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thesis entitled
ORGANIC COMPOUND SORPTION BY
KENTUCKY BLUEGRASS (POA PRATENSIS L.)
LEAVES AND THATCH

presented by
DARIN WAYNE LICKFELDT

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of the requirements for

Masters of ScieagfreeinCrop & Soil Sciences

 

 

 

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ORGANIC COMPOUND SORPTION BY
KENTUCKY BLUEGRASS (Poa pratensis L.) LEAVES AND THATCH

BY

Darin Wayne Lickteldt

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTERS OF SCIENCE

Department of Crop and Soil Sciences

1 994

ABSTRACT

ORGANIC COMPOUND SORPTION BY
KENTUCKY BLUEGRASS (Poa pratensis L.) LEAVES AND THATCH

By
Darin Wayne Lickfeldt

Soil organic matter and plant litter layers are largely responsible for the
immobility of organic compounds in agronomic environments. The objective of
this research was to determine organic compound sorption by turfgrass leaves
and thatch based upon water solubilities (Sw) and octanol/water partition
coelficients. Batch-suspension experiments for five nonionic organic
compounds (phenanthrene, fenarimol, 1,2,4—trichlorobenzene, ethoprop and
acetanilide) with Sw from 1 to 5405 mg L'1 were completed on Poa prafensis
L. cv. Touchdown leaves and thatch. The resulting linear sorption isotherms
suggest the sorption mechanism between organic compounds and leaves or
thatch is consistent with a partitioning sorption mechanism. Sorption
coefficients for thatch were less than those from leaves for the 4 most
hydrophobic compounds, which suggests thatch is a more polar sorbent than
leaves. Acetanilide was more strongly sorbed and phenanthrene was less
strongly sorbed by leaves when mixed with a commercially used pesticide

formulation than when it was not formulated.

To Jodi and Jason,

Do not forsake wisdom. and she will protect you;
love her, and she will watch over you. (Proverbs 4:6)

ACKNOWLEDGEMENTS

I want to wish my most sincere thanks to Dr. Bruce Branham for his
unending guidance. financial assitance and friendship. The professional
contributions of my committee. Dr. Stephen Boyd. Dr. Donald Penner. Dr. Paul
Rieke. and Dr. Matthew Zabik were greatly appreciated. Thanks are extended
to Rhone Poulenc and DowElanco for the gift of 14C labeled ethoprop and
fenarimol. My association with the turfgrass graduate students of Michigan

State University will never be forgotten.

TABLE OF CONTENTS

List of Tables
List of Figures
Introduction

Literature Review
Pesticide Sorption in Soils
The Importance of Soil Organic Matter
No-Tillage vs. Conventional Tillage
The Nature of Turfgrass Thatch
Pesticide Fate in Turf
The Pesticide Formulation's Effect on Pesticide Fate
The Importance of Plant Cuticle

Materials and Methods
Sorbents
Model Organic Compounds
lsotherm Determinations
Formulated Compound Isotherms

Results
Batch-Suspension Isotherms
Sw and Kow Correlations
Formulated Compounds

Discussion

Appendices
Appendix A - Grinding Effects on Thatch Sorption
Appendix B - Radio-labeled Compound Purity and Specific Activity
Appendix C - Time to Equilibrium
Appendix D - Microbial Activity in Batch-Suspension Vials
Addendix E - Batch Suspension Method's Effect on PH
Appendix F - Structures for Nonionic Model Organic Compounds
Appendix G - Log Koc Values for Organic Compounds on Various

Sorbents

Bibliography

vi

vii

13
14
17
22
23

27
28
29
33

34
36
37

51
60
61
65

69
7O

71
72

LIST OF TABLES

Table 1. Characteristics of thatch and underlying soil
Table 2. Properties of nonionic organic compounds

Table 3. Compound purity. specific activity. and concentration range
evaluated as Ci

Table 4. Sorption coefficients, y-int., and r2 values for nonionic organic
compounds on leaves and thatch

Table 5. K. Koc. and Kom values for leaves and thatch

Table 6. Comparison of Koc values for leaves and thatch with empirical
estimations of Koc from the literature

Table 7. Log Kow. log Koc (thatch) and log Koc (soil) from Dell et al. (1994)

Table 8. Log Kow, log Koc (thatch) and log Koc (soil) for nonionic organic
compounds

Table 9. The effect of thatch grinding on acetanilide sorption

Table 10. Log Koc values for organic compounds on various sorbents

vi

28
29

30

35
49

50
55
55

60
71

LIST OF FIGURES

Figure 1. lsotherrns for acetanilide on Poa pratensis L. leaves and thatch 38

Figure 2. Isotherms for ethoprop on Poa pratensis L. leaves and thatch 39

Figure 3. Isotherms for 1,2,4-trichlorobenzene on Poa pratensis L. leaves and
thatch 40

Figure 4. Isotherms for fenarimol on Poa pratensis L. leaves and thatch 41

Figure 5. Isotherms for phenanthrene on Poa pratensis L. leaves and thatch 42

Figure 6. Koc values for nonionic organic compounds on Poa pratensis L.

leaves and thatch 43
Figure 7. Relationship between Kow and super-cooled liquid Sw 44
Figure 8. Relationship between Sw and experimentally determined K values

for Poa pratensis L. leaves and thatch 45
Figure 9. Relationship between Kow and experimentally determined K values

for Poa pratensis L. leaves and thatch 46
Figure 10. Phenanthrene formulated with Primo® blank 47
Figure 11. Acetanilide formulated with Primo® blank 48

Figure 12. Time to reach equilibrium with the batch-suspension method on
leaves 66

Figure 13. Microbial degradation of acetanilide during isotherm determination
68

vii

INTRODUCTION

Estimates of total pesticide usage in the United States are around 500
million kg applied to 150 million ha (16% of the total land mass of the US.)
(Pimental and Levitan. 1986). The environmental fate of pesticides is an issue
of increasing concern. Due to increasing environmental awareness in the last
two decades. the US. Environmental Protection Agency conducted a survey of
drinking water wells in order to clarify the degree of contamination from
pesticides (USEPA, 1990).

When cultural and other options have not been effective for the control of
massive insect. disease and weed infestations. pesticide treatments are the
only option for maintaining healthy crops (Potter and Braman, 1991).
Unfortunately, most of the pesticide fate research has addressed more
traditional forms of agriculture such as crop production and largely ignored
more urban forms of agriculture. There are many dissimilarities between the
turfgrass system and the crop production system. The no-tillage crop
production, forage and turfgrass systems are probably the most similar. No
other system has a dense monostand and thatch layer such as that which
occurs in turf.

The application of pesticides to crops. which are not feeding the world,
may seem unnecessary, but all one has to do is evaluate the importance of the
turfgrass industry in a state such as Michigan to realize that this industry has a
strong place in state economics. As of 1988 there were 2.9 x106 acres of

turfgrass in Michigan (MTF, 1988). Sixty-six percent of this land area was in

2
lawn care/landscaping. 9% was in golf courses and 9% in highways. In that

same year. lawn care operators spent $297 x 105, and golf courses spent $80.3
x 106 on turf maintenance in Michigan.

Soil organic matter and plant litter layers are largely responsible for the
immobility of organic compounds in agronomic environments (Boyd et al.. 1990;
Chiou. 1989). The turfgrass thatch layer and grass leaves (verdure) should
also act as a strong partitioning media for most pesticides.

The objectives of this research were to determine nonionic organic
compound sorption (partitioning) by turfgrass leaves and thatch; secondly, to
correlate water solubilities and octanol/water partition coefficients (Kow) of the
compounds with leaf and thatch sorption; and finally to determine the effect of
surfactants in an EC formulation on nonionic organic compound sorption by

leaves.

LITERATURE REVIEW

E I"| S I' .51

The following review addresses sorption of organic compounds to
sorbents in agronomic systems such as conventional and no-tillage production
agriculture and the relationship of this information with turfgrass systems. The
term sorption can be defined as the absorption, adsorption, desorption to
sorbents such as soil organic matter, or partitioning of a compound into a
sorbent such as soil organic matter.

Sorption is usually determined by one of four methods (Green et al..
1980). Three of these methods were developed to leave soil columns intact
and apply a flow of solution containing the solute of interest to the column.
These methods help simulate the real world. but they may label nonwater
soluble compounds as mobile. when sorption coefficients tell otherwise (e.g..
Hurto and Turgeon, 1979). Soil column methods have their advantages, but
the batch-suspension method has become the most widely used and is
accepted for pesticide registration purposes (USEPA, 1975). The batch-
suspension method involves placing a solute at a known concentration in a
solution (usually water). and then suspending the sorbent in the solution. The
solution containing the solute and sorbent are then shaken until equilibrium is
reached and the quantity sorbed is determined. The advantages of this method
far outweigh the disadvantages. The batch-suspension method reaches
equilibrium quickly, requires very little specialized equipment, and desorption
can also be readily determined (Green et al., 1980). The disadvantages of this
method are that only one data point for sorption determination can be derived

from one suspension, there is poor precision at very low and high solute

4
concentrations. soil (sorbent) aggregate structure is not maintained, and the

method tells nothing about sorption kinetics (Green et al.. 1980).

Sorption is usually quantified with sorption isotherms, which relate a
quantity sorbed (Os) to a quantity remaining in solution at equilibrium (Ce)
(Singh et al., 1990). Most isotherms for sorption of organic pestcides by soil
can be derived from the empirical Freundlich equation, 05 = KCel’" (Hassett
and Banwart. 1989; Weber and Miller, 1989). Where K is the sorption
coefficient, and n is an empirical constant. When n=1. the isotherm is linear,
and this occurs frequently for nonionic pesticides (Chiou. 1989). Weber and
Miller (1989) give thorough descriptions of nonlinear isotherms and when they
occur. The K value (slope) is derived from a plot of the linear isotherm (when
n=1), and it is independent of the solute concentration at concentrations from
10-80% of a compound's water solubility (Chiou. 1989). The primary factor
determining the magnitude of the K value is the water solubility (Sw) of the
solute (Hance, 1976; Chiou. 1989; Doust and Huang. 1992). lsotherm
determination is not strongly temperature dependent from 15-35’C (Chiou.
1989; Hassett and Banwart, 1989).

Wauchope et al. (1983). Singh et al. (1990), and Mingelgrin and Gerstl
(1983) have cautioned against back transformations from experimentally
determined log K values in the literature. which are often reported in the log
form, to quantities sorbed by soils. Differences in techniques. sorbents. and
experimental conditions could lead to variations as large as 75% from what
would be expected. Chiou (1989) has noted how large molecular weight
solutes will often behave unpredictably due to magnification of incompatibilities
between the large solute and the organic sorbent. which he views as

analogous to an organic solvent.

5
The ratio of sorbent(soil):solution in a batch-suspension experiment can

also have a significant effect on K values (Grover and Hance, 1970). They
observed a 5 fold increase in linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-
methylurea(N '-(3.4-dichlorophenyl)- N-methoxy-N—methylurea), Sw = 75 mg L'1
(WSSA. 1989)] adsorption at a soil:solution ratio of 1:10 from that observed at
4:1. The ratio did not have as large an effect on atrazine [2-chloro-4-
(ethylamino)-6-(isopropylamino)-s-triazine, Sw = 33 mg L'1 (WSSA. 1989)]
sorption (Grover and Hance. 1970) or the sorption of several insecticides
(Bowman and Sans, 1985a). Most batch-suspension methods are now
conducted at sorbentzsolution ratios of at least 1:10 (Chiou, 1989; Singh et al.,
1990), and the sorbentzsolution ratio must always be reported.

