Land application of food processing wastewater, a common practice, can cause mobilization of metals such as arsenic, manganese, iron, chromium and lead by creating continuously saturated and anaerobic soil conditions. Conversely, aerobic conditions may promote nitrification and leaching of nitrate into groundwater. Poplar trees have great potential to decrease metals and nitrate mobilization, allowing increased loadings of food processing wastewaters without negatively impacting soil treatment. Land application, unlike previous phytoremediation applications, utilizes uncontaminated sites and prolonged irrigation of fields with organic-carbon rich wastewater that is relatively low in other nutrients. Therefore, this research evaluated the ability of poplar plantation to reduce the problem of metals and nitrate mobilization to ground water and the phyto-processes that are expected to reduce metals and nitrate mobilization to groundwater during land treatment of food processing wastewaters. The research evaluated the central hypothesis that poplar trees reduce metal and nitrate mobilization that can occur when carbon-rich wastewaters are land applied through major processes (plant uptake and evapotranspiration) and minor processes (increased microbial activity in the rhizosphere and oxygenation of soils). The research used i) laboratory-scale small soil columns, ii) pilot-scale large soil columns and iii) field experimentation to investigate the plant associated processes that influence leaching of metals and nitrate due to microbial mobilization at wastewater application sites. The small-scale columns utilized 15 cm diameter columns (total 15) to assess effects of wastewater on poplar trees and effects of poplar trees on treatment of carbon, nitrogen and metals. The large-scale columns (56 cm diameter, total 15) were used to assess the evapotranspiration, redox dynamics, moisture dynamics and treatment of carbon, metals and nitrate during application of food processing wastewater utilizing a combination of laboratory analysis of water, soil and plant samples and data collected from oxidation-reduction, moisture and temperature sensors. Field experiment consisted of an acre of actual land application site where poplar trees were planted at 3.05 m spacing and the effects of poplar trees on drainage, evapotranspiration, carbon and nitrogen treatment and reduction in metal mobilization were assessed utilizing laboratory analysis of leachate water, soil and plant samples and data from draingage and moisture sensors.Results varied with the scale of experimentation. At small-scale columns, poplar trees showed no signs of toxicity under food processing wastewater application, enhanced the soil moisture and reduced the mass removal of organic carbon, metals and nitrogen. In large-scale columns, plants enhanced the microbial biomass, reduced the soil moisture at 30 cm and 91 cm or 122 cm below depth by virtue of high evapotranspiration, increased the carbon removal in fine textured soils and contributed to nitrate reduction in leachate waters. However, oxygenation in the soils and reduction in metal mobilization was not observed despite the high uptake of metals in the plant tissues. Field studies corroborated the large column study as plants enhanced biomass in the rhizosphere and decreased soil moisture with high crop coefficient. However, poplar trees did not contribute to the decrease in metal mobilization and minimally to decreasing nitrate leaching despite metal and nitrogen uptake. These results demonstrate the benefits of poplar trees, but need further consideration when the trees grow to full maturity.
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