Nature and Rates of Selenium Transformations: A Laboratory Study of Kesterson Reservoir Soils

The speciation and association of Se in soil determines its availability to biota. Oxidized forms are generally more biologically available. Surface and subsurface soils from Se‐contaminated Kesterson Reservoir were incubated for 2.5 yr at temperatures of 15, 25, and 35°C in order to measure Se oxidation rates and help identify the most readily oxidizable fractions. A range of moisture contents was maintained. Six Se fractions: soluble Se(IV), soluble Se(VI), adsorbed Se, organic Se, carbonate Se, and refractory Se, were monitored via periodic extraction. Oxidation rates were estimated based on changes in the relative proportions of these fractions with time. No significant changes were noted in soils from a depth of 0.45 to 0.55 m. Transformations were observed in surface soils (0–0.10 m). The most prominent change was the oxidation of up to 50% of refractory Se to soluble Se(VI). A smaller portion of the organic Se pool was also oxidized. X‐ray absorption spectroscopy indicated the dominance of Se(0) in the refractory pool. First‐order oxidation rates were between 0.058 and 0.29 yr −1 for organic Se, and between 0.11 and 2.4 yr −1 for refractory Se, with a strong positive correlation with temperature. No losses of total Se were observed, indicating the relative insignificance of methylation. An apparent plateau in oxidation of the refractory pool suggests heterogeneity of this pool, with some components more readily oxidized than others. With time, currently reduced Se will be oxidized, although some fractions are likely to remain biologically unavailable for decades.