Redox Chemistry and Transformation of Arsenic and Selenium in Agricultural Drainage Disposal Ponds

Evaporation ponds are being used for disposal of agricultural drainage waters in the San Joaquin Valley (SJV) of California since there is no option for disposal outside of the valley. The drainage water contains elevated levels of salts and trace elements including arsenic (As) and selenium (Se). The objective of this research was to evaluate evaporation pond biogeochemistry, chemical data of As and Se in water and sediment, and the potential impact on the long term use of evaporation basins. The study site is a 10-cell evaporation basin facility with a surface area of 726 ha in the Tulare Lake Drainage District of the SJV. Field sampling for water and sediments were conducted along water flow from cell to cell as well as within the initial drainage water-receiving cell and a cell towards the terminal end of the facility. Redox sensitive elements and As and Se speciation for water samples, and sediment fractionation data were determined. Salinity increased from evaporation and reducing conditions developed along water flow from cell to cell. Reducing conditions were facilitated by high concentration of dissolved organic carbon, which also increased linearly with increased salinity. Reduction of both As and Se was observed and resulted in more soluble As and immobilized Se into sediments. Total As concentration in pond water increased from cell to cell with a positive correlation to Cl concentration or EC while Se concentration was lower than the fresh drainage water. Reduction of Se was found in the initial pond and As reduction occurred towards terminal cells. There were also differences in partitioning to different solid phases between As and Se. Continuous accumulation of As in the ponds is a concern and may have impact on the sustained use of evaporation ponds for disposal of drainage water in the future.