As noted earlier, the soil aggregate size is disrupted with the batch-
suspension method. and sorption studies conducted on nondisrupted soils
show how aggregate size and soil moisture at time of treatment can greatly
affect sorption (Chiou, 1989; Grover and Hance, 1970; and Hance, 1976).
Dispersion of soil aggregates is much greater at lower soilzsolution ratios. and K
values were lower for linuron and atrazine on large soil aggregates than were
predicted (Grover and Hance. 1970). Another study (Hance, 1976) showed that
soil aggregate size affected sorption of more polar compounds such as
metribuzin [(4-amino-6-(1,1-dimethylethyI)-3-(methlythio)-1 ,2,4-triazin-5(4 H)-
one). Sw = 1220 mg L'1 (WSSA, 1989)]. but a less polar compound such as
simazine [2-chloro-4.6-bis(ethylamino)- s-triazine. Sw = 6.2 mg L'1 (Wauchope
et al., 1992)] showed no dependence on aggregate size. In this same study.
metribuzin was not as easily removed from a soil that was wet when treated as
from a soil that was dry when treated. The opposite was true for the simazine.
which was not as easily removed from a soil that was dry when treated as from

a soil that was wet when treated (Hance, 1976). It will be explained later how

6
nonionic compound sorption by hydrated soils is linear and largely dependent

upon the concentration of soil organic matter (SOM), but when soils are dry or
subsaturated nonlinear isotherms can be expected.

Chiou and Schmedding (1982) were one of the first to propose the
theory of nonionic compound partitioning in soils rather than an
adsorption/desorption mechanism, and this theory has remained largely
unopposed with only a few exceptions (Mingelgrin and Gerstl, 1983). The
evidence for a partitioning mechanism is as follows: 1) compounds nearly
always exhibit linear sorption isotherms over a large concentration range
approaching their water solubilities, 2) there are no competitive effects when
two solutes are placed in the same solution (Chiou et al., 1983, and Chiou
1989). and 3) the lack of temperature dependence on isotherm determination
indicates small exothermic or endothermic enthalpies are involved as would be
expected for a partitioning process (Hassett and Banwart. 1989; Chiou, 1989;
Wauchope et al., 1983). Wauchope et al. (1983) and Wauchope and Koskinen
(1983) have given a thorough review of binding mechanisms. They feel the
important measurement is the free-energy change (AG) associated with the
transition from one state to another. such as from the aqueous state to the
adsorbed state for a given solute. Both of these papers also discuss the
changes in entropy and enthalpy of the solvent close to the "binding site", and
how this can be used to investigate sorption. Weber and Peter (1982)
hypothesized that the binding of acetanilide herbicides was probably planar H
bonding and charge transfer across it bonds with SOM. Wauchope and
Koskinen (1983) demonstrated through thermodynamic analysis that nonpolar
herbicides behave similarly when binding to the soil surface, and the binding

mechanism is non-specific.

7
Partitioning of organic compounds in soils is often compared with the

partitioning of a solute between two immiscible solvents such as octanol and
water (Chiou and Schmedding, 1982 and 1983; Chiou. 1989; Hassett and
Banwart. 1989; Schwarzenbach and Westall, 1981; Gerstl and Mingelgrin.
1984). The partitioning between the two solvents determines a partition
coefficient (Kow) for the two solvent system. which is a constant for a given
solute (OECD, 1981). The Kow is a better modeling parameter than water
solubility (Sw) because of large inconsistencies in reported Sw values in the
literature. Kow can be accurately determined for all but extremely polar
compounds (Gerstl and Mingelgrin, 1984). and the lower a given compound's
water solubility, the better the correlation between its high Kow and soil sorption
(Mingelgrin and Gerstl, 1983).

There is a linear inverse relationship between log Kow and log Sw
(Mflller and Klein. 1992), but this relationship improves when Sw is converted
to super-cooled liquid water solubility for compounds with a melting point
greater than 25°C (Chiou and Schmedding, 1982; Miller and Klein, 1992;
Briggs. 1981; Dell et al. 1994). Chiou and Schmedding (1982) explained the
ideal linear inverse relationship would have a slope of -1. and conversion to
super-cooled liquid water solubility brings the slope closer to the ideal -1. He
evaluated 36 aromatic liquids and solids and correlated log Kow with log Sw
(super-cooled Sw) to get an r2 of 0.994 and a slope of -0.862. The very slight
solubility of water in octanol and octanol in water cannot be ignored because
they have a large influence on the solubility of extremely hydrophobic
compounds in each phase (Mackay. 1977; Chiou and Schmedding, 1982;
Briggs. 1981).

Nearly all sorption studies draw correlations between Kow values and

experimentally determined K values from isotherms (Chiou, 1989). This has

8
proven effective on soil organic matter (Chiou. 1989), lake sediments (Means et

al., 1980). plant cuticle (Kerler and Schonherr. 1988a) and even more complex
lipids such as those found in fish fats (Chiou. 1985) and the organic rich linings
in earthworm burrows (Stehouwer et al., 1993). Mingelgrin and Gerstl (1983)
point out the limitations of using parameters such as Kom, Kow and Sw for
correlations with soil sorption. Correlations between Kow. percent organic
carbon and Sw parameters to sorption by subsoils has proven ineffective in at
least one instance (Ainsworth et al., 1989) due to differences in the structure
and type of organic carbon. Dell et al. (1994) and Veith et al. (1979) have
described the strong linear relationship between Kow values and retention

times from reverse-phase HPLC.

II I I [S 'I D . II II

Soil organic matter (SOM) is the soil fraction that is of primary importance
for organic compound sorption (Weed and Weber, 1974). The SOM includes
polymeric humic and fulvic acids. which are usually defined by their
extractablility from soils (Stevenson, 1982). There is some disagreement as to
when the percent clay may become important in organic compound sorption
(Hassett and Banwart. 1989; Schwarzenbach and Westall, 1981; Chiou. 1989;
Chiou et al., 1983; Gerstl and Mingelgrin, 1984; Wilson et al., 1981; Wietersen
etal.. 1993; Weber and Peter, 1982). but water holding capacity and hydraulic
conductivity are important also (Wietersen et al.. 1993). In general, the percent
clay becomes important when the % SOM is less then 1-2% and the percent
clay is greater than 10-12%. The type and content of the clay minerals in dry
soils can be important (Chiou. 1989; Ainsworth et al.. 1989; Singh et al., 1990;
Loux et al.. 1989). Rutherford and Chiou (1992) explained how "the main effect

of water in drying-wetting cycles is the suppression of adsorption by minerals

9
rather than partitioning into organic matter". Subsoils will show widely varying

sorption characteristics due to different degrees of weathering when the percent
organic matter present is minimal (Ainsworth et al.. 1989).

The differences in sorption of nonionic organic compounds to different
soils and between different studies are probably due to the differences in
quantity and polarity of the SOM (Gerstl and Mingelgrin, 1984; Rutherford et al..
1992; Manilal and Alexander, 1991). For this reason many researchers will
report partition coefficients normalized for soil organic matter (Kom) or organic
carbon (Koc) (Hassett and Banwart. 1989; Chiou, 1989). Kom and Koc values
are determined by dividing the K value from the isotherm by the fraction of soil
organic matter or organic carbon. Therefore, Kom and Koc increase linearly
with increasing organic matter and organic carbon contents of soils. and the
prediction of K values from Koc of soil (K = Koc x organic carbon fraction) is best
when the organic carbon content is moderate to high (Mallawatantri and Mulla.
1992). Mallawatantri and Mulla (1992) showed that every compound does not
have a single Koc value due to the wide variability present in the content and
nature (polarity) of SOM and soil minerals. Karickhoff (1981) presented the
equation Koo = 0.411 Kow.

The affinity of some hydrophobic compounds for soil organic matter is so
strong that the concentration of dissolved organic matter (DOM) in solutions can
not be ignored (Chiou et al., 1986 and 1987; Kile and Chiou. 1989). Organic
matter is dissolved into soil water as water infiltrates through a soil profile, and it
can mobilize compounds that are normally immobile (Stahnke et al., 1991;
Gschwend and Wu, 1985). Chiou et al. (1986) concluded DOM is only
important for the most water insoluble organic compounds such as DDT [1,1'-
(2,2,2-trichloroethylidene)bis(4-chlorobenzene) Sw = 1.2-5.5 pg L'1

(Montgomery, 1993)]. when the DOM is nonpolar, and when the concentrations

10
of DOM are 100x greater than the Sw of the compound of interest. An organic

solute can form micelles with DOM (Kile and Chiou, 1989). and the micelles are
similar to those formed in pesticide formulations (WSSA, 1982). The solubility
enhancement of hydrophobic organic compounds may make the compound
more mobile in the soil's aqueous phase, but the formation of micelles did not
make a hydrophobic compound more easily degradable by microorganisms for
Laha and Luthy (1992).

Caution must also be taken when modifying the solution (water) with
solvents such as organic alcohols. Such modifications may allow an extremely
hydrophobic solute such as DDT to dissolve in the solution, but this modification
will change the solvent strength relative to the sorbent thereby changing the
affinity of the solute for the sorbent (Hassett and Banwart. 1989; Kile and Chiou.
1989). This effect will be more pronounced on nonpolar solutes. One must
also pay attention to whether micelles can be formed with the modifying
solvents and the solute (Hassett and Banwart. 1989; Kile and Chiou, 1989).

Spencer et al. (1988) and Taylor and Spencer (1990) gave thorough
descriptions of the factors involved in organic compound volatilization from soil.
They concluded that compounds accumulate at the soil surface by convection
through soil water before they evaporate. Volatilization increases as the
concentration of a compound increases at the soil surface. Volatilization of
compounds is usually modeled using the Henry's Law Constant (KH). which is
the equilibrium constant for a compound between vapor and a solution.
Chemicals with low Henry's Law Constants have been shown to accumulate at
the soil surface while water is evaporating (Spencer et al.. 1988). This
accumulation was shown to result in increased volatilization over time.
Volatilization of compounds with low KH is determined by the still-air boundary

layer above a soil. water evaporation rate and the K14. When a compound has

11
a high KH, volatilization is largely controlled by the soil properties. Spencer et

al. (1988) accurately predicted the volatilization of two very different compounds
using their KH values.

The one factor in pesticide fate research that is most variable is microbial
degradation (Hance, 1988). Scribner et al. (1992) found that aged simazine
residues did not desorb from a soil as readily as newly added residues.
Therefore, the aged residues did not enter the aqueous phase as readily as
new residues where they could be degraded. As is the case with many
pesticide sorption studies (Bowman and Sans. 1985b). the desorption
mechanisms were not clear, but the longer the simazine remained in the soil.
the less accurate were the predictions of partitioning between soil and water.
After 3-4 months only 5% of the simazine residues that were predicted to be in
soil solution were actually present there. Therefore. the sorbed simazine
residues remained in the soil much longer than expected (based on their half-
life and partition coefficients). because the sorbed residues could not be
degraded. Koskinen and Harper (1990) explain how a compound's bonding to
soils can get stronger as residence times increase. Laha and Luthy (1992)
showed that hydrophobic compounds could be easily mobilized in soils with
surfactants. but cpompounds in micelles were fairly non degradable.

Models have been developed for predicting pesticide sorption (Green
and Karikhoff, 1990) and fate (Wagenet and Rao, 1990). Green and Karikhoff
(1990) presented numerous equations for predicting Koc from Kow and/or Sw
taking the characteristics of the solute into consideration. One of the first fairly
accurate models was PRZM (Pesticide Root Zone Model) (Carsel et al., 1984),
but it was soon discovered that PRZM had its limitations (Wagnet and Hutson,
1986). Wagnet and Hutson (1986) devised a leaching model of their own and
called it LEACHMP (Leaching Estimation and Chemistry Model-Pesticides).

