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Researchers from the Lawrence Berkeley National Laboratory, U.S. Geological Survey and University of Illinois studied ratios of selenium stable isotopes found from samples taken at the San Francisco Bay Estuary to investigate the dynamics and anthropogenic inputs of selenium.

Selenium poisoning is a significant health problem in parts of Punjab, India, which is an area of intense agricultural productivity. To determine the complex soil dynamics that control distribution of Se in this area, we measured concentrations and δ82/76Se of bulk Se and individual Se pools in four soil profiles. This was compared against δ82/76Se of crops and groundwater used for irrigation. The isotopic composition of bulk Se and component Se pools reveal spatial heterogeneity. The bulk δ82/76Se show progressively lower values with increasing soil depth indicating the preferential migration of isotopically lighter Se downward through the soil profile. The δ82/76Se of water-soluble Se is isotopically heavier than δ82/76Se of adsorbed Se, suggesting Se isotope fractionation by reduction prior to scavenging by reactive minerals in the soil. The organically bound Se is isotopically lighter than water-soluble Se and correlates with the C/N ratio at different soil depths. Thus, Se immobilization by redox cycling controls the biogeochemical Se cycle in the soil. Se isotope ratios help to trace biochemical processes of Se in agricultural seleniferous soils and provide an important assessment for better soil management mitigating Se concentrations of ecotoxicological levels. [ABSTRACT FROM AUTHOR]

Cr stable isotope measurements can provide improved estimates of the extent of Cr(VI) reduction to less toxic Cr(III). The relationship between observed 53Cr/52Cr ratio shifts and the extent of reduction can be calibrated by determining the isotopic fractionation factor for relevant reactions. Permeable reactive barriers (PRB) made of Fe0 and in situ redox manipulation (ISRM) zones effectively remediate Cr-contaminated aquifers. Here, we determine the isotopic fractionations for dominant reductants in reactive barriers and reduced sediments obtained from an ISRM zone at the US DOE's Hanford site. In all cases, significant isotopic fractionation was observed; fractionation (expressed as e) was -3.91‰ for Fe(II)-doped goethite, -2.11‰ for FeS, -2.65‰ for green rust, -2.67‰ for FeCO3, and -3.18‰ for ISRM zone sediments. These results provide a better calibration of the relationship between Cr isotope ratios and the extent of Cr(VI) reduction and aid in interpretation of Cr isotope data from systems with reactive barriers. [ABSTRACT FROM AUTHOR]

The natural abundance of stable Se isotopes may reflect sources and formation conditions of methylated Se. We aimed at (l) quantifying the degree of methylation of selenate [Se(VI)] and (hydro)selenite [Se(IV)] by the fungus Alternaria alternata at pH 4 and 7 and (2) determining the effects of these different Se sources and pH values on 82Se/76 Se ratios (δ82/76 Se) in methylselenides. Alterenaria alternata was incubated with Se(VI) and Se(IV) in closed microcosms for 11-15 days and additionally with Se(IV) for 3-5 days at 30 °C. We determined Se concentrations and δ82/76Se values in source Se(VI) and Se(IV), media, fungi, and trapped methylselenides. In Se(VI) incubations, methylselenide volatilization ended before the 11th day, and the amounts of trapped methylselenide were not significantly different among the 11 - 15 day incubations. In 11 - 15 days, 2.9-11% of Se(VI) and 21-29% of Se(IV) were methylated, and in 3-5 days, 3-5% of Se(IV) was methylated. The initial δ82/76 Se values of Se(VI) and Se(IV) were -0.69 ± SD 0.07%o, and -0.20 ± 0.05%o, respectively. The δ82/76Se values of methylselenides differed significantly between Se(VI) (-3.97%o to -3.25%o) and Se(IV) (- 1.44%o to -0.l6%o) as sources after 11-15 days of incubation; pH had little influence on δ82/76 Se values. Thus, the δ82/76 Se values of methylselenide indicate the source species of methylselenides used in this study. The strong isotope fractionation of Se(VI) is probably attributable to the different reduction steps of Se(VI) to Se(-II) which were rate-limiting explaining the low methylation yields, but not to the methylation itself. The shorter incubation of Se(IV) for 3-5 days showed a large Se isotope fractionation of at least -6%o before the biomethylation reaction reached its end. This initial Se isotope fractionation during methylation of Se(IV) is much larger than previously published. [ABSTRACT FROM AUTHOR]

