Reduction of metal leaching by poplars during soil treatment of wastewaters: Small-scale proof of concept studies

Land application of food processing wastewaters, a common practice, can create continuously saturated and reducing soil conditions that potentially mobilize native soil metals such as arsenic, manganese, and iron, contributing to contamination of groundwater. Small-scale column studies were used to evaluate the potential of poplar trees to decrease groundwater contamination from land application of wastewaters. Research assessed growth of poplar trees during land application of food processing wastewater, the evapotranspiration by poplar trees under such conditions, and treatment of chemical oxygen demand (COD), nitrate, iron, and manganese in columns with and without poplar trees for four months. In addition, plant tissues and soil were analyzed for metals, macronutrients, and micronutrients at the conclusion of the experiment. Poplar growth, as measured by shoot height, shoot mass, number of leaves, number of shoots, mass of leaves, and root length, was not affected by application of fruit and vegetable processing wastewater at the rate of 1–2 times the highest reported application rate in Michigan. However, significantly less root mass and fewer root branches were observed in columns receiving wastewater at the end of the experiment. Poplar trees withstood continuous saturation of soils and evapotranspired significantly more water than no plant controls, with an average evapotranspiration coefficient of 3.25. Mass removal rate of chemical oxygen demand (COD) was significantly higher in columns with poplar trees than in soil-only columns. Additionally, leaching of nitrate and iron was lower in columns with poplar trees than in soil only columns. Poplars accumulated arsenic, manganese and iron, and translocated these metals into above-ground tissues. Therefore, poplar trees demonstrated the ability to decrease saturation, potentially modify redox conditions, and uptake metals—resulting in decreased leaching of COD, nitrate, iron, and manganese during land application of food processing wastewaters.