12
They tested their model in the field on the herbicide aldicarb [2-methyl-2-

(methylthio)-propionaldehyde O-(methylcarbamoyl)oxime, Sw = 6000 mg L'1
(Wauchope et al., 1992)] and the results were very promising. Jury et al. (1983,
1984a,b,c) thoroughly described the processes involved in building a model for
pesticide fate in soils. They grouped chemicals into three broad classifications
based on their properties, and determined the necessity for estimations and
determinations of the following parameters to get an accurate picture of the
most probable loss pathway: 1) Organic carbon partition coefficient (Koo). 2)
Saturated vapor density (KH and/or vapor pressure), 3) water solubility (Sw).
and 4) half-life (t1/2). They tested their model on 35 organic compounds, many
of which were pesticides, and most chemicals performed as expected. Jury et
al. (1984c) also predicted the major loss pathway for five organic compounds,
and the model accurately predicted sorption based on reported data for the five
compounds. Other models such as GLEAMS (Ground water loading effects of
agricultural management systems) evaluate soil-pesticide-climate interactions
(Truman and Leonard, 1991). The changes in half-life while a compound is
interacting with the soil causes the largest problems for predicting its late
(Truman and Leonard, 1991).

Boesten and van der Linden (1991) presented a pesticide sorption
model, which coupled Kom values with pesticide transformations. They
showed that at low transformation rates, leaching was most sensitive to Kom,
but at high Kom values the predictions were most sensitive to transformation
rates. Boesten and van der Linden (1991) explained that a change by a factor
of 2 in the Kom value or transformation rate could change the prediction for
leaching by as much as a factor of 10. This same study and Gold et al. (1988)

showed increased pesticide leaching potential in the autumn versus the spring.

13

Wm

The crop residue (litter) layer of a no-tillage corn field has been
evaluated for its ability to immobilize organic compounds (Boyd et al., 1990;
Mallawatantri and Mulla, 1992). Boyd et al. (1990) found 35-60 times higher
sorption of benzene (Sw = 1780 mg L'1), ethylbenzene (Sw = 153 mg L'1)and
1,2,3-trichlorobenzene (Sw = 16.3 mg L'1) to the crop residue layer than to a
soil's A horizon. The high concentration of soil organic matter on low-input
farms would be expected to immobilize the more hydrophobic organic
compounds (Mallawatantri and Mulla, 1992). All Boyd et al. (1990) isotherms,
including binary-solute isotherms, were linear. The high K values for the
carbon rich layers were found to be caused by degrading plant cuticular waxes
in the plant residues, and K values for isolated plant cuticle were 5-15 times
higher than those of the crop residue layer. When K values were converted to
Koc, taking the percent organic carbon of each sorbent into consideration. the
sorbents were indistinguishable. Dell et al. (1994) found similar Koc values for
fungicides on turfgrass thatch and the underlying soil. Boyd et al. (1990)
hypothesized that it may not be necessary to distinguish between crop residue
carbon and SOM carbon for the purpose of predicting K values.

In another study on a no-tillage wheat field, the wheat stubble reduced
the quantity of metolachlor [2 - chloro - N - (2-ethyl-6-methylphenyl) -N - (2-
methoxy -1-methylethyl)acetamide, Sw = 530 mg L'1 (WSSA, 1989)], alachlor
[(2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide, Sw = 242 mg L'
1 (WSSA, 1989)], and acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-
methylphenyl)acetamide, Sw = 223 mg L'1 (WSSA, 1989)] reaching the soil by
50-90% (Petersen et al., 1988). Sixty percent of an application of atrazine was
intercepted by the wheat stubble of a no-tillage field (Ghadiri et al., 1984). After

the first few rainfalls (50 mm in 3 weeks) only 20% of the original application

14
was still in the wheat stubble. Ghadiri et al. (1984) recovered 89% of the initial

application from either stubble or underlying soil at 3 weeks after treatment.

The infiltration rates for no-till systems have proven to be higher than the
infiltration rates of conventional tillage systems. and this is probably the result of
an increase in the number of macropores or macrochannels (Hall et al., 1991;
Andreini and Steenhuis, 1990; and Dick et al., 1989). Hall et al. (1991) had
more leaching of atrazine and simazine occurring in a no-tillage system than in
the conventional tillage system, especially following the first irrigation. They
also evaluated cyanazine {2-[[4-chloro-6-(ethylamlno)-1,3,5-triazin-2-yl]amino] -
2-methylpropanenitrile, Sw = 171 mg L'1 (WSSA, 1989)} and metolachlor. but
neither of these herbicides leached as readily in either no-tillage or
conventional tillage fields. Therefore, the carbon rich plant layer has a strong
affinity for the more hydrophobic pesticides, but the abundance of macropores
may override the ability of the nonpolar tissues to immobilize organic
compounds.

These no-tillage agronomic systems also lessen the movement of
organic compounds by runoff (Hall et al., 1991; Andreini and Steenhuis, 1990;
and Dick et al., 1989), but Kentucky bluegrass sod has proven to be the most
effective method of limiting runoff in at least one instance (Krenitsky and Carroll,

1993).

IbeNaturecflurfgrasslhatcb

Recent research in Cape Cod. MA has shown that pesticide applications
on golf courses do not contribute to ground water contamination (Cohen et al.,
1990). The primary reason pesticides are immobilized in the turfgrass root
zone is the thatch layer (Dell et al. 1994). Thatch was defined by Beard (1973)

as "a tightly intermingled layer of dead and living stems and roots that

15
develop(s) between the zone of green vegetation and the soil surface" of turf.

The layer contains "sclerified vascular strands of stems and leaf sheaths, intact
fibrous roots, nodes and crown tissue" (Hurto et al., 1980). Turgeon et al.
(1975) showed that the abundance of thatch in a turf caused increases in
disease incidence, wilting tendency. and reduced shoot and root growth.
Because thatch has a high lignin (carbon) content (Ledeboer and Skogley,
1967), one might think infiltration rates through the thatch will be minimal, but
that was not the case as Taylor and Blake (1982) showed that once a thatch
was wet (2 cm water applied) the infiltration rate through the thatch layer
equaled that of the underlying soil. Since thatch is an organic rich sorbent, one
would expect significant pesticide sorption in the thatch layer relative to the
underlying soil (Dell et al., 1994; Ledeboer and Skogley, 1967). Thatch does
have a large quantity of macropores (higher porosity) and low bulk density
relative to soils (Hurto et al., 1980; Dell et al., 1994). Hurto et al. (1980)
demonstrated the presence of a discontinuity at the thatch-soil interface that
slowed the unsaturated water flow between thatch and soil.

Sears and Chapman (1979) demonstrated the persistence of
insecticides when they found 60% of a chlordane treatment was still present in
an annual bluegrass fairway's thatch 56 days after treatment (DAT). They
attributed this to lower populations of microorganisms and to high moisture
contents in the thatch layer. When acetanilide herbicides were applied to the
wheat stubble of a no-tillage field. Petersen et al. (1988) found that cooler
temperature under the wheat stubble increased the herbicides persistence.
They also found that acetanilide herbicides were not as easily removed
(desorbed) from wheat stubble that was wet when treated as it was from wheat

stubble that was dry when treated.

16
Thatch is a great environment for microbial activity (Balogh and

Anderson, 1992, p. 278), but the population of microorganisms might be
drastically altered by successive applications of pesticides (Felsot, 1989).
Diazinon [o,o-diethyl-o-(2-isopropyI-4-methyl-6-pyrimidinyl)phosphorothiote,
Sw = 60 mg L'1 (Wauchope et al., 1992)] has been shown to degrade very
readily in irrigated turf where there is a thatch layer present (Branham and
Wehner, 1985). Hurto et al. (1979) first observed that degradation of benefin
[N-butyl-N-ethyl-mma-trifluoro-2,6-dlnitro-p-toluidine, sw = 0.1 mg 1:1
(Wauchope et al., 1992)] and DCPA [dimethyl 2,3,5,6—tetrachloro-1,4-
benzenedicarboxylate, Sw = 0.5 mg L'1 (Wauchope et al., 1992)] in a thatch
was much greater than in a soil under identical conditions. Turgeon et al.
(1975) observed that treatments of bandane (polychlorodicyclopentadiene)
and calcium arsenate (Sw = 130 mg L'1) were immobilized by thatch layers,
and this was correlated with a lack of earthworms and earthworm burrows. The
bandane and calcium arsenate caused an accumulation of thatch (Turgeon et
al., 1975).

Thatch decomposition and turfgrass growing conditions can be improved
by cultivation methods, which incorporate soil into the thatch layer (Hurto et al.,
1980; Danneberger and Turgeon, 1986). Soil incorporation into thatch layers
is also accomplished by earthworms that use the plant tissues as a food source
(Randell et al., 1972). Randell et al. (1972) showed that applications of dieldrin
[3,4,5.6,9,9-hexachloro-1 a,2,2a,3.6,6a,7,7a-octahydro-2,7:3,6-dimethanonaphth
[2,3-bjoxirene, Sw = 0.14 mg L'1 (Montgomery, 1993)] and chlordane [a-
1,2,4,5,6,7,8,8-octachloro-3a,4,7,7a-tetrahydro-4,7-methanoindan. Sw = 51 pg
L'1 (Montogomery, 1993)] resulted in thatch accumulations, by decreasing the
number of earthworm bUrrows near the surface. Cohen et al. (1990) found

chlordane in the ground water of Cape Cod indicating it may be mobile, but

17
they strongly implied that the chlordane came from improperly installed

monitoring wells.

Potter et al. (1990) proved unequivocally that treatments of benomyl
[methyl 1-(butylcarbamoyl)-2-benzimidazolecarbamate Sw = 2.0 mg L"1
(Wauchope et al., 1992)], ethoprop [o-ethyl-s,s-di-n-propylphosphorodithioate,
Sw = 750 mg L'1 (Wauchope et al., 1992)], carbaryl [1-naphthyl-N-
methylcarbamate, Sw = 120 mg L'1 (Wauchope et al., 1992)] and bendiocarb
[2,2-dimethyl-1,3-benzodioxol-4-yl methyl carbamate, Sw = 40 mg L'1
(Wauchope et al., 1992)] at label rates reduced earthworm populations by 60-
99% for 20 weeks. This decrease in the earthworm population was correlated
with a decrease in thatch decomposition. They also observed a decrease in the
population of ants after treating with diazinon, isazofos [o-5-chloro-1-isopropyl--
1H-1,2,4-triazol-3-yl o,o—diethylphosphorothioate, Sw = 69 mg L'1 (Wauchope
et al., 1992)] and chlorpyrifos [0.o-diethyl o-(3,5.6-trichloro-2-pyridyl)-
phosphorothioate, Sw = 0.4 mg L'1 (Wauchope et al., 1992)]. Potter et al.
(1990) observed no decreases in the population of earthworms following
herbicide applications. Even nitrogen fertilization (NH4N03) greater than 5 g N
W2 yr'1, which lowered soil/thatch pH. has caused a decrease in the biomass
of earthworms present, thereby increasing thatch accumulation (Potter et al.,
1985). Smiley and Fowler (1986) hypothesized that successive fungicide
applications increased the rate of tissue production rather than decreasing the
rate of thatch decomposition thereby causing thatch accumulation, but Potter et

al. (1990) showed that the thatch decomposition rate was the dominant factor.

E l' '| El . I I
Balogh and Anderson (1992) presented a thorough list of commonly

used turfgrass pesticides with their water solubilities, Kow, and Koc values (p.

18
268). They also list half-lives (p. 275) and Henry's Law constants (p. 253) for

these pesticides.

Several researchers have evaluated the use of enclosed chambers to
model the fate of pesticides (Branham et al., 1985; Nash and Beall, 1980). In
both of these studies, it was found that the thatch layer and/or the top 2 cm of
soil immobilized the pesticides evaluated. and then they were readily degraded
or metabolized. Because these were enclosed chambers, and 002 evolution
could be quantified, these experiments provided a means for obtaining a mass
balance of the pesticide applied. Unfortunately, the environment within a
chamber does not represent the "real world" due to higher temperatures, lack of
air movement and the filtering out of wavelengths of light. Intact monolith
lysimeters are probably the most realistic method of determining pesticide
mobility (Miltner et al., 1990).

