladesh’s

groundwater system under the original, natural conditions.

188

6.4 inﬂuence of aquifer thickness on the arsenic concentration of water
entering a pumping well

The fourth research issue of this study explained the inﬂuence of aquifer
physical properties on the arsenic concentration of a well being pumped. It was
important to understand the aquifer’s inﬂuence on arsenic concentration to

determine the validity of the oxidation theory of arsenic release in groundwater.

Research issue

 

o What is the inﬂuence of aquifer physical properties (thickness of the aquitard,

conductivity) on the arsenic concentration in an irrigation well during pumping?

Strategic questions and discussions

1. What is the inﬂuence of aquitard thickness (b’) on the amount of water (Qt)
entering the well screen and on the arsenic concentration in the water being
pumped (Ct)?

2. What is the inﬂuence of aquitard vertical conductivity (K’) on the amount of
water entering the well screen and the arsenic concentration in the water being
pumped?

3. How can aquifer physical properties (aquitard thickness and conductivity) and

their inﬂuence be used to describe the validity of theories of arsenic release?

189

It is not illogical to argue that if arseno-pyrite oxidation were the source of
arsenic release in Bangladesh’s groundwater, the upper layer would have to be
contaminated with arsenic because it was constantly in touch with atmospheric

oxygen. Therefore, in the case of a leaky aquifer, the concentration of water

entering a pumping well from an overlaid, arsenic-contaminated layer (L1) (Fig.
5.1.b) must not be the same at any time as the pumped-water concentration (Ct)

due to the mixing of the stratiﬁed arsenic concentration with pumped water (Q) of

different concentrations inside the well casing.

In a leaky or semi-conﬁned aquifer system, a deep tube-well initially

extracts water from the screened aquifer’s layer (L3) where the hypothetical
arsenic concentration (03) may be less than the layer one (L1) concentration
(C1) or may even be zero (Fig. 5.1.b). When pumping starts, the concentration of

water entering the well, 0,, is to be equal to C'3, reﬂecting the leaky aquifer’s
initial concentration. As pumping continues, the overlaid contaminated aquifer

(L1) supplies more water with a concentration of O1 through vertical leakage (01)
across the aquitard depth (b'). Eventually, C’t progresses from C'3 to O1,
reﬂecting the increasing contribution of leakage (Oi) to the pumped water (Qt).
As the radius of inﬂuence R, increases over time, the ratio of partial discharge

Q3/Qt decreases and 01/03 increases.

190

From Equation 5.1 in chapter 5, it appears that when all of the pumped
water 0, comes only from the leaky aquifer layer L3, then Q = Q3. This implies
that the arsenic concentration of the pumped water is equal to the concentration
in the leaky conﬁned aquifer (L3) water and C'( = C3. When no water comes from
the leaky conﬁned aquifer L3. it implies that all water in Q was contributed by
leakage factor Q1 from the top layer L1 (Fig. 5.1.b). When Q3= 0, the
concentration C’t in pumped-discharged water approaches 01. This means that
immediately after starting to pump, the Q comes from a mixture of C1 and C3, but
in the case of a leaky aquifer (like in Bangladesh), it takes time for Q to reach the

equilibrium concentration with C1. This means that there should be a

concentration gradient during pumping.

6.4.1 What is the inﬂuence of aquitard thickness (b’) on the amount of water
entering the well screen and concentration in the pumping water?
A numerical analysis was done to illustrate the impact of aquitard

thickness (b’) and vertical conductivity (K') on the arsenic concentration or water

entering a well being pumped (C’t). The initial hypothetical arsenic concentration

at the top layer L1 and screened layer L3 were assumed to be 100 and 50 ppb,

respectively (Fig. 5.1.b). For the simplicity of the problem, convective ﬂow of
arsenic was assumed, as the groundwater velocity was higher near the vicinity of

well screen. it was assumed that the arsenic concentration of the pumped water

191

and the arsenic concentration of the water entering a well were equal.

Arsenic concentration of water enterimLa well (0;) varied inversely with the

 

thickness of the aqpitard (b')

 

The results of this analysis were graphed in Fig. 6.8.a. It appears from that
ﬁgure that the arsenic concentration of water entering into a well (C’t) varied
inversely with the thickness of the aquitard (b’). For example, while comparing
graph 'b' and ’d' in Fig. 6.8, it was found that aquitard type 'b' (depth b'=65 ft.)
resulted in a higher arsenic concentration (80 ppm) in pumped water (C't) than
the aquitard type ’d’ (depth b'=150 ft) where C'( was predicted to be 65 ppb. Both

K'=1.1 ft/day and R(t)=1500 it were kept constant for aquitard types b, c and d.

The shallower the aquitard depth, the greater the rise in the arsenic
concentration entering well water (C't) (Fig. 6.8.a). This is because in the case of
a thick aquifer, more energy is required to let the leakage (Q1) pass through the
deeper layer, and as a result partial discharge Q1 was less and eventually the
ratio of 03/0, was higher to satisfy the equation of continuity. it was shown in
Equation 5.1 that if the ratio of 03th were as high as unity, C't would be equal to
C3. For a shallower depth of aquitard type ' b', the ratio of leQt was less and C,
would be close to 01. That is how thinner aquitards cause higher arsenic

concentrations in well water. Also, the concentration of water entering a well

varies directly with the aquitard conductivity (Fig. 6.8).

192

A one-dimensional model analysis (Fig. 6.8.a) clearly demonstrates that

the arsenic concentration increases directly with the conductivity. This analysis

was also consistent with the results of three-dimensional models.

6.4.2. What is the inﬂuence of aquitard conductivity on the amount of

water entering the well screen

While comparing graph "a" and "d" in Fig. 6.8, it was observed that the

arsenic concentration in well water (C1) varied directly with the vertical hydraulic

conductivity of the aquitard.

 

Anenlcconcentratlonlnthepumpaapm)

 

120 “r“-

1

 

  

imamdmwmmmum
mmmdmm

.... ..--mu ...-m...— -..a... o. ”- n ion-”...... .

 
 

and-1501. ic- tune-y
bib-ssh, K‘- 1.1enldoy :
c)b‘-1mit, lo- 1.1snlday l
d) u-isort, ic- 1.18m I

 

500 1” 15“ m m m m
‘ mothluwtcetfnmtlme

 

 

Fig. 6.8.a inﬂuence of aquitard thickness and permeability on arsenic

concentration in a well being pumped

193

Aquitard types "a" and "d" were of the same thickness but the conductivity
‘K' of aquitard type "a" was ten times higher than that of aquitard type "d " (11.8
ft/d vs. 1.18 ft/d). As a result, the arsenic concentration in 'a' was more than that
in 'd’. The results of this analysis explain why arsenic concentrations vary from
layer to layer and aquifer to aquifer as a function of hydraulic conductivity and

aquitard depths.

6.4.3 How aquifer physical properties (aquitard thickness and conductivity)
can be used to describe the validity of oxidation theory of arsenic release.
Fig. 6.8.a shows that it will take time for the concentration of the upper
aquifer to be equal to the concentration of the water entering a well, depending
on the hydrologic parameters like aquitard thickness b’ and conductivity K. As the
radius of inﬂuence changes over time, it takes time to reach the equilibrium
pumping concentration. if the pyrite oxidation theory were the real cause of
arsenic release, the concentration of arsenic would always have been higher in
the upper aquifer L1 (Fig. 5.1.b). As it will take time for water from the upper
aquifer to reach the well screen, there must be a concentration gradient observed
while pumping for a long time in order for this theory to be an accurate
description of Bangladesh’s situation. The data do not show a concentration
gradient, and so the reduction theory of arsenic release is more acceptable than

the oxidation theory of arsenic release.

194

6. 5 Inﬂuence of extraction at a rate greater than that of aquifer recharge
(SAR) on groundwater movements, ﬂow patterns, and the likelihood of

arsenic release

The ﬁfth research issue of this study was to examine the relationship
among aquifer recharge (SAR), groundwater movement, ﬂow patterns, the
extraction rate, and the likelihood of arsenic migration. Certainly the extraction
rate (Q) and the amount of SAR available would have an impact on groundwater
movement, ﬂow patterns and the likelihood of arsenic migrating into the deeper

layers of the aquifer.

Research issue

 

- The impact of groundwater withdrawal (Q) at a rate greater than that of
shallow aquifer recharge (SAR) on the dynamics of groundwater movement, ﬂow
patterns and the likelihood of arsenic migration from the currently contaminated

layers to the uncontaminated deeper layers of the aquifer.

Strategic questions:

 

1. What is the interaction between the extraction or pumping rate (Q) and the

availability of shallow aquifer recharge (SAR) in a groundwater system?

2. What is the inﬂuence of cropping patterns on the amount of shallow aquifer

recharge (SAR) available?

195

3. What impact do irrigation system capacities have on the potential recharge or

surplus volume (SUR)?

4. What is the inﬂuence of aquifer recharge (SAR) on groundwater movement,

ﬂow patterns and the likelihood of arsenic migration?

Concept of shallow aguifer recharge (SAR)

For an aquifer with a virgin or un-pumped condition, the water balance or
mass balance concept can be deﬁned as R0 = Do, where the virgin recharge rate
(R0) is equal to the virgin discharge rate (Do). But when wells (Q) are introduced

to the system (as was done in Bangladesh), the above water balance concept is

no longer true. Therefore,
Ro-Do-Q¢0 (6.5.1)

The virgin discharge rate (Do) remains unchanged until the pumping (0) causes

a signiﬁcant change in the hydraulic gradient (h) at the boundary or periphery of
the aquifer. When Q causes a signiﬁcant change in the hydraulic gradient (h),

then a new condition arises and the mass balance equation (6.5.1) becomes

(R0 + ARO) - (Do + ADO) - Q + (dvldt) = 0 (6.5.2)

196

where ARO, ADD, and (dvldt) are the amount of change in recharge, discharge,
and aquifer storage volume, respectively. As the initial recharge, R0, is equal to

Do, Eq. 6.5.2 under an invaded condition can. be expressed as
dvldt = Q - ARC, + ADO or Q = ARC - ADO + dvldt (6.5.3)

Here, the discharge prior to the pumping condition (ADO) is termed “Capture."

