Environ. Sci. Technol. 2000, 34, 2075-2079
Selenium Stable Isotope Ratios as Indicators of Sources and Cycling of Selenium: Results from the Northern Reach of San Francisco Bay THOMAS M. JOHNSON* Department of Geology, 245 Natural History Building, MC-102, University of Illinois, Urbana-Champaign, Urbana, Illinois 61801 THOMAS D. BULLEN Water Resources Division, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 PETER T. ZAWISLANSKI Earth Sciences Division, Mail Stop 90-1116, E. O. Lawrence Berkeley National Laboratory, Berkeley, California 94720
Selenium stable isotope ratios can serve as indicators of Se sources and reduction of Se oxyanions, much as sulfur and nitrogen isotope ratios do in sulfur and nitrogen biogeochemical studies. A new analytical method, which allows precise Se isotope ratio measurements on 500 ng of Se, greatly enhances analysis of environmental samples. This paper presents the first environmental study to use Se stable isotopes. 80Se/76Se ratios, relative to a provisional 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 extracts designed to recover Se0 are slightly (about 2‰) enriched 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 incorporated into the sediments. Consistent isotopic differences between riverine and refinery inputs were not observed, and thus tracing of refinery inputs with Se isotopes is not possible in this system.
Introduction Studies of selenium cycling and transport typically focus on the complex biogeochemistry of Se (e.g., refs 1-3). Se(VI), as SeO42-, is soluble, mobile, and similar to sulfate. Se(IV), as SeO32- and HSeO3-, is also soluble but may be strongly sorbed. Elemental selenium, Se0, is insoluble and stable over a wide Eh/pH range and is slow to dissolve when exposed to oxidizing conditions. Accordingly, reduction of the soluble oxyanions to Se0 greatly reduces the mobility and bioavailability of selenium. Reduction of selenium oxyanions is probably microbially mediated; some bacteria respire using * Corresponding author phone: (217)244-2002; fax: (217)244-4996; e-mail:
[email protected]. 10.1021/es990187y CCC: $19.00 Published on Web 04/21/2000
2000 American Chemical Society
selenate and selenite as terminal electron acceptors (e.g., ref 4). Under highly reducing conditions, Se(-II) may be present. A variety of Se-bearing organic molecules are found under various conditions and may constitute a significant fraction of the total Se inventory. Because the oxidation state of Se is critical in determining its mobility and bioavailability, redox reactions must be identified and quantified. Measurements of spatial and temporal variation of Se speciation are commonly used to infer the types and rates of chemical reactions in the studied systems (e.g., refs 2, 3, and 5). This type of study yields important information but has limitations. For example, observed decreases in selenate concentration over time in surface water, or along a flow path in groundwater, could result from reduction reactions or dilution by Se-poor water. Similarly, the presence of elemental Se in a soil may be used to infer in situ reduction of selenate and selenite, but shortterm variations in the reduction rate (e.g., monthly variations) are not resolvable with this approach. Stable isotope ratios can serve as indicators of the occurrence and extent of redox reactions and are known to be sensitive to the reduction of soluble oxyanions. S and N isotope ratios have been used as biogeochemical indicators of reduction reactions (e.g., refs 6 and 7). Reduction of sulfate or nitrate results in a shift of the 34S/32S or 15N/14N ratio. The reduced S or N is enriched in the lighter isotope, and the unreduced fraction is enriched in the heavier isotope. According to studies completed to date, selenium isotope systematics are broadly similar to those of sulfur (8-13). The notation we use for unambiguous reporting of Se isotope ratio variations is defined by
δ80/76Se )
(80Se/76Se)sample - (80Se/76Se)standard (80Se/76Se)standard
× 1000 (1)
This notation helps to distinguish several Se isotope pairs that could be measured. Various studies, summarized in Table 1, have demonstrated that reduction of Se(VI) or Se(IV) results in an enrichment of lighter isotopes in the reaction products, whereas isotopic fractionation for other reactions is smaller or nonexistent. Of particular importance are the results from a study in progress (8). Se isotope fractionations of 2.8 and 6.5‰ were observed during reduction of Se(VI) and Se(IV), respectively, by incubated sediments from the field site studied in the present paper. If shifts in δ80/76Se are observed in natural systems, they should be useful as evidence that reduction reactions are occurring. In addition, Se isotope ratios may serve as tracers of anthropogenic Se sources if they are isotopically distinct from the natural background. The northern reach of the San Francisco Bay Estuary receives Se-rich wastewater from several oil refineries; this has raised concerns that the added Se may have adverse effects on the ecosystem (e.g., refs 14 and 15). The fluxes of Se from riverine, refinery, and municipal waste inputs into the estuary have been determined (16-18), and refinery inputs dominate the Se budget of the northern reach seasonally when river flows are low. However, Se dynamics in sediments, and particularly in tidal flats and wetlands where significant ecosystem impacts are anticipated, are not well-known. Marshes and other sediments could act as Se sinks if the decay of organic matter creates sufficiently reducing conditions to reduce dissolved Se to insoluble, recalcitrant Se0. Velinsky and Cutter (3) studied Se dynamics in a salt marsh on Delaware Bay and concluded that seasonal oxidation and reduction occurred and that a net loss of Se VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Instantaneous Isotopic Fractionation, E, Caused by Se Transformations ref
transformation
8 8 9 9 10 11, 10, 12 13, 21 13 33 unpub data 13 13 a
reacting agent
E (δ80/76Se, ‰)a
MRP sediment slurry -2.8 MRP sediment slurry -6.5 bacterial cultures -0.9 to -4.8 bacterial cultures -7.3 to -8.3 HCl, 25 °C -12 NH2OH or ascorbic -7 to -13 acid Se(0) oxidation incubated soil
