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Chromium stable isotope ratios show promise at indicators of Cr(VI) reduction in groundwater, but no published study has yet demonstrated that expected relationships between 53Cr/52Cr and Cr(VI) concentration, position, and time occur in an actual groundwater plume. We present an extensive data set from a point-source plume in Berkeley, CA; data extend over 5 years and 14 locations covering the entire plume. We interpret the data using a Rayleigh distillation model with an effective fractionation factor that incorporates an intrinsic fractionation factor determined from incubations of site sediments and accounts for reservoir effects in the restricted subsurface zones where Cr(VI) reduction is thought to occur. The groundwater 53Cr/52Cr and Cr(VI) concentration data are consistent with a scenario where the system has reached a steady state: Cr(VI) reduction continues, the extent of reduction at any point is constant over time, reduction proceeds to completion at the downgradient edge of the plume, and the plume is no longer advancing. The overall consistency of the results with a reasonable model for the site supports the use of Cr isotope-based estimates of reduction, hut we discuss current uncertainties and limitations of the approach as well. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Analyzes the effectiveness of selenium stable isotope ratios as indicators of selenium sources and reduction of selenium oxyanions in the San Francisco Bay Estuary in California. Environmental samples; Reduction of soluble selenium from the overlying water; Consistent isotopic differences between riverine and refinery inputs with selenium isotopes.

Measurements of chromium (Cr) stable-isotope fractionation in laboratory experiments and natural waters show that lighter isotopes reacted preferentially during Cr(VI) reduction by magnetite and sediments. The 53Cr/52Cr ratio of the product was 3.4 +/- 0.1 per mil less than that of the reactant. 53Cr/52Cr shifts in water samples indicate the extent of reduction, a critical process that renders toxic Cr(VI) in the environment immobile and less toxic.