The mathematical deﬁnition of capture (ADC) is

L
1),, - j Kha—hdl. = AD

as
where K is hydraulic conductivity, h is the height of the water table, (6h! 63) is the

gradient, L is the length of the section, and D0 is the original discharge rate.

6.5.1 The interaction between the extraction or pumping rate (Q) and the

availability of shallow aquifer recharge (SAR)

in the extraction situation, the pumping rate (Q) in Eq. 6.5.3 has to be
balanced by three components: the aquifer recharge rate under the new pumping
condition (ARo), the capture or change in new discharge (ADO), and the storage
component (dvldt). in order to maintain a steady-state water table and maximize

the pumping, limit, the storage change (dvldt) and amount of capture (ADO) must

be. zero.

197

6.5.1.a Maximum pumping limit, aquifer recharge rate (SAR) and
lowering of the water table
When "capture” (ADQ) is equal to zero in Eq 6.5.3, the maximum value of

pumping limit Qmax is found to be equal to

in Eq. 6.5.4, Qmax has two components: change in storage volume (dvldt) and
change in recharge under the invaded condition (ARO). So long as the volume of
irrigation withdrawal (Qmax) remains equal to the volume of recharge in this
condition of extraction (ARC), there will be no vvithdrawai from the aquifer storage
volume (dvldt) and the water table will remain steady state over time. Whenever
Qmax exceeds the new aquifer recharge rate (ARC), that excess amount must
come either from (dvldt) or (ARC). For a sustainable pumping condition,
withdrawal from storage (dvldt) mustbe zero. if the new aquifer recharge rate
(ARO) is not able to meet the excess “Q,“ there will be a drop in the aquifer’s

water table head and, over time, the water table. Therefore, pumping (Q) at a
rate greater than that of aquifer recharge (SAR) will lower the water table unless

the excess water comes from induced recharge sources.

198

6.5.1.b. The lower limit of aquifer recharge (SAR), potential recharge and

amount of surplus (SUR)

in section 4.5.2, the amount of shallow aquifer recharge (SAR) was found to be

(SAR) = [Surplus (SUR) — Surface Runoff] (6.5.5)

From Fig. 4.15, it was evident that the amount of surplus (SUR) was contributing

to surface runoff and shallow aquifer recharge (SAR), and

Surface Runoff = Outﬂow - Pumpage (Q)

Substituting this value of surface runoff into Eq. 6.5.5 yields

SAR = SUR - Outﬂow + Pumpage (Q) (6.5.6)

Without extraction (Q = 0), Equation 6.5.6 gives the lower limit of shallow aquifer

recharge (SAR). Then, SAR is only equal to
SAR = SUR - Outﬂow or Surplus (SUR) = (SAR) + Outﬂow (6.5.7)

Potential recharge and its impact on the water table

From the irrigation development point of view, potential recharge is an important

term deﬁned as

Potential Recharge = Actual Recharge + Rejected Recharge

199

Potential recharge is conceptually equal to SUR and can influence the water
table position.

6.5.1.c. Seasonal ﬂuctuation of the water table and computation of shallow
aquifer recharge (SAR)

The amount of SAR was computed to be 471 to 498 mm per year. The
computation is presented in Appendix .B. The average of ﬁve years of water
table ﬂuctuation records in the study area (SW zone at Kustia) is shown in Fig.

6.8.b.

 

WT-Fluctuatlon mge at SW- zone ( Kustia, 19118-2001)

WeekinaYear

 

Water Level(m)

 

 

 

 

 

Fig. 6.8.b Evaluation of shallow aquifer recharge (SAR) under ﬁeld conditions in

the SW zone study area of Bangladesh

Validity of the shallow aquifer recharge (SAR) in the ﬁeld condition:

The maximum range of seasonal lowering of groundwater in the study area was
found to be 5.25 m (Fig. 6.8.b) and the computed SAR was between 478 and
498 mm per year. The actual groundwater storage coefﬁcient in that region was 9
to 11%.The computed SAR agrees with real ﬂuctuation levels in the ﬁeld (5.25 *
1000 mm * 0.09 = 472 mm, which agrees with the lower limit of the computed

SAR).

6.5.2 inﬂuence of cropping patterns on the amount of shallow aquifer

recharge (SAR) available

The amount of shallow aquifer recharge (SAR) was evaluated using Eq.
6.5.6 for the study area‘s irrigation system with a high-yielding variety of rice
(HW-rice) as the crop. The average seasonal ﬂuctuation of water table records
was computed from ﬁeld data and the actual amount of SAR matches the

computed SAR values for the study area.

6.5.2.a Estimation of the amount of surplus volume (SUR) under rain-fed
and irrigated HYV-rice growing conditions in the study area

It is well known that HYV-rice consumes more water than the normal, rain-
fed, local rice variety. This study suggests that growing HYV-rice decreased the

amount of surplus volume (SUR) and eventually the aquifer recharge (SAR)

201

decreased. The surplus volume (SUR) under both rain-fed rice cropping and
irrigated HW-rice cropping systems in the study area (SW zone of Bangladesh)

was evaluated.

 

Soil moisture, SUR, rainfall, and storage in the study area
non-irrigated * ----

 

350
300 -
250 -

F' 200— _ _
UX Sorl Morsture

("”150-
m)
100- calm!
50- _ » E
0_ . .--——

1 3 15 22 29 36 43 50 57 64 71 78 85 92 99105113120
Timein3Days

  

 

 

 

 

 

 

Fig. 6.9 Amount of SUR, rainfall and soil moisture balance under a rain-fed
system in the SW zone of Bangladesh. The solid green line is rainfall and the

blue line indicates the volume of SUR (716.26mm/yr).

HW-rice and the amount of suEplus SUR

The amount of surplus volume (SUR) was computed using the .soil
moisture balance. The results were presented in Appendix 4.6.4. The results

indicate that growing HW-rice decreases the amount of SUR in a system. The

202

amount of SUR computed with rain-fed local rice was equal to 716 mrnlyr. (Fig.
6.9, Appendix. D), and the amount of SUR available under an HW-rice irrigation
system was 201 mm/yr. (Fig. 6.10, Appendix .C). The amount of SUR available
under the HYV-rice irrigation system was about 72% less than that available
under the rain-fed system.

The above ﬁndings show that growing crops demanding large amounts of
water would reduce the amount of surplus volume (SUR). Eventually, the amount
of aquifer recharge (SAR) available would be reduced in absence of any induced
recharge source, and would lead to lowering of the water table. if there are no
induced recharge sources (like ﬂoodwater or a river), the pumping rate (Q) must

not exceed the amount of capture or ARO in Eq. 6.5.4.

 

Amount of SUR , and storage opportunity under

300 - irrigated system in SW zone of Bangaldesh
250 *1, . , ‘- — .. ..a-3
" I“ 'a i ,A, ‘\ z 5 '\
' ‘ . j. :1. ; K Soil Moisture Status 5‘. ;‘.

FIUZOOH .\‘.‘ : ‘3 5.“.- : ‘3 .21: E - I .

 

m 1504
m)

1001

50 4," ...:

 

 

1713192531374349556167737985919710101112
3 9 51

 

 

Time in 3- days in a cropping year
Fig. 6.10 Amount of SUR and soil moisture under an HYV-rice irrigation system.

The blue line was the amount of SUR under the HYV-n'ce irrigated system.

203

6.5.3. Impact of irrigation system capacities on the potential recharge or

surplus volume (SUR)

Irrigated rice, in particular, is a heavy consumer of water: Rice consumes

7650 m3/ha and wheat consumes only 4,000 m3/ha (David, 1995). As a general

rule, a growing rice crop needs 10 millimeters of water a day. This is equivalent
to 100 tons of water per hectare. To provide 1 hectare with supplementary water,
a capacity of 166 liters per minute (44 US gallons per minute) is needed if
operated 10 hours per day, or 70 liters per minute (18.6 gallons per minute) is
needed if operated 24 hours per day. This means that the irrigation project
system design capacity would be around 1.16 Usec/ha for rice and 0.38

L/sec/ha.

6.5.3.a Design capacity of irrigation systems and its inﬂuence on surplus
volume (SUR)

Like cropping patterns, the irrigationsystem capacities also inﬂuence the
amount of surplus volume (SUR) or potential recharge because it affects the
water level in excess of climatic and soil moisture demand. in this study, the
amount of SUR was computed with 70%, 80% and 90% levels of system
capacity. As shown in Fig. 6.11, the lowest system capacity level (70%) produced
the highest amount of SUR available .It was very likely that if the supply were

reduced, the balance would be reﬂected in the volume of SUR.

204

 

Amount. ofSUR at different. ayaem capacity levels in the My use at. SW
zone ofB angledesh

    

35 -
30 —
HighestSUR Q
25 70 system
.- ' capacty
E
é
:20 -«
=
,w
515 —
O
E
‘10 —
5 ..
U

 

 

xe<kmh¢pxbg§bb¢ﬁs§$§>°~0x

 

 

 

Fig. 6.11 Amount of SUR computed under different system capacity levels

What is the effect of system capacity reduction on the soil moisture balance and
crop yield reduction?

