SELENIUM IMMOBILIZATION IN A POND SEDIMENT AT KESTERSON RESERVOIR

Kesterson Reservoir, Merced County, CA, a disposal facility for agricultural drain water, became the object of intense scientific investigation following discovery in 1983 that Se-laden agricultural drain water was having serious effects on the reproductive success of waterfowl. A remedial measure involving permanent flooding with low-Se water, aimed at taking advantage of low Se solubility under reducing conditions, was proposed as a means of limiting Se movement into groundwater and biota. A field experiment was undertakent to evaluate the feasibility of the proposed remedial measure, its impact on th quality of shallow groundwater and for quantifying Se immobilization and transport through a newly flooded pond bottom soil. Extensive soil water and groundwater sampling demonstrated that although initial soluble Se concentrations in the top 1.22m (4ft) of soil typically ranged from approximately 1000 to 3000 μg L-1, Se concentrations declined dramatically after flooding and elevated concentrations below 1.22 m were observed at only one of five sampling sites. Analysis of the temporal and spatial changes in the distributin of dissolved Se and Cl- indicated that 66 to 108% of the initial soluble Se present in the top 1.22 m was immobilized shortly after flooding. These estimates were consistent with the low Se concentrations observed in shallow monitoring wells. The extent to which Se immobilization occurred correlated inversely with average pore water velocity. Data presented suggest that reducing conditions in the newly flooded soils lead to the microbially mediated transformation of selenate to less soluble or mobile forms.