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Selenium Transformations in Ponded Sediments

Earth Sciences Division, Lawrence Berkeley Lab., Berkeley, CA 94720

Ingrid J. Pickering

Stanford Synchrontron Radiation Lab., Stanford, CA 94309

Gordon E. Brown, Jr.

Dep. of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305

*Corresponding author (tktokunaga{at}lbl.gov).

ABSTRACT

Features of the sediment contamination process that occurred during disposal of seleniferous agricultural drainage waters at Kesterson Reservoir (California) were simulated in the laboratory. Selenium was introduced by ponding 3.0 mM solutions of Se(VI) over previously uncontaminated soils. Analyses of soil waters and ponded waters with conventional laboratory methods [hydride-generation atomic absorption spectrometry (HGAAS) and redox electrodes] and with x-ray absorption spectroscopy (XAS) measurements on sediment and water samples provided direct Se speciation of the dominant valence states. The XAS demonstrated that Se in a contaminated sediment from Kesterson Reservoir occurs primarily as the red, monoclinic elemental form. Selenium (VI) reduction to Se(IV) and removal of both species from the aqueous phases (ponded and pore waters) was observed with HGAAS. Reduction of Se(VI) to Se(0) in the sediments was confirmed with XAS. The XAS on all shallow sediments at the end of the ponding period (48 to 51 d) demonstrated that 99 ± 5% of the Se occurred as Se(0). Rapid reoxidation of Se(0) was observed in some of the previously ponded sediments. About 60% of the Se inventory was reoxidized to a mixture of Se(IV) and Se(VI) in unamended sediments within 2 d after sampling. Selenium in sediments that were amended with organic matter remained in the Se(0) state during this short period.

Received for publication June 8, 1995.

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