Soil moisture status mainly depends upon the amount and frequency of
rainfall in a n irrigation system. von Bernuth et al. (1984) investigated the
relationship between soil moisture depletion and irrigation system capacity. Their
results suggested that a system capacity existed above which no beneﬁt was
gained. This was known as the maximum useful system capacity. The maximum
useful system capacity can be smaller or variable depending on the location and

as a function of the probability of soil moisture depletion and yield reduction (von

205

Bernuth et al.,1984). As the irrigation system capacity is a management decision

variable, the amount of SAR can be regulated through system control policy.

This demonstrates that adjusting the irrigation system capacity and

cropping patterns could increase the volume of SUR or SAR. For example,

growing two HYV-rice crops in a particular year reduces the available SUR to 48

mm/yr. from 211 mm/yr. when two years return period rainfall was considered

(Table 6.5.1). By adjusting to crops with low water requirements, the amount of

SUR can be increased.

Table 6.5. 1 impact of cropping patterns on the amount of surplus volume

 

 

 

Cropping 3 Days Max (33 3 Days Max (2 3 Days Max (5 Irrigation
Pattern years) years return years return Wlthdrawal
period) , period) Requirement
(mm)
Rain SUR Rain SUR Rain SUR 33 2
(mm) (mm) (mm) (mm) (mm) (mm) years years
Rain-fed
local rice 1566 716 693 21 1 2481 1 541 0 0
HW
Rice-HYV
Rice 1 566 201 693 48 2481 944 962 1632

 

206

lnﬂu_ence40f induced recharge on water table elevation in the study area

 

An analysis of water table records of the study area shows that the water
table level’s ﬂuctuation remained consistent (within 5 in) (F lg 6.8.b) over the
period from 1994 to 1999, despite a signiﬁcant rise in the number of shallow
wells to 0.62 million. from 0.093 million in 1994 (Table 3.5). The number of
shallow wells had increased by 58% but there was no evidence of signiﬁcant
decline in the water table position (Fig. 6.8). It is suggested that the study area
must have been recharged by induced source such as seasonal ﬂoodwater.

It was shown (Eq. 6.5.4) that if the pumping rate (Q) increases without
any change in storage volume (dvldt), the excess water has to come from. the
new recharge source in order to keep the water table elevation constant. The
amount of induced recharge helps maintain a consistent water table throughout
the period. Now, it appears from the analysis that increasing the recharge rate
(SAR) might have a signiﬁcant inﬂuence on groundwater movement and the ﬂow

pattern and can increase the risk of arsenic migration into deeper layers.

6.5.4 inﬂuence of SAR on groundwater movement, ﬂow patterns, and

likelihood of arsenic migration

The aquifer system in Bangladesh is complex in terms of sedimentoiogical

properties, both laterally and with depth. it is difﬁcult and expensive to build at:

207

hydrogeologic model that could incorporate all complexities to address the reality
of the ﬁeld conditions. However, in this study, a simple and uniform model (M-1),
relatively complex models (M-2 & M-3) and a site-speciﬁc model were developed

to reﬂect the reality of the ﬁeld conditions of the study area.

Basic issues in groundwater modeling

 

In order to have a proper understanding of arsenic movement in the
groundwater, one must understand the:

a) inﬂuence of shallow aquifer recharge (SAR) on groundwater movement
and ﬂow patterns.

b) inﬂuence of aquifer conductivity on downward ﬂow through deeper layers
0) Variation in groundwater movements in response to natural groundwater
flow conditions and extraction conditions

d) Impact of extraction or pumping on the depth of flow, ﬂow patterns,

dispersivity and likelihood of arsenic migration with moving water.

6.5.4.a. Inﬂuence of shallow aquifer recharge (SAR) on groundwater
movement and ﬂow patterns

This analysis was performed using the IGW-Groundwater modeling tools.
Three types of conceptual models, M—1, M-2, and M-3 as previously described ,

were used to estimate the ﬂow patterns and arsenic migration .

208

inﬂuence of shallow aguifer recharge (SAR) on the groundwater ﬂow pattern

Fig. 6.12 and Fig. 6.13, presented below, illustrate the simulated ﬂow
patterns obtained from the uniform model (M-1). These ﬁgures were examined to
comprehend the inﬂuence of the aquifer recharge (SAR) rate on groundwater
ﬂow patterns. Basically, it gave an indication of how groundwater typically moves.
it was found from the model simulation that doubling or halving the amount of
aquifer recharge (SAR) had little or no impact on the groundwater ﬂow patterns in

homogeneous, uniform model M-1 (Fig. 6.12).

     

1-

.30-5.” . :2 *
SAR=38I mm/yr. SAR=762 mm/yr.

Fig. 6.12. Doubling and halving the amount of aquifer recharge (SAR) in the

uniform (M-1) model had little or no impact on the groundwater ﬂow pattern.

209

However, when the conductivity at the lower shallow aquifer (L2) in the M-

2 model was increased to 50 m/day from 15 m/day, the streamlines moved

deeper, just below the 315.03 level (Fig.6.13).

 

 

 

 

 

Fig. 6.13. Streamlines moved deeper in the M-2 model after increasing the
conductivity, showing that the lowest streamline moved below the line of 315.03

m beneath the surface.

i1 .
[54:1

  
   
 
 
 
 

' * .- —. or . :I'ilf. ..r.t.“4"llknsrtlnoﬂw,

. , .,. . . . 'l ’9 (292m-

 

(' ” ,ll '5“ rl Lir‘it: 9’"

'54—.

:il’J ".l! "i 1.53935 ”)8V3m ”INN-'7"
. w”

  
  

’1

~ . . '_.I.

3.! w, m Grill r-rti no: t. . '.:.t‘ q'llil'll"-"u’.‘" 2.3-3on am 16!" M -
r-

0i:

_ _, f L. ‘t'-—'e

6.5.4.b. inﬂuence of aquifer conductivity and the aquifer recharge rate
(SAR) on the downward ﬂow through deeper layers

Increasing conductivity drew water to the deeper layers (Fig. 6.13). it is a
great concern in Bangladesh that presently uncontaminated layers may be
contaminated by arsenic as a result of such a groundwater movement. in order to
understand the likelihood of arsenic migration through the groundwater due to

extraction, it was necessary to estimate the amount of ﬂow passing downward

through each of the deeper layers (L2 & L3) under natural groundwater ﬂow and

extraction conditions. Mass balance for each aquifer layer(L1, L2, and L3) (Fig.

5.2.c) of all conceptual models (M-1, M-2, and M-3) were computed using the
lGW modeling tools, and the results are presented in Table 6.5.1. Table.6.5.2
was derived from the mass balance presented in Table.5.1, Table.5.1.a, and

Table.5.2. The ﬁndings are discussed below.

Model ﬁnding 1

 

increasing the conductivity of deeper layers increases the downward ﬂow rate

through the deeper layers.

Simulation result of model M-2

 

The three-layer model (M-2) reﬂects a condition in which the upper shallow layer

(L1, K1= 15) and deeper layer (L3, K3 = 15) were signiﬁcantly less permeable than

211

the middle layer (L2, K2 = 50 m/day). This results in an increase in downward ﬂow

to the middle layer (L2) to 15.12% under pumped conditions from 12.82% in the
M-2 model under natural ﬂow conditions (Col. 3, Table 6.5.2). This is due to the

increase in conductivity of L2 from 15 m/day to 50 m/day in M-2 model.

Simulation result of model-M-3

 

The conceptual model MS was characterized to incorporate the second

condition, which reﬂected the upper ﬁning sequence of aquifer system,

K3>K2>K1. The simulation results indicate that increasing the conductivity of
deeper aquifer layers (in model M-3) drew more water to the deeper aquifer L3

and L2 under both natural ﬂow (Col. 7, Table 5.2) and pumped conditions (Col.
7, Table 5.3).

Vertical ﬂow of water in the deeper aquifer layer (L3) under natural ﬂow
conditions was increased from 4.71 to 11.79% (Col. 3, Table 6.5.2). This
indicates that the more highly conductive layers (L2 & L3) in the M-3 model drew

water deeper into the aquifer under natural ﬂow and pumped conditions.
it was found from the mass balance results presented in Table 5.2 that

the downward ﬂow (with aquifer recharge rate SAR=381 mm/yr.) passing through

aquifer layers L2 and L3 (in model M-3) under natural ﬂow conditions were 350

and 230 m3/day respectively (shown in Col. 4, Table 5.2). Whatever amount of

212

water came as inﬂow to aquifer layers L2 and L3 (Col. 4, Table 5.2), passed
vertically downward as “flow from top to bottom” at the same rate of 350 or 230
malday (Col. 9, Table 5.2). The percentages of flow through each layer were
computed as 17.94% (350 I 1950 * 100 = 17.94%) and 11.79% (230/ 1950 * 100
= 11.79%), respectively (Col. 3 of Table 6.5.2).

These results clearly demonstrate that higher conductivity drew water to
the deeper layers in model M—3 under pumped and un-pumped conditions. it
implies that upper ﬁning sequential aquifers (M-3) are subjected to a higher risk
of arsenic migration than the type of aquifer represented by M—2, because
arsenic can migrate more with groundwater moving from currently contaminated

layers to uncontaminated layers.

6.5.4.c. Groundwater movement variations under natural groundwater ﬂow
conditions and extraction conditions

The mass balance results presented in Table 5.1.a and Table 5.3 were
used to estimate the components of flows passing through the deeper layers (L2
and L3) of the M-1 and M-3 models at a pumping rate of Q = 5000 malday. While
comparing Table 5.2 and Table 5.3, it was evident in Col. 4 and Col. 9 that the
amount of flow passing downward through the aquifer layers (L2 and L3) in Table

5.3 was signiﬁcantly higher under the inﬂuence of extraction in the M-3 model.