At Idaho National Laboratory, Cr(VI) concentrations in a groundwater plume once exceeded regulatory limits in some monitoring wells but have generally decreased over time. This study used Cr stable isotope measurements to determine if part of this decrease resulted from removal of Cr(VI) via reduction to insoluble Cr(III). Although waters in the study area contain dissolved oxygen, the basalt host rock contains abundant Fe(II) and may contain reducing microenvironments or aerobic microbes that reduce Cr(VI). In some contaminated locations, 53Cr/52Cr ratios are close to that of the contaminant source, indicating a lack of Cr(VI) reduction. In other locations, ratios are elevated. Part of this shift may be caused by mixing with natural background Cr(VI), which is present at low concentrations but in some locations has elevated 53Cr/53Cr. Some contaminated wells have 53Cr/53Cr ratios greater than the maximum attainable by mixing between the inferred contaminant and the range of natural background observed in several uncontaminated wells, suggesting that Cr(VI) reduction has occurred. Definitive proof of reduction would require additional evidence. Depth profiles of 53Cr/53Cr suggest that reduction occurs immediately below the water table, where basalts are likely least weathered and most reactive, and is weak or nonexistent at greater depth. [ABSTRACT FROM AUTHOR]

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]

Wetlands remove many dissolved pollutants from surface waters by various mechanisms. Stable isotope ratio measurements may provide a means of detecting and possibly quantifying certain removal processes, such as reduction of SeO42-, Cr(Vl), NO3-, and HCIO4-, that fractionate isotopes. However, the magnitude of the isotopic fractionation for a given reaction depends on the setting in which it occurs. We explore the case where isotope ratio shifts in surface waters are used to detect or quantify reactions occurring in pore waters of underlying sediments. A series of SeO42- reduction experiments reveals that the effective isotopic fractionation, observed in the water column as a result of SeO42- diffusion into underlying, Se-reducing sediments, is weaker than the intrinsic fractionation induced by the same reduction reactions in well-mixed systems in which reaction sites are not separated from measured SeO42-. An intact sediment core yielded an effective ϵ (≈ δreact — δinstantaneous prod) of 0.20‰, whereas the intrinsic ϵ was 0.61‰. These results are consistent with previously published reactive transport models. Isotopic studies of sediment-hosted reactions in wetlands and other surface water systems should use the smaller effective fractionation values, which can be estimated using the models. [ABSTRACT FROM AUTHOR]

Chromium stable isotope values can be effectively used to monitor reduction of Cr(Vl) in natural waters. We investigate effects of sorption during transport of Cr(Vl) which may also shift Cr isotopes values, complicating efforts to quantify reduction. This study shows that Cr stable isotope fractionation caused by sorption is negligible. Equilibrium fractionation of Cr stable isotopes between dissolved Cr- (VI) and Cr(Vl) adsorbed onto γ-Al2O3 and goethite is less than 0.04%o (53Cr/52Cr) under environmentally relevant pH conditions. Batch experiments at pH 4.0 and pH 6.0 were conducted in series to sequentially magnify small isotope fractionations. A simple transport model suggests that adsorption may cause amplification of a small isotope fractionation along extreme fringes of a plume, leading to shifts in 53Cr/52Cr values. We therefore suggest that isotope values at extreme fringes of Cr plumes be critically evaluated for sorption effects. A kinetic effect was observed in experiments with goethite at pH 4 where apparently lighter isotopes diffuse into goethite clumps at a faster rate before eventually reaching equilibrium. This observed kinetic effect may be important in a natural system that has not attained equilibrium and is in need of further study. Cr isotope fractionation caused by speciation of Cr(Vl) between HCro4 and CrO4² was also examined, and we conclude that it is not measurable. In the absence of isotope fractionation caused by equilibrium speciation and sorption, most of the variation in δ53Cr values may be attributed to reduction, and reliable estimates of Cr reduction can be made. [ABSTRACT FROM AUTHOR]

U isotope fractionation may serve as an accurate proxy for U(VI) reduction in both modern and ancient environments, if the systematic controls on the magnitude of fractionation (ε) are known. We model the effect of U(VI) reduction kinetics on U isotopic fractionation during U(VI) reduction by a novel Shewanella isolate, Shewanella sp. (NR), in batch incubations. The measured ε values range from 0.96 ± 0.16 to 0.36 ± 0.07‰ and are strongly dependent on the U(VI) reduction rate. The ε decreases with increasing reduction rate constants normalized by cell density and initial U(VI). Reactive transport simulations suggest that the rate dependence of ε is due to a two-step process, where diffusive transport of U(VI) from the bulk solution across a boundary layer is followed by enzymatic reduction. Our results imply that the spatial decoupling of bulk U(VI) solution and enzymatic reduction should be taken into account for interpreting U isotope data from the environment.