The earliest pesticide fate studies on turf determined the lengths of time
required between time of treatment and when reentry to the lawn or golf course
was safe (Goh et al., 1986a,b; Kuhr and Tashiro, 1978; Sears and Chapman,
1979). These studies all evaluated insecticides, which are usually the most
toxic (with the exception of nematicides) of all pesticides used in turfgrass. In all
of these studies, very little downward movement of the insecticides below the
thatch was ever detected. . Removable chlorpyrifos (Sw = 0.4 mg L”) residues
dropped below the estimated safe level of 0.5 pg cm'2 immediately if watered in
and within 2-6 hours if not watered in (Goh et al., 1986a,b). Kuhr and Tashiro
(1978) found only 10 ppm chlorpyrifos remaining on Kentucky bluegrass leaves
3 weeks after treatment (WAT) regardless of the formulation applied (EC or G).
Kuhr and Tashiro (1978) also found posttreatment irrigation was largely
ineffective for removal of chlorpyrifos and diazinon (Sw = 60 mg L'I) from a turf

canopy. Dichlorvos [phosphoric acid 2,2-dichloroethenyl dimethyl ester, Sw =

19
16 g L'1 (Montgomery, 1993)] levels on leaves were not reduced below safe

levels (0.06 119 cm'2) with 1.2 cm posttreatment irrigation (Goh et al.. 1986a,b).
When irrigated, the dichlorvos did not dissipate below safe levels until 2 hours
after treatment (HAT), and was not detectable by 23 HAT (Goh et al., 1986b).
When not watered in, dichlorvos required 14 hours before it dropped below
safe levels (Goh et al., 1986a). Reducing the spray volume to half the
recommended rate had no effect on chlorpyrifos and dichlorvos fate (Goh et al.,
1986a).

There is evidence that hydrophobic organic compounds will permeate
into the plant cuticle when they are immobilized (Willis et al., 1992a and 1994).
Willis et al.(1994) showed that permethrin [(3-phenoxyphenyl)-methyl(1Fi,S)-
cis,frans-3-(2,2-dichloroethenyI)-2,2-dimethylcyclo-propanecarboxylate, S w =
0.006 mg L'1] and sulprofos [O-ethyl O-(4-methylthiophenyl)-S-propyl
phosphorodithioate, Sw = 0.31 mg L‘I] were immobilized by cotton leaves.
These compounds were more tightly sorbed the longer they were allowed to
remain on leaves before posttreatment irrigation (51 mm h'1 for 60 min) was
applied.

Dell et al. (1994) determined sorption isotherms on Poa pratensis L.
thatch and the underlying soil for the fungicides triadimefon [1-(4-
chlorophenoxy)-3.3-dimethyl 1-1-(1 H -1,2,4-triazol-1-g-1), sw = 260 mg L'1],
vinclozolin [3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3—oxazolidine-2.4-dlone,
Sw = 1000 mg L'1], chloroneb [1,4-dichloro-2,5-dimethoxybenzone, Sw = 8.0
mg L4). The K values for thatch were nearly ten times the K values for the
underlying soil for each compound. The isotherms for vinclozolin on thatch and
triadimefon on soil were not linear (unlike the other compound/sorbent
combinations) due to the ionic nature of the compounds and sorbents. Dell et

al. (1994) also estimated sorption coefficients for numerous other turfgrass

20
fungicides from Kow values (Karickhoff, 1981), but predictions were poorly

correlated with experimentally determined values. Sears and Chapman (1979)
found diazinon and isazofos were nondetectable in an annual bluegrass
fairway's thatch 14 DAT, but Niemczyk and Krueger (1987) had high levels of
isazofos remaining in the thatch at 8 WAT. The lack of mobility for diazinon
applied to turf has already been established (Branham and Wehner, 1985).

Niemczyk and Krueger (1987) determined posttreatment irrigation was
effective for removing isazofos from the turf canopy, and ineffective for moving
the insecticide below the thatch (96-99% of the application could be recovered
from the thatch). Niemczyk (1987) found the same to be true for isofenphos [1-
methylethyl-2-[[ethoxy[(1-methylethyl)-amino]phosphinothioyl]oxy]benzoate, Sw
= 24 mg L’1 (Wauchope et al., 1992)] where 92-99% of the original treatment
could be recovered from the thatch following posttreatment irrigation. This
study was also important in showing that posttreatment irrigation did not make
the immobilized compound more effective for control of PopiIlia japonica.
Therefore, grub control that does occur from these immobilized insecticides
must be due to the grubs coming up to feed on the treated thatch (Niemczyk,
1987), and/or very small amounts of the insecticide could be toxic. Branham
and Miltner (1993) observed no isazofos residues in leachate from intact
monolith lysimeters in turf, and this was also the case for chlorothalonil
[tetrachloroisophtalonitrile, Sw = 0.6 mg L'1 (Wauchope et al., 1992)] and
fenarimol [a-(2-chlorophenyl)-a-(4-chlorophenyl)-5—pyrimidi-nemethanol, S w =
14 mg L'1 (Wauchope et al., 1992)].

Since turfgrass leaves and thatch layers are usually effective for
immobilizing insecticides. the highly water insoluble preemergence herbicides
must also be immobilized by these sorbents. Stahnke et al. (1991) found nearly

all of a pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine,

21
Sw = 0.275 mg L" (Wauchope et al., 1992)] treatment could be recovered from

Poa pratensis L. 'Touchdown' leaves (95%) or thatch (5%). It took 42 days for
the amount of pendimethalin on leaves to drop below that in the thatch and 168
days before no pendimethalin was recovered from the leaves. A Poa pratensis
L. 'Baron' turf, which had a less dense canopy than 'Touchdown', had more
pendimethalin in the thatch relative to the leaves. Stahnke et al. (1991) never
found any pendimethalin residue below 30 cm on either turf, and had a 3 pg L"
pulse of the compound attached to soil colloids in collected leachate following
heavy rainfalls 6-14 DAT and 95 DAT. Krause and Niemczyk (1989) found 77-
100% of pendimethalin, bensulide [s-(0,0-diisopropyl)phosphorodithioate
ester, sw = 5.6 mg L" (Wauchope et al., 1992)], and oxadiazon [3-[2,4-
dichloro-5-(1 -methylethoxy)phenyl]-5-( 1 ,1-dimethylethyl)-1 ,3,4-oxadiazol-2(3 H)
-one, Sw = 0.7 mg L" (Wauchope et al., 1992)] applications in the thatch layer
after treatment. When thatch was not present, 82.5—99.6% of these same
applications was found in the top 2.5 cm root zone of the turf.

Other studies show only minimal leaching (<24 pg L") of 2,4-D [2,4-
dichlorophenoxyacetic acid, Sw = 900 mg L" (Wauchope et al., 1992)] and
dicamba [amine and sodium salts of 2-methoxy-3,6-dichlorobenzoic acid, Sw =
400 g L" (Wauchope et al., 1992)] (Branham and Miltner, 1993; and Gold at al.
1988), but posttreatment irrigation has proven effective for removal of 2,4-D
from leaves (Thompson et al., 1984). Gold et al. (1988) never observed more
than 100 pg L" of either 2,4-D or dicamba in leachate even though suction was
applied and the turf was overwatered. In fact, 2,4-D can be very tightly held by
Poa pratensis L. leaves so that even with vigorous wiping less than 10% of an
application will be removed (Thompson et al., 1984).

Hurto and Turgeon (1979) observed better weed control for many

preemergent herbicides when a Poa prafensis L. thatch was present, but

22
greater mobility of most herbicides through a turf with a thatch was observed

than through a turf with no thatch present. This does not seem consistent with
what would be expected with water insoluble (hydrophobic) compounds. In
batch-sorption experiments Hurto and Turgeon (1979) found more sorption of
benefin, bensulide and DCPA to thatch than to soil. They concluded injury to
the Poa pratensis L. only occurred when a thatch was present, and the
herbicides were moving through the thatch to injure the roots below. It may be
that the density of the thatch was small enough to allow bulk flow of the
herbicides through the thatch before they could reach equilibrium, or there may
have been many roots in the thatch. Roots have been shown to prefer growing

in thatch rather then growing into underlying soils (Hurto et al., 1980).

IIEI"IE ||"Elf| El"|E

Preemergence herbicides, the most water insoluble pesticides used in
turfgrass, would be considered immobile based on their water solubilities and
one might expect them to be completely immobilized by hydrophobic waxes on
leaves. Recommendations call for turt managers to water these compounds in,
and this should only prove effective if the water solubility of the compounds are
greatly increased by the formulation.

Numerous studies conducted on cotton have demonstrated the removal
of pesticide residues from leaves decreases with time, but usually greater than
50% of the mass balance of residue on the leaves could be removed with as
little as 7 mm rainfall (McDowell et al., 1985) when irrigated within-24 h after
treatment (Pick et al., 1984; Smith et al., 1987; Southwick et al., 1983; and Willis
et al., 1991, 1992b, 1994). The type of formulation (emulsifiable concentrate
(EC), wettable powder (WP) or vegetable and mineral oils) had little to no effect

on the retention of pesticide residues on leaf surfaces when applications were

23
followed with rainfall (Baker and Shiers, 1989; Smith et al., 1987; and Willis et

al., 1991), but Pick et al. (1984) observed slightly better retention with an EC
formulation than with a WP. Compounds applied as emulsifiable concentrates
or wettable powders permeate plant cuticles readily, and are difficult to desorb
(Baker et al., 1983). All of the pesticides in these formulation studies were
easily removed from leaves by rainfall occurring within 24 h after the time of
treatment even though the active ingredients were water insoluble.

Bryson (1987) found 10 herbicides were made much less efficacious for
johnsongrass control with posttreatment irrigation, regardless of the water
solubility of the herbicide and the time interval between treatment and the onset
of rainfall (0-240 min). Southwick et al. (1983) observed better leaf penetration
for formulations in water tank-mixes than with formulations in oil. Also, the use
of ultra-low-volume application techniques has been proven ineffective for
improving pesticide retention to cotton leaves (Southwick et al., 1983; and Willis
et al., 1992b). Therefore. pesticides in formulations are not as tightly bound to
leaves as might be expected based on their hydrophobicity. The effect of the
surfactants, solvents and emulsifiers in formulations appears to be particularly
important in determining the sorption/washoff from leaves and may result in a
deeper initial penetration of the pesticide/formulation complex than would be

expected from the active ingredients properties.

IbeJmportanceoLElamCuticle

The cuticular membrane (CM) of plants covers all above ground portions
of a plant, and the specific nature of the cuticular membrane is different for
every species (Holloway, 1982). The membrane is formed as a layer of wax,
and the wax particles are oriented perpendicular to a leaf surface (Baker,

1982). The waxes within a cutin matrix "consist of a variety of long-chain even-

24
numbered (ng-C24) primary alcohols, acetates, aldehydes and fatty acids,

and their hydroxy- and oxyderivatives, and odd-carbon-numbered (C17-C35)

hydrocarbons, secondary alcohols. ketones. ketols, and beta-diketols" (Baker,
1982). The primary function of this membrane is protection of internal leaf
organs, and the prevention from leaf dehydration. Therefore, above ground
portions of plants are relatively impermeable to water and constructed of water-
insoluble compounds. The relative thickness and morphological characteristics
of the cuticle are also dependent upon the stage of development of the plant,
and the climactic conditions under which it is being grown (Baker, 1982).
Immature leaves have been shown to have twice as much wax as more mature
leaves (Baker et al., 1983). Kerler and Schdnherr (1988a) estimate 0.1 m3 of
cuticle in 1 ha of grass with a 1 pm thick cuticle and a leaf area index of 0.5.
This represents a large abundance of cuticle in a turf, and it may be expected
that hydrophobic compounds will readily sorb to this hydrophobic sorbent.