This ﬁnding requires further investigation.

213

Finding 2
Extraction influences the groundwater ﬂow dynamics in the following two areas:
1) Flow between the river and the aquifer system, and

2) Downward ﬂow through the deeper layers

Flow between the river and the aquifer system

 

Comparing the simulation as run with and without the extraction rate (Col. 4 and

Col. 9, Table 5.2 & Table 5.3) clearly demonstrates that

o Pumping increases the ﬂow from the river to the aquifer when the
shallow aquifer recharge (SAR) is less than the extraction rate (Q)

- Pumping decreases the ﬂow rate from river to the aquifer system
when the SAR rate is greater than the extraction rate (Q)

Under pumped or extraction conditions, the reason for increased groundwater

ﬂow from the river to the aquifer was that ﬂow to the well required more water

(5000 m3/day) than was available from recharge (at a SAR = 381 mm/yr., the

total amount of SAR was 1950 m3/day for an area of 1.95 kmz).

Influence of extraction on downward ﬂow through deeper layers

. Extraction generally increases the ﬂow through deeper layers more than

natural ﬂow conditions (Table 6.5.2).

214

o Extraction increased the ﬂow rate through deeper layers when the

pumping rate (Q) was greater than the aquifer recharge rate (SAR).

For example, without extraction, water flow from the river to the aquifer in L2 and
L3 was 350 and 230 m3lday (Col. 4, Table.5.2), whereas, with extraction, the ﬂow
in L2 and L3 were 1700 and 1900 m3/day, respectively (Col. 4, Table 5.3).

. Extraction proportionately decreased the ﬂow rate through the deeper

layers when aquifer recharge (SAR) rates were increased.

What happened when the extraction rate (Q) was less than the SAR?

 

It was found in the simulation results of model M-3 (Table 5.3) that when

the extraction rate (Q = 5000 m3/day) was less than the amount available from
aquifer recharge (Col. 5, Table 5.3, SAR of 1000 mm/yr. = 5180 m3/day for the
mass balance area of 1.95 kmz), the ﬂow from the river to the aquifer system

decreased. Also, the amount of downward ﬂow through L2 and L3 was

proportionately decreased (Col. 9, Table 5.2).
For example, it was shown in the mass balance computation (Col. 4,

Table 5.3) that when SAR=1000 mm/yr., the downward water ﬂow rate through

L2 was 620 m3/day, whereas the ﬂow from the river through L2 to the aquifer was

1700 m3/day when the SAR=381mm/yr.

215

Why the increased aquifer recharge @AR) rate decreased the downward ﬂow

rate and flow from river to the aquifer

 

Finding 3

The more the pumped-water share comes from the recharge source (SAR) or
leakage, the less water ﬂows from the river to the aquifer system. That condition
leads to a proportionate decrease in the amount of downward ﬂow through the
deeper layers. However, in general, extraction conditions increased the
downward ﬂow to a rate higher than that of natural groundwater ﬂow conditions

(Table 6.5.2).

M-1 model simulation results

 

Simulation of the uniform homogeneous conductivity model (M-1), the results of

which are presented in Col. 3, Table 6.5.2, indicates that the lower shallow
aquifer layer (L2) and deeper aquifer layer (L3) receive 12.82% and 4.71% of total
flow, respectively, under natural ﬂow conditions. Under extraction conditions, with

average recharge rate (SAR) equal to 381 mm, the vertical ﬂow through L2 and

L3 were increased to 18% (Col. 4, Table 6.5.2).

216

Model M-3 simulation results

 

Extraction also increases the downward ﬂow through the deeper layer at L3 .The

percentage of ﬂow increased to 38% (Col. 4, Table 6.5.2) from 11.79% (Col. 3,
Table 6.5.2). The share of groundwater ﬂow through the deeper layers is
presented in Table 5.3. While comparing Table 5.3 (Col. 9) with Table 5.2 (Col.
9), it was evident that “ﬂow from top to bottom” increased signiﬁcantly under

abstraction.

Finding4
Under natural groundwater ﬂow conditions, increasing the aquifer

recharge rate (SAR) increases the downward ﬂow through the deeper aquifer
layers (L2 & L3) in models M-1, M-2 and M-3.

The simulation results presented in Table 6.5.2 indicate that increasing the
average recharge rate (SAR) from 381 to 625 mm/yr. increases the vertical ﬂow
through the lower shallow aquifer (L2) and deeper aquifer layer (L3) under natural
flow conditions in models M-1, M-2 and M-3 (comparing Col. 3 & Col. 5 in Table
6.5.2). Flow through L2 and L3 in model M-1 was increased to 15.21% from

12.82%, and in model M-2, in the same layer L2, ﬂow increased from 15.12 to

20.96% (Table 6.5.2).

Finding 5

217

o The vertical hydraulic gradient and local hydraulic gradient were also
changed due to the inﬂuence of extraction.

0 Extractions signiﬁcantly changed the flow pattern in the groundwater
system. When the pumping rate Q is more than the SAR rate, groundwater ﬂows

from the river to the aquifer, changing the local hydraulic gradient.

 

 

 

 

 

Fig. 6.14. Contrasting groundwater ﬂow patterns between un-pumped (left

side)and pumped conditions(right side)

Wh um in at a rate reater than that of shallow a uifer rechar e SAR

increases the risk of arsenic migration

The mass balance records of 3-D models indicate that pumping at a rate

greater than the SAR increases ﬂow from the river to the aquifer in order to

218

maintain pumping rate Q. This is also supported by the one-dimensional model

analysis presented in Chapter 5.1.1. It was shown that when the ratio of the

partial discharge (03th in Fig. 5.1.b) is decreased, part of the pumped water Qt
comes from the boundary recharge sources from top layer L1. When the aquitard
thickness (b’) is less and 03th is near zero, the contaminant concentrations in

pumped water (Ct) becomes equal to the concentration of water in layer L1, C1

(as shown in Eq. 5.1). The simulated mass balance results in Table6.5.2 also
demonstrated that pumping can increase the vertical ﬂow rate more than natural
ﬂow condition. The groundwater velocity vectors are more dominant around the
pumping well screen where water along with contaminants move with advective
velocity.

This implies that contaminant water from boundary recharge sources
moves through the layers to the well screens with the velocity of moving
groundwater. Therefore, arsenic can also move with advective velocity from the
contaminated upper layers to the uncontaminated deeper layers during heavy

pumping. This will certainly increase the risk of arsenic migration.

219

Table 6.5.2 Percentage of total ﬂow associated with each layer in the geological

cross section of the conceptual model.

 

Types of Conceptual

Models and Aquifer

Depth (m)

Model (1) Layer
(2)

M-1 L1(0-40m)
L2(40-140)
L3 (140-400)

M-2 L1(0-40m)
L2 (40—140)
L3 (140-400)

M-3 L1(0-40m)
L2 (40-140)

6.5.4.d.

L3(14O-4OO)

migration of arsenic

Flow from the surface through the layers

 

 

Average Recharge

(381 mm)

Natural
conditions

% of total ﬂow

(3)

100
12.82
4.71
100
15.12
1.91
100
17.94
11.79

ﬂow Pumped
conditions

% of total
ﬂow (4)

18
18
100
27.4
1.12
100
34
38

Medium Recharge

(625 mm)

Natural ﬂow Pumped

conditions

°/o Of total
ﬂow (5)

100
15.21
5.81
100
20.96
3.22
100
0.4

2

conditions

% of total
ﬂow

(6)
100
16.4
18.6
100
18.66
8.41
100
26.4
34.4

inﬂuence of aquifer pumping and dispersivity on the vertical

Migration of arsenic is inﬂuenced by the factors controlling the downward

ﬂow of groundwater. Downward ﬂow of arsenic is inﬂuenced by the aquifer

recharge rate (SAR), depth of well screens and the aquifer conductivity,

dispersivity and distribution coefﬁcient (Kd). The inﬂuence of the pumping rate on

220

arsenic dispersion and arsenic migration in particulate form in the no-ﬂow

boundary zone (Fig. 5.15) was analyzed.

 

 

 

 

Fig. 6.15. Arsenic dispersion after 8400 days with pumping rate of 2448 malday

(left) and after 4250 days with 4896 m3/day pumping rate (right).

Finding 6
inﬂuence of extraction on arsenic migration

The above ﬁgure (Fig. 6.15) shows that arsenic migration is signiﬁcantly

inﬂuenced when the pumping rate (Q) is doubled (from 2448 to 4896 m3/day)

while keeping the lateral and transverse dispersion ratio (DLIDT =5) the same .

221

 

inﬂuence of disgrsion ratio on arsenic migration
The ﬁgure below (Fig. 6.16, middle and left) clearly demonstrates the impact on

arsenic movement of increasing the dispersivity ratio( DL/DT ) from zero to 5
when the extraction rate (Q) remains constant. The arsenic migration trend after
5000 days was found to be greater when a higher dispersive ratio was added in
Fig. 5.7 (DL/DT =5). Inﬂuence of extraction on dispersivity was also dominant.
Doubling the pumping rate Q (from 2448 to 4896 mslday) signiﬁcantly increases

the dispersion (comparing the middle and right ﬁgures of Fig. 6.16).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

llll

 

 

 

(21’ "

(1) DL/Dr=0. (2) DL/DT =5. (3) DL/DT =5,
=2448m3/day Q=2448 m3/day o=4896m37day
Fig. 6.16. inﬂuence of dispersivity ratio (DLIDT) and extraction rate (Q) on arsenic

migration as a particle

222

6.6 Placing of tube wells screens in the deeper layers and its inﬂuence on
arsenic movement from contaminated layers to the uncontaminated layers.