High-sensitivity and high-precision (2 SD ≤ 0.06‰) measurement of chromium (Cr) isotopes at the 10 ng level was successfully carried out using double spike multiple-collector inductively couple plasma mass spectrometry (MC-ICP-MS). To enhance the signal sensitivity and stability, the Aridus II desolvating nebulizer system was improved by placing its waste gas trap bottle in an ice chamber (5 °C cold trap). This setup, beyond Cr isotope analysis, can be applied to most heavy metal isotope measurements. The sensitivity of the 52 Cr signal is ≥300 V mg -1 L (with a 10 11 Ω amplifier and a 110 μL min -1 uptake rate), an enhancement of ≥1.5 times compared to the Aridus II without the cold trap. In addition, the relative standard deviation of the 52 Cr signal varied ≤4% over 8 h, demonstrating high stability. The δ 53 Cr values of common geological reference materials determined using 10 ng of Cr are in excellent agreement with results measured at 25 ng and 50 ng and are consistent with previous determinations, validating the accurate and precise Cr isotope ratio measurements. An empirical method is proposed to correct for the residual (after subtraction) effect of Fe interference on δ 53 Cr determination. This method relies on a linear relationship between the [Fe]/[Cr] and δ 53 Cr shift within one analytical session. Finally, we report the δ 53 Cr values of 19 new reference materials, ranging from -0.44‰ to 0.49‰. Among them, GSS-7 (-0.44 ± 0.02‰, 2 SD, n = 5), GSS-4 (0.48 ± 0.02‰, 2 SD, n = 5), and GSD-10 (0.49 ± 0.05‰, 2 SD, n = 5) can be used as candidate reference materials for interlaboratory comparisons to complement existing ones that are mostly isotopically unfractionated from the bulk silicate earth.

Biostimulation to induce reduction of soluble U(VI) to relatively immobile U(IV) is an effective strategy for decreasing aqueous U(VI) concentrations in contaminated groundwater systems. If oxidation of U(IV) occurs following the biostimulation phase, U(VI) concentrations increase, challenging the long-term effectiveness of this technique. However, detecting U(IV) oxidation through dissolved U concentrations alone can prove difficult in locations with few groundwater wells to track the addition of U to a mass of groundwater. We propose the 238 U/ 235 U ratio of aqueous U as an independent, reliable tracer of U(IV) remobilization via oxidation or mobilization of colloids. Reduction of U(VI) produces 238 U-enriched U(IV), whereas remobilization of solid U(IV) should not induce isotopic fractionation. The incorporation of remobilized U(IV) with a high 238 U/ 235 U ratio into the aqueous U(VI) pool produces an increase in 238 U/ 235 U of aqueous U(VI). During several injections of nitrate to induce U(IV) oxidation, 238 U/ 235 U consistently increased, suggesting 238 U/ 235 U is broadly applicable for detecting mobilization of U(IV).

Groundwater samples were collected from the Integrated Field Research Challenge field site in Rifle, Colorado, over the course of a bicarbonate-induced U desorption-adsorption experiment. Uranium concentrations and high precision U isotopic compositions ((238)U/(235)U) of these groundwater samples were determined and used to assess the impact of bicarbonate-induced U(VI) desorption from contaminated sediments on the (238)U/(235)U of groundwater. The (238)U/(235)U of groundwater was not significantly impacted by bicarbonate-induced desorption of U(VI) from mineral surfaces or by adsorption of advecting U(VI) from upgradient locations onto those surfaces after the treatment. Assuming this absence of a significant shift in U isotopic composition associated with desorption-adsorption applies to other systems, reduction of U(VI) to U(IV) is expected to be the dominant source of U isotopic fractionation associated with removal of U(VI) from pore water as a result of natural and stimulated reductive pathways. Thus, changes in the (238)U/(235)U composition of uranium-bearing fluids should be useful in quantifying the extent of reduction.

The attenuation of groundwater contamination via chemical reaction is traditionally evaluated by monitoring contaminant concentration through time. However, this method can be confounded by common transport processes (e.g., dilution, sorption). Isotopic techniques bypass the limits of concentration methods, and so may provide improved accuracy in determining the extent of reaction. We apply measurements of 238U/235U to a U bioremediation field experiment at the Rifle Integrated Field Research Challenge Site in Rifle, Colorado. An array of monitoring and injection wells was installed on a 100 m2 plot where U(VI) contamination was present in the groundwater. Acetate-amended groundwater was injected along an up-gradient gallery to encourage the growth of dissimilatory metal reducing bacteria (e.g., Geobacter species). During amendment, U concentration dropped by an order of magnitude in the experiment plot. We measured 238U/235U in samples from one monitoring well by MC-ICP-MS using a double isotope tracer method. A significant approximately 1.00 per thousand decrease in 238U/235U occurred in the groundwater as U(VI) concentration decreased. The relationship between 238U/235U and concentration corresponds approximately to a Rayleigh distillation curve with an effective fractionation factor (alpha) of 1.00046. We attribute the observed U isotope fractionation to a nuclear field shift effect during enzymatic reduction of U(VI)(aq) to U(IV)(s).