Work conducted by Kerler and Schbnherr (1988b) showed that
permeance coefficients for compounds applied to isolated cuticular membranes
(CM) with log Kow values from 2 to 7.9 could be estimated to within an order of
magnitude of values determined experimentally. More importantly, the
precision of the predictions was best when "the molar volumes of the test
compounds were also taken into consideration by relating the log P
(permeance coefficient) to log Kcm I molar volume", where Kcm = the partition
coefficient between CM and water. The permeance coefficient can be
determined from the slope of a plot of quantity diffused vs. time. Kerler and
Schonherr (1988b) showed that with increasing Kow. increasing permeance
was observed, as would be expected based on partitioning theory. They
presented an equation for predicting permeance coefficients (r2 = 0.98) without

experimentation.

25
Kerler and Schonherr (1988b) explained that compounds with low

polarities will tend to dissolve in soluble cuticular lipids, which are also
relatively nonpolar. If this is the case, then as the polarity of a given solute
decreases (water solubility decreases), the differences in permeance between
species should decrease.

Kerler and Schonherr (1988a) explained that the similarities in the
chemical properties of octanol and cutin arise from the fact that they both have
an 8:1 ratio of nonpolar to polar groups. Prediction of partitioning in cuticles will
only improve when our knowledge of the chemical nature of cuticle improves
(Kerler and Schonherr, 1988a). More data is needed comparing the nonpolar
solvents and naturally occurring nonpolar organic compounds such as cutin or
humus. The nature of the soluble cuticular lipids and non-lipid compartments
within a cuticular membrane (CM) will also lead to better understanding of
partitioning and diffusion processes for CM's. But Kerler and Schcnherr
(1988a) noted that sorption differences between species were smaller than
differences between predicted and experimentally determined Kcm-

Schreiber and Schonherr (1992) considered the uptake of organic
compounds by intact needles from five coniferous tree species. They observed
consistent biphasic uptake of all compounds tested, the first phase of which was
attributed to sorption to cuticular surfaces and was complete in 30 minutes. The
second phase was attributed to uptake into the needle and was much slower to
reach equilibrium. The isotherms for all five species tested had the same shape
and were non-linear. Rates of sorption of a given species and a given
compound differed considerably among individual needles tested. The
differences between the sorption of the organic compounds tested for a given
species are attributed to the Sw of the compounds. Schreiber and Schbnherr

(1992) found only 81-98% of a given compound could be desorbed. because it

26
was trapped within the CM. Therefore, the compounds apparently diffused

through the CM or were compartmentalized.

Boyd et al. (1990) evaluated sorption of non-ionic organic compounds by
isolated plant cuticle, corn residues (in a no-tillage field), the soil/residue layer
immediately underlying the corn residue, and the A horizon of the soil. By
evaluating sorption coefficients for all of the above sorbents, Boyd et al. (1990)
was able to speculate on the important role of cuticular sorption of organic
compounds once the cuticle becomes part of the leaf litter. The observed
uptake of the organic compounds by cuticle were 5 to 15 times higher than for
the corn residue layer. This is consistent with a hypothesis that claims cuticle is
a major partition medium. As expected, sorption was inversely related to water
solubility. Kom values were similar for all four sorbents tested, and they closely
resembled the Kow values. Boyd et al. (1990) also evaluated binary-solute
uptake but observed no competitive effects between solutes. This is consistent
with a partitioning sorption mechanism, and as noted earlier (Kerler and
Schbnherr, 1988a) the organic solvent octanol appears to be analogous to the
CM when considering sorptive capabilities. Future research should consider
the effects of further plant decomposition on plant cuticle sorption. Since the
organic compounds Boyd et al. (1990) evaluated did not differ between organic
binding sites on different sorbents (similar Kom values), we may be able to
conclude that cuticular sorption of organic compounds is nearly identical to

sorption by humic materials in the soil.

MATERIALS AND METHODS

Sorbents

Leaves and thatch were collected from an 8-year old 'Touchdown' Poa
prafensis L. turf. The turf was grown over a Mariette sandy loam soil (fine-
loamy, mixed, mesic Glossoboric Hapludalfs), received 195 kg N ha" yr", and
at least 2.5 cm irrigation or rain per week. The 5.1 cm of leaf tissue (verdure)
was removed in late August and stored at -9° C. Thatch and soil were also
collected with a 15-cm diameter golf green cup cutter to at least 7 cm deep. The
thatch-soil boundary (4 cm deep) was identified by the lowest depth at which
rhizomes could be seen. The thatch was separated from the soil (4 cm deep).
Thatch plugs were floated in distilled/deionized H20 and gently agitated to
remove as much soil as possible. The thatch was allowed to dry at 21° C for 96
h and was stored at -9‘ C. Table 1 shows the pertinent thatch characteristics (%
organic carbon, % organic matter, moisture, particle size. pH, CEC, and bulk
density) before and after washing. Stored thatch and leaf samples were oven-
dried (70° C for 48h) to determine percent moisture. The fresh weight was
corrected for percent moisture to determine dry weights for isotherm
calculations.

The washed thatch was not ground because grinding could release
colloidal material into solution and create more surface area with additional
sorption sites. Initial experiments showed that grinding of thatch and removal of
soil did not alter acetanilide sorption (Appendix A). Sorbents evaluated in the

initial experiment were thatch ground to 2 mm and unground thatch.

27

28
Table 1 - Characteristics of thatch and underlying soil

 

 

 

Characteristic * Underlying Soil Thatch
Unwashed Washed
% OM" 82.5 :1: 6.7 82.1 :1: 4.0
% OC" 2.7 :l: 0.4 38.6 :I: 3.9 39.8 a: 0.9
% Moisture by Wt.” 5.5 :f: 0.5 7 a: 2 79 :1: 2
3qu density (9 cc") 1.74 :t 0.02 0.38 :l: 0.04 ---
CEC 7
% Clay 16
% Silt 15
% Sand 69
pH 7.5 7.5 7.5

 

‘ Determined Michigan State University Soil Testing Laboratory.
“ Percent OM and DC of leaves was 76.3 :I: 1.0 and 37.8 +/- 2.8, respectively.
“‘ Determined by drying at 70‘ C for 48 h.

Modelmanicmmpounds

The nonionic organic compounds (MOC's) chosen were acetanilide. ethoprop
(o-ethyl s,s-dipropyl phosphorodithioate), 1,2,4-trichlorobenzene, fenarimol [a-
(2-chlorophenyl)-a-(4-chlorophenyI)-5-pyrimidinemethanol], and phenanthrene.
The water solubilities of these compounds are 5405, 750, 49, 14 and 1 mg L",
respectively. Table 2 shows water solubilites (Sw), octanol/water partition
coefficients (Kow). molecular weight, melting and boiling points of MOC's used.
The purity of all MOC's was verified by thin-layer chromatography (Appendix B)
(Touchstone and Dobbins, 1983). All radio-labeled compounds were stored at
-9° C while non-labeled compounds were stored at 21° C. The Kow for
acetanilide was determined experimentally (OECD, 1981). The Kow values for
all other compounds were retrieved from the literature (Table 2). These
compounds were also chosen for their chemical stability to minimize

degradation losses during sorption measurements and to give a range of Sw.

The structure of all five compounds were compiled into Appendix F.

29
Table 2 - Properties of nonionic organic compounds

 

 

MOC Sw log Sw' log Kow MOI. Wt. Mt. Pt. Bl. Pt
(rm L") (moi L") (C) (C)

acetanilide 5405" (-1.19) 1.74'" 135.2 114 304
ethoprop 7501 -2.51 3.59§ 242.3 20 88
1,2,4-TCB 491 -3.57 4.021 181.5 17 213
fenarimol 141 (-3. 12) 3,401 331 .2 1 15 NA
phenanthrene 11 (-4.48) 4,571 178.2 100 340

 

" Numbers in parenthesis represent super-cooled liquid water solubilities
for compounds that are solids at 25° C (Chiou and Schmedding, 1982).
“ Windholz et al., (1983).

“‘Determined by standard method from OECD (1981 ).

T Chiou (1989)
1 USEPA (1992)
§ Rhone Poulenc Technical Bulletin for Ethoprop - CHIPCO@ MOCAP® Brand 10G Pesticide.

lsotbemfleterminations

Batch-suspension experiments were conducted to determine sorption
isotherms. The initial concentration (Ci) of all compounds ranged from 10-80%
of the compounds water solubility. The Ci included both 1"'C-labeled and non-
labeled MOC. The range of Ci values evaluated, purity and specific activity for
each MOC are listed in Table 3. The low Ci's of hydrophobic compounds
(phenanthrene, fenarimol, and 1,2,4-trichlorobenzene) were verified by HPLC.
Three grams of leaves or 12.0 g of thatch were placed in a 30 ml Corex (Baxter
Sci.) centrifuge tube. Twenty-six ml of solution containing the MOC were
dispensed into leaf tubes and 18 ml into thatch tubes. The sorbentzsolution
ratios were 1.2/10 and 6.7/10 for leaves and thatch, respectively. Head space
in tubes was minimized to lessen volatilization. Three tubes (replications) for
each Ci were equilibrated simultaneously. The tubes were covered with foil
and Teflon-lined screw-caps, and placed in a horizontal position on a rotary

platform shaker. They were shaken at 120 rpm for 24 h in the dark. The

30
temperature was maintained at 25° C in order to maintain the predetermined

water solubility of each solute.

Table 3 - Compound purity, specific activity, and concentration range evaluated
as Ci

 

 

MOC Purity' (96) 14c Spa, Act % Sw Range
(Bq mmol 4) Evaluated as C i
acetanilide 97 2.2 x 109 10 - 80
ethoprop 96.2 1.8 x 108 10 - 80
1,2,4-TCB 99 2 x 106 7 - 62
fenarimol 97 2,5 x108 12 - 83
phenanthrene 99 3,1 x108 10-80

 

" Reported by Sigma Chemical Co., verified by thin-layer chromatography or HPLC (Appendix B)

The initial MOC concentrations were verified by collecting 1 ml
subsamples as solutions were dispensed into the tubes. After the 24 h
equilibration time, the thatch tubes were centrifuged at 9264 g for 20 min.
Centrifugation was not necessary for tubes containing leaves. Three 1-ml
aliquots of the supernatant were removed from each tube, and the mean
equilibrium concentration (Ce) was determined. Initial concentrations (01) and
Ce were quantified by liquid scintillation counting for 10 minutes on a liquid
scintillation counter (Packard Tri-Carb Model 1500) with 20 ml aqueous (Safety
Solve) counting cocktail (Research Prod. Int. Corp, Mt. Prospect, IL.) in each
vial to determine disintegrations per minute (DPM). Ouenched standards
(Dupont) were used to derive a quench curve, which related the degree of
quenching to a tSlE (Transformed Spectral Index of the External Standard
Spectrum) value derived from the Compton spectrum induced in the sample by

the external standard source (133 Ba).

31
The quantity sorbed (Os) in mg kg" was determined by 05 = [(01 -

Ce)Vs] I Ms, where C1 and Ce are the initial and equilibrium supernatant
concentrations (mg ml"); V5 is the volume of solution added to each tube (ml);
and Ms equals the mass of sorbent (kg). All isotherms were determined twice to
verify reproducibility of results. This method gave 24 points for the regression
(isotherm) plot.

Volatile organic compounds (ethoprop and 1,2,4-trichlorobenzene) and
fenarimol, which can photodecompose under artificial light, were added to
tubes containing no sorbent. These tubes were treated identically as tubes
used in earlier batch-suspension experiments to determine losses during
equilibration. Equilibration concentrations (03) were corrected for losses
before 05 was calculated.