Research issue:

 

The sixth research issue of this study was to understand the impact of placing

deep tube well screens at the deeper layers of the aquifer.

Strategic questions:

 

1. What is the impact on the components of groundwater ﬂow in the mass
balance and arsenic migration processes when placing deep tube well screens at
deeper layers?

it appeared from the field experience that placing well screens at different depths
might have an impact on the groundwater ﬂow components and arsenic

migration process.

6.6.1 impact on the ﬂow components and likelihood of arsenic migration
of placing deep tube well screens at deeper layers

This issue was investigated from the computation of mass balance results
presented in Table 5.4 and Table 5.5. it was necessary to analyze the
components of groundwater ﬂow and local groundwater gradients to the river to

address the issue of arsenic migration in the aquifer. The results of placing well

screens at different aquifer layers [L1 (-25 to -35 m), L2 (-100 to 120 m) and L3 (-

223

300 to —320m)] of the uniform M-1 and complex site-specific models were

evaluated.

Installation of wells at L3 indicates that a signiﬁcant amount of water was

drawn from the river (Col. 7 of model M3 in Table 6.6.1). While comparing the
mass balance in Table 5.5 (for model M-2) and Table 5.5.3 (for model M-3), it
was evident that the M-3 model drew more water into the deeper layers (Col. 9,
Table 5.5.3). Flow through the vertical layer was increased more in the M-3
model when aquifer sediments were more transmissive (in M3 and M-2, in Col. 9

of Table 6.6.1). Therefore, it is very likely that the placing of a tube well screen at
the deeper layer (L3) of the study area aquifer would result in increased ﬂow to
the deeper layers, and with this moving groundwater, arsenic could migrate by
the advection process. Therefore, placing well screens at deeper layers will

increase the risk of arsenic migration from the currently contaminated layers to

the uncontaminated layers.

224

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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225

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Conclusion

 

The overall conclusions are that irrigation wells with realistic extraction rates
signiﬁcantly and beneﬁcially affect the ﬂow patterns within the groundwater
system. The aquifer recharge rate (SAR) also signiﬁcantly induces groundwater
movement and ﬂow patterns. Extraction also signiﬁcantly changed the
groundwater movement and ﬂow patterns. Placing deep tube well screens at

deeper layers is not a safe approach to obtaining arsenic-free water

6.7. Administrative and management decision variables to reduce the risk
of arsenic concentration in the case of further irrigation development

The seventh research issue in this study is to identify the management
decision variables that can help reduce further risk of arsenic contamination in
the case of groundwater irrigation development policy. There are management
decision variables such as the irrigation system capacity, cropping patterns,
rainwater harvesting, and soil-based arsenic-contaminated water treatment
systems can help improve the availability of shallow aquifer recharge (SAR) in

groundwater.

Strategic questions

 

1. How can irrigation system capacity reduction and adjustments in cropping
patterns or seasons help increase the amount of aquifer recharge in the

groundwater system?

226

2. How can rainwater harvesting and soil-based arsenic-contaminated

groundwater treatment systems be used to reduce the risk of contamination?

6.7.1 How irrigation system capacity reduction and adjustments in
cropping patterns or seasons can help increase the amount of aquifer
recharge

The impact of irrigation system capacity reduction on the amount of
shallow aquifer recharge available is discussed in section 6.5.3.a. Decreasing the
irrigation system capacity can increase the availability of amount of SAR but it is
a trade-off between yield reduction and increasing SAR.

Section 6.5.2 describes the inﬂuence of cropping patterns on the amount
of SAR available. The amount of SUR available with rain-fed rice (no-pumping
condition) was equal to 716 mm/yr. (Appendix D and Fig.6.9) using 33 years of
average rainfall data. But when intensive HYV-rice were grown in two seasons
throughout the study area, the amount of SUR was decreased from 716 to 201
mm (Appendix C, Fig. 6.10) which is about 71% less than the rain-fed condition.

This section will graphically investigate how adjusting the cropping season

can help to increase SAR availability in the ﬁeld.

By adjusting the cropping season, the irriggtion withdrawal (Q) could be

reduced from 791 to 250 mm in Kharif-1 season

227

By adjustment in the cropping patterns (introducing crops with low water
requirements in cropping seasons Rabi, Kharif-1 and Kharif-2, starting the
transplanting date earlier in the Kharif-1 season (Fig. 6.17), and maximizing the
use of monsoon rainfall) , the irrigation withdrawal volume for rice can be

reduced from 791 to 250 mm in the Kharif-1 season (without land preparation).

 

Av. Monthly Rainfall, ETo and Crop Season in
the Ganges Basin
1800 ~

1603‘

   
   
     

Maximum Monthly Rainfall
1400 1

12(1)“

10(1)‘

Amounthrxn)

Normal Monthly Rainfall

 

 

 

O 1 r r r
RABI KH4ARlF-1

i 2 3 5 8 7

 

 

8 ““3”“ 10 11 12

 

Fig. 6.17 By adjusting the cropping season and cropping patterns, the best use of

rainfall can save 500 mm of pumping in a year

Therefore, by introducing crops with low water demands, 541 mm of water could

be saved, and that would eventually contribute to groundwater and cause a rise

228

in the water table. Equation 6.5.6 indicates that, when SUR is less, outﬂow would
be greater, and when pumpage (Q) is greater, the amount of shallow aquifer

recharge (SAR) required is also higher.

6.7.2. How rainwater harvesting and soil-based arsenic-contaminated
groundwater treatment systems can be used to reduce the risk of
contamination

It is evident in Fig. 4.5 and the discussions in section 6.2.1 that the redox
potential values decrease with depth. But the upper 0 to 10 m layer is relatively

less arsenic contaminated because it is rich in dissolved oxygen.

6.7.2.1 How rainwater can be used to reduce the risk of arsenic
contamination

As this oxygenated water comes from the available rainfall, it has higher
redox potential (Eh) values (800 mv). Each year, about 500 mm of rainfall is

accumulated on the top of the summer water table, which is arsenic-free. This
500 mm of rainwater can develop a 4 to 5 m water table head, giving locations
for small-scale pumps or wells to be placed to obtain arsenic-free water.
Therefore, aquifer delineation is extremely important in terms of the supply
of safe irrigation and drinking water. It is strongly suggested that the top 10 m of
the aquifer be kept for installation of small scale dug wells or hand tube wells in

order to ensure a long-term supply of arsenic-free water (Fig. 6.18). The upper

229

layer receives rainwater, which is high in redox potential values and should be

safe for drinking; extraction for irrigation must be restricted.

 

 

  
 

2"I Layer (1 0-150 m)

  

11503093093040 m)
Eh = 350 .100 my ,
Arsenic Cows.- s50 ppb

    
     
 

Dueto TEAP 9 Eh -

< 511.10. <-1oo my
Arsenic Conc.= §Q to 2000 ppb

Arseric Cone.- 450 ppb
high DO- Content

 

 

 

Fig. 6.18 Aquifer delineation for long-tenn supply of arsenic-free, safe water

on the basis of redox potential values (Eh)

6.7.2.2 How soil-based

‘ ' ‘ ‘ groundwater can be used to

use for irrigation to reduce the risk of further arsenic contamination

With presently available technologies, contaminated water can be treated

up to any trace amount. However, the low-cost arsenic removal techniques are

still limited by their inability to completely remove residual As (ill) from water.

Adding external chemical oxidants sometimes degrades the water quality.

230

Therefore, this study presents two low-cost techniques as soil-based treatment

systems that make use of locally available materials. These are small-scale and

medium-scale treatment systems for rural community people in Bangladesh.
Arsenic-contaminated groundwater in Bangladesh is rich in dissolved iron

content. Mixing of arsenic-contaminated groundwater with atmospheric oxygen
would easily oxidize the dissolved iron (Fe2+) to Fe (OH)3 within about 20

minutes. Under this oxic condition, dissolved arsenic (V) species will be adsorbed
onto iron oxides but As (ill) will remain in the water in dissolved form because
dissolved oxygen has no effect on the oxidation rate of As (Ill) (Scott and
Morgan, 1995). The contaminated water will then pass through a ﬁlter bed made
of low—cost ferruginous manganese ore ( FMO). In order to decrease the
detention time, "t", the FMO will be used to chemically oxidize the residual As (ill)
to As (V). Eventually, arsenic (V) will be adsorbed onto iron oxides. After laterally
ﬂowing through the ﬁlter bed, the arsenic content in afﬂuent water will fall below
the level of 10 ppb.

How does the system work?

The velocity head can easily be used to mix oxygen with contaminated
water. The aerated, oxygenated water will help to form a coating of iron oxides ‘
around the sand particles and create a surface of metallic cations. Anions of
arsenate (As (V)) are less mobile and stable in high oxic conditions, so they will
tend to co-precipitate with metallic cations of iron oxides. Then the contaminated

water will pass through the ﬁlter bed containing FMO and local river sands. The

231

residual arsenic (ill) would be oxidized to arsenic (V) followed by adsorption on to
iron oxides. Aﬂer ﬁltration through FMO and local sands, eventually arsenic

concentration will be reduced below the level recommended by the WHO (<50

ppb)

 

in this study, two different approaches are considered for low-cost arsenic
removal techniques. The shallow tube well based treatment system is for small-
scale community levels and the Subsurface Flow (SF) constructed wetland
system is for medium-scale communities. The design procedure is described
below.
Shallow tube well-based arsenic treatment system
in order to keep the system cost low, the shallow tube well-based arsenic
treatment system was considered because it is most available source of cheap

energy. as well as a potential source of arsenic-contaminated water.

