Selenium stable isotope ratios as indicators of sources and cycling of selenium: Results from the northern reach of San Francisco Bay

Selenium stable isotope ratios can serve as indicators of Se sources and re duction of Se oxyanions, much as sulfur and nitrogen isotope ratios do in s ulfur and nitrogen biogeochemical studies. A new analytical method, which a llows precise Se isotope ratio measurements on 500 ng of Se, greatly enhanc es analysis of environmental samples. This paper presents the first environ mental study to use Se stable isotopes. Se-80/Se-76 ratios, relative to a p rovisional standard, were measured in water, oil refinery wastewater, total sediment digests, and sediment extracts from the Carquinez area in the San Francisco Bay Estuary. Se isotope ratios in total sediment Se and in extra cts designed to recover Se-0 are slightly (about 2 parts per thousand) enri ched in the lighter isotope relative to local bay water Se. This difference is smaller than the isotopic fractionations expected upon reduction of Se( VI) or Se(IV) to Se(0) and suggests that reduction of soluble selenium from the overlying waters is not the dominant process by which Se is incorporat ed into the sediments. Consistent isotopic differences between riverine and refinery inputs were not observed, and thus tracing of refinery inputs wit h Se isotopes is not possible in this system.