Preliminary experiments showed equilibrium for acetanilide and
phenanthrene was reached in 4 h (Appendix C). The most widely used
equilibration time in published isotherm studies is 24 h, and because there was
no further sorption between 4 and 24 h in our experiments, we chose a 24 h
equilibration period.

Another experiment was conducted to determine if microbial
decomposition of acetanilide occured in the 24-h equilibration period.
Decomposition would lead to changes in structure that would change sorptive
characteristics. Microbial activity can be retarded in batch-suspension solutions
by the addition of sodium azide (Wahid et al., 1980). This addition stops
microbial degradation without drastically changing the solvent characteristics.
There was no significant degradation of acetanilide in this experiment
(Appendix D).

The change in supernatant pH was determined after the 24-h

equilibration period for leaves and thatch in distilled/deionized H20. Solution

32
pH decreased from 7.48 :1: 0.48 to 5.20 :1: 0.94 during equilibration with leaves

and from 7.68 :1: 0.39 to 7.28 :t 0.09 for thatch (Appendix E), which suggests it
may be necessary to buffer the solutions in batch-suspension experiments with
these sorbents. Buffering was not necessary for the M005 evaluated here
since they are non-ionizable.

Solubility enhancement (Sw') can be estimated by a compound's
partition coefficient for dissolved organic carbon (K000) and the concentrations
of dissolved organic carbon (DOC) with the equation Sw' = Sw (1 + K009)
(Chiou and Schmedding, 1982). Based on this equation, phenanthrene was
the only compound evaluated that may experience water solubility
enhancement at the relatively large [DOC] present in the supernatant solutions
(300 mg L" for thatch and 450 mg L" for leaves). Phenanthrene's Sw would
be increased to 1.1 mg L" (a 10% increase), and this would not drastically
change the isotherm for the compound. This correction is often necessary for
extremely hydrophobic compounds such as DDT (Sw = 1.2-5.5 pg L").

The sorption coefficients (K) for each compound on thatch and leaves
were determined by plotting 05 and Ce and finding the slope of the regression
line. The K values were then plotted versus other easily determined
parameters (Kow. Sw, Koo. Kom) to evaluate all relationships that may exist.
The K values for all compounds can be converted to Koc and Kom values using
the relationship of Koc = K I (organic carbon fraction) or Kom = K / (organic
matter fraction). The percent organic carbon (00) was 37.8 :1: 2.8% for leaves
and 39.8 :1: 0.9% for thatch, while the percent organic matter (OM) was 76.3 :t
1.0 for leaves and 82.1 :1: 4.0 for thatch.

33
EormulatedfiompoundJsotberms

Initial phenanthrene and acetanilide solutions were spiked with 0.5% v/v
Primo® (Ciba Geigy) formulation (emulsifiable concentrate) blank to determine
how the presence of a formulation may affect isotherms. Formulation
information is usually proprietary, but Ciba Geigy disclosed the following
constituents: primarily aromatic petroleum hydrocarbons (C-11 and C-12), less
then 8% light mineral oil, and less then 10% anionic and nonionic emulsifiers.
Phenanthrene and acetanilide solutions of approximately 10, 40, 60 and 80%
of Sw were made, and leaf isotherms were determined as before. All samples
were left in the dark for 1 h before liquid scintillation counting was conducted.
because the Primo @ formulation caused counting cocktail to fluoresce in the

presence of artificial light.

RESULTS

Batchfiuspensimlsotberms

All 5 nonionic organic compounds (acetanilide, ethoprop. 1,2,4-
trichlorobenzene, fenarimol, and phenanthrene) had linear (r2 > 0.922) sorption
isotherms for the concentration ranges tested (Figures 1-5). Values of K are the
slope of the sorption isotherm plotted as a regression line through the data
points for quantity sorbed (05) and the equilibrium concentration (Ce). All
compounds, with the exception of phenanthrene, had y-intercept values far from
the ideal of zero. All figures and graphs are compiled at the end of the results
section.

The results of the isotherm determinations are summarized below and in
Table 4. Acetanilide (Sw = 5405 mg L") had a K value for Poa pratensis L.
leaves of 3.53 a: 0.42, while the thatch K value was 6.32 :1: 0.28. The regression
equations for acetanilide 0n Poa pratensis L. were Os = 3.53 Ce + 325 (r2 =
0.939) for leaves and 05 = 6.32 Ce + 252 (12 = 0.991) for thatch (Figure 1).
Ethoprop (Sw = 750 mg L") had a K value for Poa pratensis L. leaves of 32.4
:1: 2.40, while the thatch K value was 16.2 1 0.69. The regression equation for
ethoprop 0n Poa pratensis L. leaves was 03 = 32.4 Ce - 134 (r2 = 0.971) and
the equation for thatch was 05 = 16.2 Ce - 39.5 (12 = 0.943) (Figure 2). The K
values for 1,2,4-trichlorobenzene (Sw = 49 mg L") were 167 :1: 7.29 for leaves
and 19.2 :1: 1.13 for thatch. The regression equations used to determine these K
values were 05 = 167 C3 - 84 (r2 = 0.990) for leaves and 05 = 19.2 03 +0.32
(r2 = 0.982) for thatch (Figure 3). Fenarimol (Sw = 14 mg L") had a leaf K

34

35
value of 155 :1: 13.3, while the K value for thatch was 86.5 :1: 6.92. The

regression equations for the isotherms were 05 = 155 Ce + 9.69 (r2 = 0.962) for
leaves and as = 86.5 Ce + 3.41 (r2 = 0.922) for thatch (Figure 4). The least
water soluble compound evaluated, phenanthrene (Sw = 1 mg L"), had very
high K values for leaves (2520 :1: 307) and thatch (793 :1: 80.4). The regression
equations for the isotherms were 05 = 2520 Ce + 0.98 (r2 = 0.929) for leaves
and 03 = 793 Ce + 0.075 (r2 = 0.952) for thatch (Figure 5).

Table 4. Sorption coefficients, y-int., and r2 values for nonionic organic
compounds on leaves and thatch.

 

Leaves firhatch
Compound K y-Int. r2 K y-lnt. £2

acetanilide 3.53 :I: 0.42 325 :t 1070 0.939 6.32 :1: 0.28 252 t 396 0.991
ethoprop 32.4 :1: 2.40 ~134 :t 462 0.971 16.2 :t 0.69 -38.5 t 140 0.943
1,2,4-tcb 167 t 7.29 -84 :1: 21 0.990 19.2 :1: 1.13 0.32 t 2.6 0.982
fenarimol 155 :1: 13.3 9.69 t 16 0.962 86.5 :I: 6.92 3.41 :1: 5.3 0.922

Jhenanthrene 2520 :t 307 0.075 :1: 0.37 0.929 793 :1: 80.4 0.98 :1: 1.7 0.952

The K values for all compounds were converted to Koc and Kom values
using the relationship of Koo = K I (organic carbon fraction) or Kom = K /
(organic matter fraction) (Figure 6, Table 5). The percent organic carbon (00)
was 37.8 1 2.8% for leaves and 39.8 :1: 0.9% for thatch, while the percent
organic matter (OM) was 76.3 a: 1.0 for leaves and 82.1 :1.- 4.0 for thatch. The
Koo, Kom and K values for sorption of all compounds by leaves were always
greater than the corresponding K values for thatch other than the most water

soluble compound evaluated, acetanilide. Therefore, the Poa pratensis L.

36
leaves appear to be a less polar (more hydrophobic) sorbent than Poa

prafensis L. thatch. The K values were always greater for less water soluble
compounds for both sorbents with only one exception. The Kleaves value for
fenarimol was lower than the Kleaves value for 1,2,4-trichlorobenzene.

The Koo values for leaves and thatch were compared with empirical
estimates of Koo compiled by Green and Karickhoff (1990) (Table 6). The
equation: log Koc = -0.68 log Sw (pg ml-1) + 4.273 originally from Hassett et al.
(1980) provided the closest estimate for the five compounds, but Green and
Karickhoff (1990) have shown that different empirical equations fit different

families of organic compounds.

Sw.and.Kow_CoLLelations

When the Sw of compounds was converted to super-cooled liquid Sw.
with the melting point correction (Chiou and Schmedding, 1982), the
relationship between log Kow and log Sw was greatly improved (Figure 7). The
super-cooled liquid water solubility was derived from the equation log Sw = (log
Kow - 0.710) [0862 derived by Chiou and Schmedding (1982) for 36 aromatic
liquids and solids that had their Sw converted to super-cooled liquid Sw (r2 =
0.994). This correction of Sw was only conducted for compounds that are
solids at 25° C. and it is a way of correcting for the energy needed to dissolve a
solute in a solution. With the conversion to super-cooled liquid Sw (mol L"),
log sw = -0.828 log Kow + 1.00 (r2 = 0.915). but when the sw (moi 1:1) was not
I converted, log Sw = -0.590 log Kow + 1.45 (r2 = 0.705) and there was no longer
a linear relationship (P=0.075). The super-cooled liquid log Sw was plotted
versus the experimentally determined K values for both Poa pratensis L. leaves
and thatch (Figure 8). The regression equations were log Kleaves = -0.842 log

sw - 0.529 (r2 = 0.974) and log Kthatch = -0.567 log sw - 0.0586 (r2 = 0.719).

37
The lower r2 for thatch is an indication that thatch is a less homogeneous

sorbent (more variety of polar and nonpolar functional groups) than leaves.
The relationship between log Kthatch and log Sw would have been significant
(r2 = 0.934) if 1,2,4-trichlorobenzene did not have such a low kthamh value.
The relationship between log Kow and the experimentally determined K
values for leaves and thatch were log Kleaves = 0.906 log Kow - 1.17 (r2 =
0.846) and log Kthatch = 0.567 iog Kow - 0.339 (r2 = 0.540) (Figure 9). The

relationship between log Kthatch and Sw or Kow was not linear (P>0.05).

Eormulatedfiompounds

When the least water soluble compound evaluated, phenanthrene, was
formulated in a tank mix with 0.5% v/v Primo® blank, the Kleaves value was
drastically reduced from 2520 :1: 307 to 59.8 :1: 13.8 (Figure 10). The regression
equation for this formulated isotherm was 05 = 59.8 Ce + 4.18. The formulated
phenanthrene isotherm also had a lower r2 (0.793) than the nonformulated
isotherm (r2 = 0.929). Acetanilide produced a higher Kleaves value (6.58 :l:
0.97) when formulated in a tank mix with 0.5% v/v Primo® blank than when it
was not formulated (3.53 :l: 0.42) (Figure 11). The variability for the formulated
acetanilide isotherm was only slightly increased (r2 = 0.899) over that of the
nonformulated isotherm (r2 = 0.939). The regression equation for the

formulated acetanilide isotherm was 05 = 6.58 Ce + 4390.

38

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DISCUSSION

Hydrophobic organic compounds are less strongly sorbed by
hydrophobic sorbents when formulated with surfactants (Kile and Chiou, 1989).
or when they are in the presence of dissolved humic materials (Chiou et al..
1986 and 1987). A formulation's surfactants form micelles that solubilize
organic compounds (active ingredients) (WSSA, 1982). When phenanthrene
was formulated with Primo® blank, the Kleaves value decreased drastically
from 2520 :l: 307 to 59.8 :f: 13.8 (Fig. 10). Acetanilide. when formulated with
Primo® blank, had its KIeaves value increased from 3.53 e 0.42 to 6.58 a: 0.97
(Fig. 11). Therefore, the surfactants had the opposite effect on acetanilide
relative to phenanthrene. Acetanilide was more strongly sorbed to leaves with
the Primo® blank; however, the increase in sorption was minimal for this the
most water soluble compound evaluated. Based on these results and work by
Kile and Chiou (1989), Pick et al. (1984) and Liu et al. (1992), the effects of
surfactants on pesticide sorption to Poa pratensis L. leaves and thatch appear
to be significant.

