Selenium Immobilization in a Pond Sediment at Kesterson Reservoir

Abstract

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 undertaken to evaluate the feasibility of the proposed remedial measure, its impact on the 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 1.22 m (4 ft) 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 distribution 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.

Hydrological and geochemical investigations of selenium behavior at Kesterson Reservoir. Progress report, October 1, 1994--September 30, 1996

“…The lack of significant Se(VI) reduction within the upper 2.0 m of this site is important to recognize. In most soils of Kesterson Reservoir, reductive removal of Se(V1) during ponding was a process of primary importance, leading to the high concentrations of insoluble Se(0) within the upper 0.15 m of profiles (Weres et al, 1989;Pickering et al, 1995), and to low soil water Se concentrations (Long et al, 1990). The inefficient reduction of Se (V1) Note that the elevated soluble Se concentrations displaced into the 2.00 and 2.25 m depths after the March 1995 rainfall events are reduced back to pre-storm levels within about 6 months.…”

“…of this report). In Kesterson Reservoir soils, water-extractable selenium includes selenate, selenite, and minor amounts of organically associated selenium (Weres et al, 1989;Long et al, 1990). All Se concentrations reported are in units of mgkg or ppm on a dry weight basis.…”

Hydrological and geochemical investigations of selenium behavior at Kesterson Reservoir

“…Waterextractable selenium and selenite were analyzed using atomic absorption spectroscopy (AAS) coupled with a hydride generator (see Section 8 of this report). Water-extractable selenium includes selenite, water-extractable selenate, and minor amounts of organically associated selenium (Weres et al, 1989;Long et al, 1990). …”

“…Recently, attention has again been drawn to this problem with Se contamination of Kesterson Reservoir, California, as a result of wildlife deaths and deformities in a wetland supplied with seleniferous agricultural drain waters (Ohlendorf, 1989). Several previous studies have provided information on Se distributions within Kesterson Reservoir soils on the macroscopic scale of individual soil profiles (Weres et al, 1989;Long et al, 1990;Benson et al, 1991;Tokunaga et al, 1991;White et al, 1991;Zawislanski et al, 1992). These macroscopic analyses provide information on Se concentrations and speciation averaged over distances on the order of about 10 to 103 mm.…”

“…Selenium transport and transformations in such environments has received considerable attention over the past ten years since the implication of Se toxicity in wildlife deaths and deformities at Kesterson Reservoir, California (Presser and Barnes, 1985;Ohlendorf, 1989;Weres et al, 1989a;Gilliom, 1989;Long et al, 1990;White et al, 1991;Tokunaga et al, 1994a). Although transport of Se across surface water-sediment boundaries is recognized, quantitative comparisons of measurements with models are needed to test the relative importance of various mechanisms.…”