Groundwater contamination at the Kesterson Reservoir, California: 2. Geochemical parameters influencing selenium mobility

Transport of selenium in groundwater at the Kesterson Reservoir in the Central Valley of California was strongly retarded because of chemical reduction and precipitation mediated by microbial activity. Under such conditions, negative correlations were documented between aqueous Se and Fe 2+ , Mn, and H 2 S. Locally, the presence of oxidizing species, notably O 2 and NO 3 , suppressed this reduction, permitting Se mobilization in the shallow aquifer. Selenate, the dominant and most oxidized form of Se, was in electrochemical disequilibrium with subordinate concentrations of selenite. Normally slow inorganic reduction rates were accelerated by microbial activity which utilizes oxidized chemical species including selenate as electron donors during the oxidation of organic matter. Two stratified redox barriers to selenium migration were documented beneath Kesterson: an underlying shallow anoxic zone underlying most of the pond bottom, characterized by high organic content and sulfate reduction, and a deeper dynamic front established by localized O 2 infiltration from the overlying ponds and Fe 2+ release from aquifer materials. The reducing nature of this deeper aquifer ultimately precludes Se transport to regional groundwater.