If the solvents. surfactants and other ingredients in a pesticide
formulation volatilize or degrade in the turf, then the effect of formulations may
be minimal. Turf managers will often water in pesticides immediately following
an application to make them less susceptible to volatilization and
photodecomposition, and this increases the likelihood that the watering in of the
formulation will effect the sorption of the applied pesticide. This also minimizes

the risk of human exposure to the active ingredients. Many of the pesticides

51

52
(active ingredients) used on turf are relatively water insoluble (Balogh and

Anderson, 1992).

The application of hydrophobic organic compounds to turf is a very
common practice. and the leaves should intercept a large percentage of an
application (Stahnke et al., 1991). Once these hydrophobic compounds come
in contact with the leaves they may not be readily desorbed because of the high
Kleaves values expected for such hydrophobic compounds. If the compounds
are desorbed. they will then come into contact with the thatch layer. The
chance of degradation is very great for such compounds because of the long
residence time that results from the tortuosity of their pathway through the
thatch (Taylor and Blake. 1982; and Hurto et al., 1980).

Numerous studies conducted on cotton have demonstrated the removal
of pesticide residues from leaves decreases with time. but usually more than
50% of the mass of residue on the leaves could be removed with as little as 7
mm rainfall (McDowell et al., 1985) when applied within 24 h after treatment
(Pick et al., 1984; Smith et al.. 1987; Southwick et al., 1983; and Willis et al..
1991, 1992b, 1994). Once the water in the formulation dries from the leaf
surface. leaving the surfactant residue and the organic solute on the leaf
surface. the effect of the formulation's surfactants should be minimal. However,
Pick et al. (1984) found pesticides in formulations were not as tightly bound to
leaves as might be expected based on the active ingredient's hydrophobicity.
Stahnke et al. (1991) found that even after posttreatment irrigation nearly all
(95%) of a pendimethalin (Sw = 0.275 mg L'1) treatment could be recovered
from Poa prafensis L. 'Touchdown' leaves. The effect of the surfactants.
solvents and emulsifiers in formulations will result in deeper initial leaf
penetration of a pesticide than would be expected from the active ingredient's

properties.

53
The linearity of the sorption isotherms (r2 > 0.922). over the

concentration (Ci) ranges tested. indicates that sorption resembles a
partitioning phenomena rather than an adsorption (surface) phenomena on
Poa pratensis L. leaves and thatch (Chiou and Schmedding, 1982). This
theory is further supported on leaves by the linear relationship between the
experimentally determined Kleaves values and Kow values. The sorption of the
organic compounds to Poa pratensis L. leaves is analogous to the partitioning
of the compound between two immiscible solvents (octanol and water) at
concentrations from 10 - 80% of the water solubility of a given nonionic organic
compound.

From Figures 1-5 and Table 4, it is apparent that most isotherms do have
a y-intercept close to zero. When the standard deviations of the y-intercept
values are considered, the regression line would include the origin without
drastically changing the K values. The isotherm for 1,2,4-trichlorobenzene
produced a y-intercept that was significantly less than zero {-84 a: 21). If the
isotherms are not linear at lower concentrations (<10 % of Sw), the nonlinear
portion of the isotherm may resemble the L-type isotherms described by Weber
and Miller (1989) where a portion of the isotherm has an exponential increase
or decrease in 03 vs. Ce at lower concentrations. Ironically, these lower
concentrations may be a more realistic portrayal of the actual quantity of
residue reaching the thatch layer in the field. If these lower concentration
ranges are nonlinear, the thatch isotherms may resemble the nonlinear
isotherms reported by Dell et al. (1994) for thatch. Additional data is needed at
these lower concentrations (<10 % of Sw) to determine the sorption
characteristics in this region of the isotherm.

The more hydrophobic (lower Sw) the organic compound, the greater the

Kleaves and Kthatch values for both sorbents, with one exception. Fenarimol

54
had a lower Kleaves value than expected for a compound with a water solubility

of only 14 mg L'1. Fenarimol did not behave as predicted by its Kow of 3.40.
Fenarimol's Kow and Kleaves were lower than the Kow (4.02) and Kleaves for
1.2,4-trichlorobenzene. Since the Koc values were usually much greater for
leaves than for thatch (acetanilide being the only exception), the Poa pratensis
L. leaves are a more hydrophobic sorbent than thatch (Figure 6). This seems
reasonable since thatch is primarily made up of "sclerified vascular strands of
stems and leaf sheaths. intact fibrous roots. nodes and crown tissue" (Hurto et
al.. 1980) that are primarily lignin (Ledeboer and Skogley, 1967) with very
limited amounts of plant cuticle. The leaves are covered with hydrophobic
cuticular waxes.

The Koc and Kom values for all 5 of the nonionic organic compounds
evaluated are listed in Table 5. The leaves and thatch had nearly identical
percent organic carbon (00) and organic matter (OM), thereby the same divisor
in the equation Koc = K I (CC fraction). The Koc and Kom values have proven
useful for comparing the hydrophobicity of sorbents with different 00 and OM
contents (Hassett and Banwart. 1989; Chiou. 1989; Boyd et al., 1990). Dell et
al. (1994) observed slightly lower Koc values for thatch than soil (Table 7) for
chloroneb (1 ,4-dichloro-2.S-dimethoxybenzone), triadimefon [1 -(4-
chlorophenoxy)-3.3-dimethyl-1-1-(1 H-1 ,2,4-trizol-1 -g-1 )butanone] and
vinclozolin [3-(3,5-dichlorophenyl)-5-methyl-S-vinyl-1,3-oxazolidine-2,4-dione]
indicating these compounds would not have as strong sorption to Poa prafensis
L. thatch organic carbon as they would to soil organic carbon. Table 8 lists the
log Kow. log Koc for thatch (39.8% OC). and log Koc for soil (see Appendix G)
for the 5 organic compounds evaluated in this study. The leaf Koc values are

always greater than the thatch Koc values with the exception of acetanilide.

55

Table 7. Log Kow. log Koc (thatch) and log Koc (soil) from Dell et al. (1994).

 

 

Wmund log Kow log Koc (Thatch) log Koc (soil)“
chloroneb 2.71 2.72 2.85
triadimefon 2.39 2.46 2.54
vinclozolin 3.01 3.14 3.20

 

‘ Poa pratensr's L. cv Wabash turf (thatch = 30% OC).
“ Silty clay loam soil with 3.1% 00.

Table 8. Log Kow. log Koc (thatch) and log Koc (soil) for nonionic organic
compounds.

 

Compound log Wow log Koc (Thatch) log Koc (leaves) log Koc (sofl)'
acetanilide 1 .74 1 .20 0.97 -----
ethoprop 3.59 1 .61 1.93 1 .85
1,2,4-tcb 4.02 1 .68 2.65 2.94
fenarimol 3.40 2.34 2.61 2.78
phenanthrene 4.57 3.30 3.82 4.59

 

‘ Koc values for various sorbents are listed in Appendix G with references.

56
Both thatch and leaf Koc values were lower than soil Koc values for

1,2,4-trichlorobenzene, fenarimol and phenanthrene. Dell et al. (1994)
observed this same result, but their Koc values for thatch were closer to both the
Koc values for soil and Kow values. The soil Koc value for ethoprop was
greater than the thatch Koc value but slightly less than the leaf Koc value. A soil
Koc value for acetanilide was not found in the literature. In both this study and
Dell et al. (1994), the thatch organic carbon was less hydrophobic than soil
organic carbon and 1-octanol. Turfgrass leaves and thatch have a much
greater mass of organic carbon than soils, but soil organic carbon (matter) is a
more hydrophobic sorbent. Turfgrass leaves and thatch will have a greater
sorption capacity than soil organic carbon (matter) because of the abundance
of organic carbon (matter) in these sorbents.

Dell et al. (1994) had a thatch with 30% OC, and the thatch in this study
was 39.8 :1: 0.9% 00. Therefore, the thatch from Dell et al. (1994) may have
been in a more decomposed state and more closely resembled soil organic
carbon than the thatch in this study. Poa pratensis L. leaves and thatch have a
strong affinity for hydrophobic organic compounds due to their high 00
contents. but the hydrophobicity of the organic carbon in the leaf and thatch
tissues seems to be less than that of soil organic carbon.

The empirical estimates of Koc compiled by Green and Karickhoff (1990)
may be successful for predicting Koc values for organic compounds it the
compound's family is taken into consideration, but the Koc range for these
empirical equations is 10 - 106 (Table 6). Empirical estimates for Koc were
usually greater than the experimentally determined Koc values. All of the Koc
equations were derived from soil sorption research. It was interesting that the
soil Koc values from the literature are also greater than the experimentally

determined Koc values in this study. Therefore. the usefulness of such

 

57
correlations with soil values must be scrutinized. Based on the results of this

study. the empirical estimates of Koc derived for soils (Green and Karickhoff.
1990) should not be used to predict Koc values for Poa pratensis L. leaves and
thatch.

The differences in sorption by cuticular waxes between species has
been shown to be negligible in at least one instance (Kerler and Schonherr,
1988a). who found sorption differences by needles from various coniferous tree
species were due to the surface area of the needles rather than the nature of
the cuticular membrane (Schreiber and Schonherr. 1992). Therefore, the
sorption of hydrophobic compounds to various turfgrass species' leaves should
be similar. Future research will have to address, the hydrophobicity and state of
decomposition of the thatch layer under dissimilar turfgrass systems.

In the batch-suspension isotherm determinations, equilibrium was
achieved in 4 h (Appendix C) for both acetanilide (Sw = 5405 mg L'1) and
phenanthrene (Sw = 1 mg L‘1). Future research should determine if
equilibrium is achieved on leaves or thatch under field conditions. Assuming
the compound does not have a long residence time on the leaves. due to
removal with water from the sprayer solution or posttreatment irrigation.
equilibrium would probably not be achieved with leaves. Equilibrium might be
achieved on leaves it a turf was treated when wet, and it remained wet for an
extended period of time. Otherwise the assumption can be made that the
compound will be deposited in the thatch soon after an application. In the
thatch, where infiltration rates are minimal until the thatch is thoroughly wet
(Taylor and Blake. 1982; Hurto et al., 1980), the residence time of an organic
compound on organic thatch tissues may be very long. Equilibrium would
probably be reached in the thatch layer, and the Kthatch values determined by

the batch—suspension method would be valid predictors of nonionic organic

58
compound sorption in the field. As stated earlier, the effect of the formulation

should be considered in all future pesticide sorption work. Determination of
nonequilibrated sorption to turfgrass leaves and thatch would be the optimum
way of determining if the batch-suspension method is an accurate predictor of
sorption in the field.

The relationships between Kow or Sw and the K values for leaves and
thatch are shown in Figures 8 and 9. The regression lines for leaves and thatch
intersect in both figures. Therefore, at log Sw values (mol L'1) greater than -2
and log Kow values less than 2.5, we can expect thatch to be a stronger sorbent
than leaves. This is contrary to what was observed for the more hydrophobic
compounds. The intersections in Figures 8 and 9 indicate that compounds with
a log Sw (mol L") of approximately -2 and log Kow of approximately 2.5 would
have equal K values for leaves and thatch.

Future research should address the differences in thatch sorption at
different sites under various management strategies. The effects of
formulations will need to be addressed. because the chemical parameters of
the active ingredients in pesticides are not the only factors involved in
distribution of a compound in the turfgrass system. Finally. there is a need for
non-equilibrium sorption studies on turfgrass to determine the applicability of

batch-suspension data to field sorption of pesticides.

