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TECHNICAL REPORTS
Heavy Metals in the Environment

Selenium Stable Isotope Ratios in California Agricultural Drainage Water Management Systems

^a U.S. Geological Survey, Water Resources Division, 345 Middlefield Road, Mail Stop 480, Menlo Park, CA 94025
^b Dep. of Geology, 245 Natural History Bldg., 1301 W. Green St., Univ. of Illinois, Urbana, IL 61301
^c Dep. LAWR, One Shield Avenue, Univ. of California, Davis, CA 95616

* Corresponding author (mjherbel{at}usgs.gov)

Received for publication August 31, 2001. Selenium stable isotope ratios are known to shift in predictable ways during various microbial, chemical, and biological processes, and can be used to better understand Se cycling in contaminated environments. In this study we used Se stable isotopes to discern the mechanisms controlling the transformation of oxidized, aqueous forms of Se to reduced, insoluble forms in sediments of Se-affected environments. We measured ⁸⁰Se/⁷⁶Se in surface waters, shallow ground waters, evaporites, digested plants and sediments, and sequential extracts from several sites where agricultural drainage water is processed in the San Joaquin Valley of California. Selenium isotope analyses of samples obtained from the Tulare Lake Drainage District flow-through wetland reveal small isotopic contrasts (mean difference 0.7) between surface water and reduced Se species in the underlying sediments. Selenium in aquatic macrophytes was very similar isotopically to the NaOH and Na₂SO₃ sediment extracts designed to recover soluble organic Se and Se(0), respectively. For the integrated on-farm drainage management sites, evaporite salts were slightly (approximately 0.6) enriched in the heavier isotope relative to the inferred parent waters, whereas surface soils were slightly (approximately 1.4) depleted. Bacterial or chemical reduction of Se(VI) or Se(IV) may be occurring at these sites, but the small isotopic contrasts suggest that other, less isotopically fractionating mechanisms are responsible for accumulation of reduced forms in the sediments. These findings provide evidence that Se assimilation by plants and algae followed by deposition and mineralization is the dominant transformation pathway responsible for accumulation of reduced forms of Se in the wetland sediments.

Abbreviations: IFDMS, integrated on-farm drainage management system • TLDD, Tulare Lake Drainage District

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				T. M. Johnson and T. D. Bullen Mass-Dependent Fractionation of Selenium and Chromium Isotopes in Low-Temperature Environments Reviews in Mineralogy and Geochemistry, January 1, 2004; 55(1): 289 - 317. [Full Text] [PDF]