232

 

 

 

 

S rinkler V
GS . ll l “W’ Q
1‘.— - 1
‘— <—
FIVIO 2 River Sand _
«ll—r e a5
L-1 Li: 4
STW
Water Table

Well-screen

 

 

 

Fig. 6.19 Shallow tube well-based groundwater arsenic treatment system using
locally available materials
Soil-based Subsurface-Flow (SF) constructed wetland systems for arsenic
treatment

Constructed wetland systems have been popular in removing
contaminants from wastewater. Generally two types of constructed wetlands are
known to be developed, Free Water Surface (FWS) and Subsurface Flow (SF)
constructed wetland systems (Crites et al., 1998).

Depending on topography and the availability of local materials, the SF-
constructed wetland systems could be efﬁciently designed to accelerate the
arsenic removal rate from contaminated groundwater at a low cost. The main

advantage of SF—constructed wetlands is that they do increase the residual

arsenic (ill) removal process without adding chemical oxidants (Fig. 6.20).

233

Introducing aquatic weeds or bioremediation plants can also manage inorganic

arsenic biologically.

 

 

 

 

 

 

 

 

 

 

 

 

 

Impermeable Bed Length L = L-l + L-2-l-L-3

‘ 1 8t 3 Gravel packed collector for inﬂow and outﬂow
‘ 2 Subsurface -Flow mixed reactor with aquatic system, made of
River sand and Ferruginous Manganese Ore (FMO) arsenic removing plant
4 collector opipe 5 Final arsenic free water collector
1 g . .'

 

 

 

 

 

Fig. 6.20 Subsurface-Flow constructed wetland system for treating arsenic-

contaminated groundwater

234

7. SUMMARY

This study began with several hypotheses that were based on the idea
that less oxygenated water at and below the water table leads to more anoxic
conditions. The lowering of the water table leads to the extension of the
unsaturated zone by irrigation withdrawal and reduction of aquifer recharge
because of upstream diversion of river water. The control mechanism is the lower
rate of diffusion of atmospheric oxygen into the shallow groundwater and the
shortage of oxygenated water in aquifer recharge.

The main objective of this study was to understand the factors that
produce high levels of arsenic in Bangladesh’s groundwater and to formulate a
quantitative irrigation development policy so that the present pace of irrigation
development could be sustained without any further risk.

This study addressed the following issues: (1) high arsenic concentration
distribution and its association with geohydrologic factors; (2) interaction between
lowering of the water table, the amount of dissolved oxygen (DO) available in
recharging groundwater, and arsenic concentration variation over the aquifer’s
depths; (3) evaluation of the present theories of arsenic release; (4) the inﬂuence
of the irrigation withdrawal rate and cropping systems on the amount of shallow
aquifer recharge available; (5) the inﬂuence of extraction rate (Q) and shallow
aquifer recharge (SAR) on groundwater movement, ﬂow patterns, and the
likelihood of arsenic migration; and (6) the impact of placing deep tube well

screens at deeper layers and increasing the risk of arsenic migration to the

235

deeper layers of Bangladesh’s groundwater. Finally, this study presents the
ﬁndings and recommends administrative decision variables for quantitative
irrigation development policy.
The relationship between high arsenic concentration distribution and
associated geohydrologic factors.

in order to establish a relationship between high arsenic concentration and
associated hydrogeologic parameters, analyses of the probability of arsenic
concentrations exceeding threshold levels (50 ppb) were done initially, and the
results were presented in Table 4.2.1 and pictured in Fig. 4.2.b. The southeast
(SE) and southwest (SW) zones were the most arsenic-affected areas, in which
the probability of arsenic exceeding threshold levels (50 ppb and 250 ppb) were
found to be 71 .4% and 59.55%, respectively (Table 4.2.1).

High arsenic concentration distribution and its relationships with surface geology

 

and physiography

 

Surface geological map analysis demonstrated that the most arsenic-
contaminated area was mostly conﬁned to the lower delta ﬂood plains of the river
Ganges, which comprised the southwest (SW) region, and the Meghna River
Flood Plain (FP), which built up the southeast (SE) region of Bangladesh (Fig.
4.3.a and Fig. 4.2.b). The surface geology of the SE and SW regions was alluvial
deposits and deltaic deposits respectively (Table 4.2.2 and Fig. 4.2.a).

The least arsenic-contaminated zones are the northwest (NW) and north—

center (NC) areas (Fig. 4.2.b) where the geomorphology is comprised of Old

236

Himalayan Fan, Tista-Jamuna Floodplains, and Madhupur Tract (Table 4.2.2 and
Fig. 4.3.a). Arsenic concentrations in the SW and SE zones were found to be
greater than those in the NW and NC regions because arsenic was inherited in
the sediments of that region through the development of different
geomorphological processes.

The physiographic map analysis in Fig. 4.3.b clearly indicates that high
arsenic concentration levels (>250 to 1665 ppb) were mostly conﬁned to the SE
region where the physiographic unit was dominated by Tippera surface (Fig.
4.3.b and Table 4.2.2).

High arsenic concentrations offered a good correlation with dissolved iron
C_0rEe_nt

Grids 5, 9, 10, 12, and 15 show the highest arsenic concentration, with R2
values between 71% and 99% (Table 4.2.4). The conﬁdence interval associated
with these R2 values is 95%. All of the highest arsenic contaminated areas
belong to the SW and SE zones.

Arsenic concentrations increase with increasing aquifer depth from 9 to 30 m

 

The spatial variations of arsenic concentration distribution were analyzed
using about 3500 arsenic-contaminated wells’ water quality data (collected by
BGS). The results indicate that the distributions of arsenic concentrations above
the threshold level (>50 ppb) are mostly located between 15 m and 150 m below
the surface (Fig. 4.5). Further depth analysis demonstrated that arsenic

concentrations are found to increase over the depth ranging from 9 m to 30 m

237

below the ground surface (Fig. 6.2.a) and the correlation between arsenic
concentration and depth are found to be positive (R2=60%) (Fig. 6.2.a).

Arsenic concentrations decrease with depth from 100 m to 350 m

 

Arsenic concentrations decrease with depth from 100 m to 350 m below
the ground (R2=72%, Fig. 6.2.b). Arsenic concentration records at the top-most
layer (within the depth ranging from 0 to 10 m) did not show any relationship with
depth. The spatial variations in arsenic concentration distribution over these
depths were investigated in the context of thermodynamics. The ﬁeld redox
potential values were computed to be within the range of 2.09 to —1.82 and are
presented in Table 4.4.1.

Redox potential (pe) values decreased linearly over aquifer depths ranging from 9

m to 30 m below the ground

 

The redox potential values at each depth were computed and a
relationship between redox potential values and aquifer depths (Fig. 6.3.a and
6.3.b) was found. Fig. 6.3.a indicates that the redox potential values decreased
linearly with increasing aquifer depth from 10 m to 30 m below the ground. On
the contrary, redox potential values increase in the deeper aquifer depths ranging
from 150 to 350 m (Fig. 6.3.b) and arsenic concentration values decrease below
50 ppb.

Evaluation of existing theories of arsenic release
The experts’ opinions are still diverging, and there are considerable

difﬁculties in diagnosing this problem. Proponents of oxidation theory argue that

238

the lowering of the water table provides atmospheric oxygen access to oxidize
the arsenic-rich pyrites and eventually arsenic is released into the groundwater
as desorption from iron oxides. The proponents of reduction theory believe that
organic carbon takes oxygen from iron hydroxides through reductive dissolution
and eventually arsenic is released.

However, the hypothesis presented in this study is in complete
disagreement with the oxidation theory and partial agreement with the
oxyhydroxide reduction theory of arsenic release .The hypothesis presented here
states that the shortage of dissolved oxygen in the recharging groundwater is the
cause of arsenic release, whereas the oxidation theory blames the exposure of
arsenic rich pyrite layers to atmospheric oxygen. in addition to this, there are
several vital questions that must be answered.

The critical difference between the reduction theory and hypothesis offered in this
stpgy

The reduction theory also has shortcomings. Investigators questioned why
arsenic contamination was a recent phenomenon if organic carbon has been
taking oxygen from iron oxides for thousands of years. Why there was not any
prior report of arsenic toxicity in Bangladesh?

Like the reduction theory of arsenic release, this study indicates that
arsenic is released due to development of a reducing condition. But this study
does not support explanations that microbial activities derive organic carbon to

remove oxygen from the iron hydroxides, releasing arsenic. Rather, this study

239

investigated the shortage of dissolved oxygen in the recharging water.

Lack of dissolved oxygen and I or shortage of oxygenated recharging water

 

at and below the water table as the major cause of arsenic release in

 

Bangladesh

 

This study analyzed both thermodynamic and numerical models to support
the hypothesis that shortage of dissolved oxygen in the recharging groundwater
is the major cause of arsenic release. Finite element simulation results show that
lowering of the water table cannot increase the oxygen diffusion rate in the
deeper layer of the aquifer.

Oxygen concentrations decrease with increasing aquifer depths

 

Analysis of the oxygen diffusion model (Table 4.3.2) suggested that
oxygen concentration decreases with increasing aquifer depths (Figs. 4.11, 4.12,
and 6.4). it is evident (in Fig. 6.2.a) that arsenic concentration increases with
increasing depth (9 to 30 m), and the redox potential values (pe) decrease (Fig.
6.3.3). This is due to a low oxygen supply (over the depth of 9 to 30 m below the
ground) that reduces the redox potential values.