APPENDICES

APPENDIX A
W

All thatch was gently washed to remove as much soil as possible and to
achieve a more uniform sorbent. Before washing the thatch was 38.6 :t 3.9%
organic carbon, and after washing it was 39.8 :t: 0.9% organic carbon. A

sample of the thatch was then ground to 2 mm. The sorption of acetanilide at

500 mg L'1 (0]) was determined by another batch-suspension experiment with
6 replications on both ground and unground thatch. The quantity sorbed (Os)
and K values for this experiment are listed in the table below. The batch-
suspension experiments showed how sorption was not effected by grinding, but
the standard deviation for both quantity sorbed and the K values was
decreased by grinding. lf sorption isotherms are consistent with a partitioning
mechanism rather than adsorption, grinding would not be expected to affect
sorption. The variability in sorption with unground thatch was acceptable. and
the chance of increasing adsorption sites was eliminated by using only
unground thatch. All thatch samples were washed but left unground for the

remainder of the batch-suspension experiments.

Table 9. The effect of thatch grinding on acetanilide sorption

 

Thatch 05 (mg kg'1) K

 

ground to 2 mm 0.87 a: 0.02 2.63 :f: 0.09
unground 0.85 a: 0.13 2.62 a: 0.65

60

APPENDIX B

:AIO- a: I Ou". L" : “I “ :.

Acetanilide

Four-5 ul spots of acetanilide dissolved in methanol were placed on a
silica gel 60A thin layer chromatography (TLC) plate and developed with
acetonezcarbon tetrachloride (3:2) for 0.5 h at 21 ’ C. The plate was then placed
in a desiccator at 21 ° C until dry. The spots was then observed under a UV
lamp at 254 nm to determine the Rf (0.58). The plate was then scraped in 0.5
cm increments into separate scintillation vials containing 20 ml aqueous (Safety
Solve) counting cocktail (Research Prod. lnt. Corp). Care was taken to place
the visible spot into a vial by itself. Quantities of 14C were then determined by
liquid scintillation counting for 10 minutes on a Packard Tri-Carb Model 1500
liquid scintillation counter. All DPM values were corrected for background and
quenching when necessary. The 14C-acetanilide sample was determined to
be >98% pure as indicated by Sigma Chem. Co.

The specific activity reported by Sigma was then verified by reverse-
phase HPLC (lsco - Model 2300). The acetanilide/methanol solution was
injected at 21° C with a flow rate of 1.75 ml min“. The mobile phase was 3:2
water:acetonitrile. while the stationary phase was a 5 pm x 3 cm x 4.6 mm, C18
column (Anspec-H1404). The samples were detected at 242 nm on a UV
detector (lsco - V4) and the peaks were integrated (Varian - Model 4290) to
determine peak area. A standard curve was employed to determine acetanilide
concentrations. The same volume that was injected into the HPLC was placed

in liquid scintillation vials to determine the 14C concentration. This method was

61

62
successful in verifying the specific activity reported by Sigma(2.2 x 109 Bq

mmol“).

EIDQDEQD

Two-5 pl spots of ethoprop dissolved in methanol were placed on a silica
gel 60A thin layer chromatography (TLC) plate and developed with
hexanezacetone (4:1) at 21" C (Rf=0.6). The plate was then placed in a
desiccator at 21° C until dry. The plate was developed in toluenezmethanol:
triethylamine (80:19:1) in the direction 90" from the first development (Rt = 0.3).
The plate was again placed in a desiccator at 21 ° C until dry. The spots were
observed under a UV lamp at 254 nm. The plate was then scraped in 0.5 cm
increments into separate scintillation vials containing 20 ml aqueous (Safety
Solve) counting cocktail (Research Prod. lnt. Corp.) Care was taken to place
the visible spot into a vial by itself. Quantities of 14C were then determined by
liquid scintillation counting for 10 minutes on a Packard Tri-Carb Model 1500
liquid scintillation counter. All DPM values were corrected for background and
quenching when necessary. The 14C-ethoprop sample was determined to be
>96% pure as indicated by Rhone Poulenc.

The specific activity of the 1‘IC-ethoprop sample reported by Rhone

Poulenc (1.8 x 108 Bq mmol'I) was not verified.

lzflflmmmbenzene

Four-5 pl spots of 1,2,4—trichlorobenzene dissolved in methanol were
placed on a silica gel 60A thin layer chromatography (TLC) plate and
developed with iso-pentane for 0.5 h at 21° C. The spots was then observed

under a UV lamp at 254 nm to determine the Rf (0.57). The plate was scraped

in 0.5 cm increments into separate scintillation vials containing 20 ml aqueous

63
(Safety Solve) counting cocktail (Research Prod. Int. Corp). Care was taken to

place the visible spot into a vial by itself. Quantities of 1"'C were determined by
liquid scintillation counting for 10 minutes on a Packard Tri-Carb Model 1500
liquid scintillation counter. All DPM values were corrected for background and
quenching when necessary. The 1”'C-1,2,4-trichlorobenzene sample was
determined to be >99% pure as indicated by Sigma Chem. Co. The above
procedure must be done with care to minimize the volatilization of the very
volatile 1,2,4-trichlorobenzene.

The specific activity of the 14C-1,2,4-trichlorobenzene sample reported
by Sigma (2 x 106 Bq mmol“) was not verified.

Eenaflmol

Four-5 pl spots of fenarimol dissolved in methanol were placed on a
silica gel 60A thin layer chromatography (TLC) plate and developed with ethyl
acetate for 0.5 h at 21° C. The plate was placed in a desiccator at 21 ° C until
dry. The spots was observed under a UV lamp at 254 nm to determine the Rf
(0.54). The plate was scraped in 0.5 cm increments into separate scintillation
vials containing 20 ml aqueous (Safety Solve) counting cocktail (Research
Prod. lnt. Corp.). Care was taken to place the visible spot into a vial by itself.
Quantities of 14C were determined by liquid scintillation counting for 10
minutes on a Packard Tri-Carb Model 1500 liquid scintillation counter. All DPM
values were corrected for background and quenching when necessary. The
14C-fenarimol sample was determined to be >97% pure as indicated by
DowElanco.

The specific activity of the 14C-fenarimol sample reported by DowElanco

(2.5 x 108 Bq mmol'1) was not verified.

64
EbenanIbLene

Four-5 pl spots of phenanthrene dissolved in methanol were placed on a
silica gel 60A thin layer chromatography (TLC) plate and developed with
carbon tetrachloride for 0.5 h at 21 ° C. The plate was placed in a desiccator at
21' C until dry. The spots were observed under a UV lamp at 254 nm to
determine the Rf (0.61 ). The plate was scraped in 0.5 cm increments into
separate scintillation vials containing 20 ml aqueous (Safety Solve) counting
cocktail (Research Prod. lnt. Corp.). Care was taken to place the visible spot
into a vial by itself. Quantities of 14C were determined by liquid scintillation
counting for 10 minutes on a Packard Tri-Carb Model 1500 liquid scintillation
counter. All DPM values were corrected for background and quenching when
necessary. The 14C-phenanthrene sample was determined to be >99% pure
as indicated by Sigma Chem. Co.

The specific activity reported by Sigma was verified by reverse-phase
HPLC (lsco - Model 2300). The phenanthrene/methanol solution was injected
at 21° C with a flow rate of 1.5 ml min'1. The mobile phase was 3:1
acetonitrilezwater, while the stationary phase was a 5 pm x 3 cm x 4.6 mm, C18
column (Anspec-H1404). The samples were detected at 254 nm on a UV
detector (lsco - V4) and the peaks were integrated (Varian - Model 4290) to
determine peak area. A standard curve was employed to determine
phenanthrene concentrations. The same volume that was injected into the
HPLC was placed in liquid scintillation vials to determine the 14C
concentration. This method was successful in verifying the specific activity

reported by Sigma (3.1 x 108 Bq mmol'1).

APPENDIX C

IlMEIQEQLflLIBBJUM

Preliminary experiments showed equilibrium for acetanilide and
phenanthrene was reached in 4 h. The initial solute concentrations for
acetanilide and phenanthrene were 2700 mg L"1 and 0.4 mg L‘I. respectively.
One-ml aliquots of phenanthrene and acetanilide supernatant were removed at
0. 0.2. 0.3, 0.5. 0.7, 0.8, 1.0, 1.3, 1.7, 2, 4, 12, and 24 h. The most widely used
equilibration time in published isotherm studies is 24 h, and because there was
no further sorption between 4 and 24 h, we chose a 24 h equilibration period
(Figure 12). These results show that equilibrium is reached very quickly for

both sorbents regardless of the water solubility of the solute evaluated.

65

66

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APPENDIX D

W

A batch-suspension experiment was conducted with acetanilide at 500
mg L'1. One gram of thatch was placed in each of 8 centrifuge tubes and half of
the tubes were spiked with sodium azide at 10.0 g L'1(Wahid et al., 1980). The
tubes were shook for 48 h with 2 subsamples removed from each tube at 2. 4. 8.
24. 32. and 48 h. Differences in sorption may be due to the change in the
solvent properties of the supernatant with the addition of sodium azide.
Therefore. further studies with sterilized water and autoclaved sorbents may be
necessary to determine if microbial degradation is a concern, but, based on
these results, shaking tubes for longer then 24 h would not be advised (Figure
13). The 24 h equilibration time chosen appears to be safe from microbial

degradation. Different results could be expected for less stable compounds.

67

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APPENDIX E

W

The change in pH of the supernatant in batch-suspension vials was
determined after the 24 h equilibration period. The supernatant was decanted
into a beaker, and the pH was determined with a pH meter (Fisher - Model
815MP) calibrated in the pH range from 4.0 - 7.0 with a 97.3% calibration
efficiency. The pH of the leaf supernatant was 7.48 1 0.48 initially and it
dropped to 5.20 :l: 0.94 after the 24 h equilibration period. The pH of the thatch
supernatant dropped from 7.68 :l: 0.39 to 7.28 :I: 0.09 in the 24 h equilibration
time. The thatch pH did not change as drastically due to the thatch's buffering
capacity and/or its cation exchange capacity (CEC). The leaf supernatants pH
decreased by the release of acidic (polar) organic matter into the solution
during equilibration. These findings indicate buffering of the solution may be

necessary when ionizable organic compounds such as 240 are evaluated.

69

C?
@W

Acetanilide

O: t‘k 0L

APPENDIX F

H" C: ;k .li'. k'

0
I
CHacHZO—é—SCHQCHZCHa
SCH ZCHZCHa

Ethoprop

.63.

Fenarimol

 

Phenanthrene

APPENDIX G

 

 

;.: o O u ; 0:0: ;x Cu“. k. 0L Eu". 0:: L
Tompound log Koc Sorbent Reference
ethoprop 1.85 soil Wauchope et al., 1992
1.2,4-tcb 0.55 river sediment (0.08% 00) Swartzenbach and Westall. 1981
1,2,4-tcb 2.94 silt loam soil (1.9% 00) Chiou et al., 1983
1.2,4-tcb 2.95 clay subsoil (0.05% 0C) Southworth and Keller, 1986
1,2,4-tcb 3.11 clay subsoil (1 .2% OC) Southworth and Keller, 1986
1,2,4-tcb 3.32 clay subsoil (0.11% 0C) Southworth and Keller. 1986
fenarimol 2.78 soil Wauchope et al., 1992
phenanthrene 2.44‘ activated carbon Walters and Luthy. 1984
phenanthrene 3.15 clay subsoil (0.11% 00) Southworth and Keller, 1986
phenanthrene 3.76 clay subsoil (0.05% 0C) Southworth and Keller. 1986
phenanthrene 3.77 clay subsoil (1.2% DC) Southworth and Keller, 1986
phenanthrene 4.28 estuary sediment (4.0% DC) Vowles and Mantoura. 1987
phenanthrene 4.36 pond sediment (3.27% 00) Karickhoff et al., 1979

phenanthrene 4.59 silt loam soil (2.74% 002 Schrap and Opgrhuizeng 1989

‘ K value determined from Freundlich isotherm for activated carbon with an unknown % CC.

71

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