Finite element analysis of oxygen diffusion model does not support the oxidation

 

theogy

The results presented in Table 4.3.2 show that the diffusive supply of
oxygen in arsenic-rich pyrite layer L5 is decreased from 2.391 to 1.235 mg/L

when the water table lowered from 2 m to 5 m below the ground surface

240

(Table.4.3.2). This analysis clearly demonstrates that lowering of the water table
cannot accelerate arsenic-rich pyretic layer oxidation, as the dissolved oxygen
content in recharging water is decreasing.

Inﬂuence of aquifer thickness (b’) and conductivity on the arsenic concentration

 

of water entering a well

 

it appears from the concentration model analysis (Fig. 5.1.b and Fig.
6.8.a) that the arsenic concentration of water entering a well varied inversely with
the thickness of the aquifer. After increasing the aquifer thickness from 65 to 100
ft while keeping the vertical conductivity the same at 1.18 ft./day, the arsenic
concentration in the water entering the well was decreased from 85 to 76 ppb
(Fig. 6.8.a, curve b and c). A 54% increase in aquifer depth decreases the
arsenic concentration of the well water by 8%.

The arsenic concentration in a well’s water varies directly with the vertical
conductivity of the aquifer. it was shown that a ten-fold increase in the
conductivity value of the aquifer produced a 26°/o rise in the arsenic concentration
of the well water.

The ﬁnite element oxygen diffusion model (Table 4.3.2) and concentration
model (Fig. 6.8.a) simulation results suggested that the pyrite oxidation theory is
not applicable to explain the arsenic release mechanism in the groundwater of

Bangladesh.

241

Irrigation system capacity and planting high yielding rice varieties (HYV—

 

rice) signiﬁcantlLinﬂuence the availability of shallow aquifer rechargg
(SAR) and potential recharge and increase the risk of arsenic migration

The amount of surplus water (SUR) and shallow aquifer recharge (SAR)
water available were discussed. The virgin amount of SAR was computed as 281
mm/yr. (Section 4.5.2 and Appendix B) and under pumping conditions, the
amount of SAR was computed to be in the range of 471 to 498 mm/yr. (Section
4.5.3).

Growing HYV-Rice decreases the surplus volume (SUR) by 71 %

The amount of surplus volume (SUR) available was computed using the
soil moisture balance. The results indicate that the amount of SUR is decreased
when HYV-rice is grown. The amount of SUR available with rain-fed rice (no-
pumping condition) was equal to 716 mm/yr. (Appendix D and Fig. 6.9) using 33
years of average rainfall data. But when intensive HYV-rice was grown in two
seasons throughout the study area, the amount of SUR was decreased from 716
to 201 mm (Appendix C, Fig. 6.10), which is about 71% less than the rain-fed
condition.

In addition to that, the irrigation requirement of HYV-rice without land
preparation was computed as 637 mm. Under the improved management
practice, this requirement could be reduced to 433 mm. it was shown that, by
adjusting the growing season or making use of monsoon rain, the groundwater

withdrawal rate (Q) could be signiﬁcantly reduced.

242

By adjusting the cropping seasoniie irrigation withdrawal (Q) could be reduced

from 791 to 250 mm in Kharif—i season

 

By adjusting the cropping pattern, the irrigation withdrawal volume can be
reduced from 791 to 250 mm in the Kharif—l season. Therefore, by introducing
crops with low water demands, 541 mm of water could be saved, which would
eventually contribute to groundwater and cause the water table to rise. Equation
6.5.6 indicates that, when the SUR is smaller, outﬂow is greater. When the
pumpage (Q) is greater, the amount of shallow aquifer recharge (SAR) to be
required is also higher. This excess water for SAR has to come from an induced
recharge source or from storage volume (dvldt). If there is no induced recharge
source, water comes from storage and eventually the water table drops. If
irrigation development continues without developing new recharge sources, the
water table will rapidly fall. If there is an induced recharge source, however, the
amount of SAR will increase.

Design capacity of irrigation system and its inﬂuence on surplus volume (SUR)

 

Like cropping patterns, the system capacities also inﬂuenced the amount
of SUR or potential recharge. In this study, the amount of SUR was computed
with a 70%, 80% and 90% level of system capacity. it was evident in Fig. 6.11
that the lowest system capacity level (70%) produced the highest amount of SUR

available after less than 3 day's maximum rainfall.

243

Inﬂuence of induced recharge on water table elevation in the study area

 

Analysis of the study area’s water table ﬂuctuation records in Fig. 6.8.b
shows that ﬂuctuation of the water table remained consistent (within 5 m) over
the period from 1994 to 1999, despite there being a signiﬁcant rise in the number
of shallow wells to 0.62 million from 0.093 million in 1994 (Table 3.5). The
number of shallow wells had increased by 58% but there was no evidence of
signiﬁcant decline in water table position (Fig. 6.8.b). This suggests that the study

area must have been recharged by an induced source like seasonal ﬂoodwater.

inﬂuence of shallow aquifer recharge (SAR) and conductivity on the

 

 

groundwater ﬂow pattern

it was found from simulation that doubling or halving the amount of aquifer
recharge (SAR) had little or no impact on the groundwater ﬂow patterns in
homogeneous uniform model M-1 (Fig.6.12).
0 Increasing the conductivity of deeper layers increased the downward ﬂow
rate through them.

A substantial extraction rate signiﬁcantly inﬂuences the groundwater ﬂow

 

dynamics and increases the likelihood of arsenic migration

Extraction, or pumping, inﬂuences the groundwater ﬂow dynamics in the
following two aspects:
1) Flow between the river and the aquifer system

2) Downward ﬂow through the deeper layers

244

Flow between the river and the aquifer system
A comparison of simulations with and without extraction rates (Col. 4 and Col. 9,
Table 5.2 8. Table 5.3) clearly demonstrates that
- Pumping increases the ﬂow from the river to the aquifer when shallow
aquifer recharge (SAR) is less than the extraction rate (Q)
o Pumping decreases the ﬂow rate from the river to the aquifer system when
the SAR rate is greater than the extracting rate (Q)
Under extraction conditions, the reason for increased groundwater ﬂow
from the river to the aquifer is that the well requires more water (5000 m3/day)
than it was available from recharge (at SAR: 381mm/yr, total amount SAR was

1950 m3/day for an area of 1.95 kmz).

Inﬂuence of extraction on downward ﬂow through the deeper layers

- Extraction or pumping generally increases the ﬂow through deeper layers

to a rate higher than that of natural ﬂow conditions.

. Extraction increases the ﬂow rate through deeper layers when the

pumping rate (Q) is greater than the aquifer recharge rate (SAR). Therefore,

under pumping conditions, the risk of arsenic migration to deeper layers is high.
It was also found that placing well screens at deeper layers increases the

flow rate in those deeper layers. When the extraction rate is greater than the

recharge rate (Q>SAR), the downward ﬂow through the deeper layers is

increased and simultaneously the risk of arsenic contamination is increased.

245

Mass balance analyses indicate that arsenic can migrate downward with moving
groundwater’s advective or hydrodynamic dispersive velocity. In order to obtain
arsenic-free water in a leaky aquifer system, irrigation wells must not be placed
into the deeper layers.
7.1 Conclusions

The groundwater arsenic contamination problem in Bangladesh is a
complex hydrogeochemical process. The arsenic source or origin in Bangladesh
is certainly natural. it appears now that arsenic release is triggered by a
misguided groundwater extraction policy. Large-scale extraction and diversion of
surface water from major rivers has caused a shortage of oxygenated recharging
water (SAR) at and below the water table, which eventually develops reducing
conditions and releases arsenic in the groundwater system.

it is still a debatable issue whether pyrite oxidation or reduction of iron
oxides controls the arsenic release mechanism in the groundwater of
Bangladesh. The oxidation or reduction process is a time-dependent
phenomenon. Therefore, especially in a heterogeneous aquifer system,
accepting or rejecting these hypotheses based on space-dependent relations
cannot well be representative.

it is unfortunate that the use of groundwater for irrigation and drinking
purposes and upstream diversion of surface water from the major river systems
began nearly simultaneously in 1970. The simultaneous development has

created confusion regarding the release mechanism of arsenic. Currently there

246

are two mutually exclusive theories of arsenic release: the oxidation theory and
the reduction theory.

Despite divergence of opinion on the arsenic release mechanism,
supporting evidence from the ﬁeld and from model results is strong enough to
reveal the inadequacy of the oxidation theory of arsenic release. At the same
time, the suitability of the hypothesis presented in this study is strong because it
is hard to deny the clear results of thermodynamic and mathematical models.

The findings of this study are contrary to the existing theories because
previous theories have ignored the contribution of aquifer recharge to
maintenance of the high redox values below the water table. Without knowing the
inﬂuence of aquifer recharge on groundwater movement and ﬂow patterns, it is
simply a misleading approach to formulate any irrigation development policy. For
example, experts working on irrigation development projects have already
suggested that tube wells should be installed reaching to deeper layers in order
to obtain arsenic-free water. However, models results clearly demonstrate that
placing well screens at deeper layer will increase the downward ﬂow and arsenic
might move with the groundwater to the deeper layers. Also, there is evidence
that extraction at a rate greater than the aquifer recharge rate (SAR) increases
the downward ﬂow through the deeper layers of the aquifer system and thus

increases the risk of arsenic migration with the moving groundwater.

247

Similarly, cropping patterns, irrigation system capacity, and extraction
rates all are management decision variables and decisions based on quantitative
results can improve controls over the arsenic contamination problem. Any
irrigation project must be accomplished by proper hydrogeochemical analysis. if
the extraction rate Q exceeds the aquifer recharge (SAR) rate, it will endanger
the groundwater ecosystem.

7.2 Findings of this study
0 A distinct regional pattern of arsenic concentration was found with the
greatest contamination in the southeast (SE) and southwest (SW) zone of the
country. The probability of arsenic exceeding threshold values (>50 ppb) was
found highest in the southeast (SE) and southwest (SW) regions, respectively
and it agrees with the ﬁndings of the other ﬁeld studies.
0 The present theory of arsenic release by pyrite oxidation is inadequate to
explain the arsenic contamination problem in Bangladesh.
- The top-most layers of the aquifer (0 to 10 m below the ground surface)
were found to be relatively less arsenic-contaminated in comparison with deeper
layers. This ﬁnding is supportive to the hypothesis offered in this study.
- Lowering of the water table to deeper layers after introducing well ﬁelds
did not increase the oxygen concentration in water; rather it decreased the
oxygen supply, because the oxygen diffusion rate (ODR) decreases with
increasing aquifer depth.

a Arsenic concentration was found to increase with aquifer depth from 9 to

248

30 m below the ground surface but to decrease with depth from 100 to 350 m
below the ground.

a The redox stratiﬁcation in the most arsenic contaminated aquifers can be
explained as a function of aquifer depth from 9 to 30 m (R2=60%) and 100 to 350
m (R2=72%) below the ground.

0 Growing high yielding variety rice twice in a year in the study area has
substantially reduced the surplus volume (SUR) available.

- Improved irrigation management practices can reduce the pumping
requirements of the study area. It appears that by adjustments in the cropping
season, the irrigation withdrawal rate can greatly be reduced. By adjustment in
the cropping pattern, the irrigation withdrawal volume can be reduced about 68%
in the Kharif—1 season.

0 By adjusting the irrigation system capacity, the amount of surplus can be
increased. The lowest system capacity level (70%) produced the highest amount
of SUR available.

o The arsenic concentration of groundwater entering into a well screen
increased linearly with the increasing rate of aquitard conductivity, and
decreased with the increasing depth of the aquitard.

- It was found from the model simulation that doubling or halving the
amount of aquifer recharge (SAR) had little or no impact on the groundwater ﬂow
patterns in a homogeneous, uniform model.

0 Increasing the conductivity of deeper layers increased the downward ﬂow

249

rate through the deeper layers in the aquifer system

. Pumping increased the ﬂow from the river to the aquifer system when
shallow aquifer recharge rate (SAR) was less than the extraction rate (Q).

- Pumping at the rate greater than available aquifer recharge (SAR) rate
decreased the ﬂow from the river to the aquifer systems when the SAR was
proportionately greater than the abstracting rate (Q)

- Extraction increased the downward ﬂow rate through deeper layers when
the pumping rate (Q) was greater than the aquifer recharge rate (SAR).

o Pumping proportionately decreased the downward ﬂow rate through the
deeper layers when aquifer recharge (SAR) rates were greater than extraction
rate (Q).

. Pumping at a rate greater than the SAR also increased the arsenic
concentration to the deeper layers.

0 Under natural groundwater ﬂow conditions, increasing the aquifer
recharge rate (SAR) increased the downward ﬂow through the deeper aquifer
layers.

0 Vertical hydraulic gradients and local hydraulic gradients are changed by
extraction patterns of the tube wells.

0 Extractions or pumping signiﬁcantly changed the ﬂow pattern in the
groundwater system. Extraction overall increases the downward ﬂow rate.
Placing well screens at deeper layers signiﬁcantly increases the downward ﬂow

rate through deeper layers.

250

7.3 Recommendations

Against the backdrop of an alarming fall in the groundwater level, the
optimal use of surface water in irrigation is a must for the country's sustainable
development. In order to make use of the contaminated aquifer, safe layers can
be delineated on the basis of redox potential values. For example, it was found
that arsenic concentration varies as a function of aquifer depth. The top most
layers from zero to 10 m are relatively arsenic-free. if an extraction of 500 mm/yr.
or an equivalent amount can be saved, a 4 to 5 m water table could easily be
developed on top of the summer water level, giving locations for small- scale
pumps or walls to be placed to obtain arsenic-free water.

The aquifer delineation is extremely important in terms of supplying safe
irrigation and drinking water. it is recommended that that the top 10 m of aquifer
layer be kept for installation of small-scale dug wells or hand tube wells in order
to ensure a long-term supply of arsenic-free water. The upper layer receives
rainwater, which is high in redox potential values and should be safe for drinking;
extraction for irrigation must be restricted in the top aquifer.

2. it is recommended that the placement of tube well screens at the deep
layers of the aquifer should be stopped in order to reduce the risk of arsenic
migration from the currently contaminated to uncontaminated deeper layers of
the aquifer.

3. It will be beneﬁcial for the aquifer system if the best use is made of

runoff or surface water resources. For example, by controlling the drainage outlet

251

structures, residual water or ﬂoodwater can be stored or used to induce
recharge. These techniques will provide oxygenated water to the aquifer and
keep the redox potential values high.

4. The traditional management practices used by farmers are less efﬁcient
in terms of groundwater use. The study suggests that adjusting the cropping
seasons and pattern both could reduce the volume of pumpage from the aquifer.
Adjusting the transplanting dates could reduce the amount of irrigation
withdrawal from 637 to 433 mm in the Kharif—1 season. Rather than growing rice
twice in a year, if farmers grow wheat in the Kharif—1 season, it can save a
tremendous amount of groundwater extraction. The cropping pattern “HYV—
wheat in Rabi and HYV-rice in Kharif-2 “ can save about 500 mm of water in a
cropping year. Therefore, for the study area of the southwest zone of
Bangladesh, it is recommended that this cropping pattern be followed in order to
increase the level of water table.

5. it was recommended that a soil-based, low-cost arsenic removal
technique could be developed using the already existing millions of shallow
irrigation wells in Bangladesh. The iron reduction and oxidation theory was
discussed. Laboratory studies have shown that arsenic (V) could be removed by
using local materials, river sands rich in iron oxides. Cheaply available
Ferruginous Manganese Ore (FMO) can adsorb arsenic (III), which otherwise
can’t be removed without chemical oxidants. Under oxic conditions, arsenic (V)

species can be adsorbed onto the river sand rich in iron oxides.

252

7.3.1 Recommendations for further research

1. An important part of this study was based on the amount of dissolved
oxygen available in the recharging groundwater. The oxygen diffusion model in
this study was completely based on an analytical approach and is a transient
simulation. This transient simulation was calculated for each of two different
water table conditions. It was not a perfect approach to use a transient model to
understand the complex geochemistry in the aquifer. Therefore it is suggested
that field-testing of dissolved oxygen concentrations under heavy pumping
condition versus limited pumping conditions would be appropriate to evaluate the

amount of dissolved oxygen concentration changes over aquifer depth.

2. The pyrite oxidation theory of arsenic release and the theory of arsenic
release by iron oxide reduction are both time-dependent phenomena. Thus,
validation or rejection of either hypothesis on the basis of space-dependent
relationships in a heterogeneous aquifer is not a reasonable representation. The
ﬁnal conclusion as to the validity of the hypotheses must be developed based on

time-dependent relationships using more reliable ﬁeld and laboratory data.

3. The exact contribution of surface water diversion to the shortage of

aquifer recharge is to be estimated by reliable ﬁeld data in order to demonstrate

the actual impact on the groundwater ﬂow dynamics and ﬂow patterns.

253

Appendix -A:

Computation of surplus volume (SUR) and soil storage capacity for the study
area in SW zone of Bangladesh

254

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Appendix -B:

Computation of amount of shallow aquifer recharge (SAR) for the
groundwater system of the study area in SW zone of Bangladesh

259

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Appendix-C:

Computation of the amount of surplus volume (SUR) under rice irrigated

condition of the study area in Bangladesh

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265

Appendix-D:

Computation of the amount of surplus volume (SUR) under rain -fed rice

condition of the study area in Bangladesh

266

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Appendix E :

Demonstration of velocity distribution patterns of site speciﬁc non
homogeneous model under natural groundwater ﬂow and pumping
condition

271

ME:

Vertical velocity distribution of site smiﬁc actual model actual ﬁeld model
under my condition at seven different layers

Water moving with a up ward velocity above the Tube well screen depth (Layer-2

and Layer-3) and below the screen depth water moves downward (Layer-3 to
Layer-7)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Layer-4 Layer-5 Layer-6

 

 

 

 

 

ndix E-2

Velocity distribution patterns in non-homogeneous model with variation in
hydraulic conductivity values under natural groundwater ﬂow or condition

 

 

 

 

 

 

 

Velocity distribution at layer-1 Velocity distribution at layer—2

 

 

 

 

 

Velocity distribution at layer-3

273

M: E:

Vertical velocity distribution of site speciﬁc actual model actual ﬁeld model
under pumping condition at seven different layers.

Water moving with a up ward velocity above the Tube well screen depth (Layer-2

and Layer-3) and below the screen depth water moves downward (Layer-3 to
Layer-7)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ix E-3

A

 

 

 

 

 

and Layer-3 whereconductivityatthemiddlelayerislﬁgerthandeperlayersM-2

Model).

Variation of hydraulic conductivity values in non homogeneous model at Layer-l , 2

 

 

Layer-1 Conductivity varies from

Layer-1 Conductivity varies from
11 to 25 111/day

25 to 50 m/day-

 

Layer-S Conductivity values varies from 4 to 12 m/day.

 

 

 

 

275

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