Development of Chromium Stable Isotopes for Chromium Transport Studies and Other Geoscience Applications

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0001153JohnsonThis project is a one-year exploratory study needed to examine the feasibility of Cr isotope measurements as indicators of Cr(VI) reduction and Cr contamination sources. Cr is a common groundwater contaminant because of its use in electroplating and other industrial processes. The complex redox chemistry of Cr determines its mobility and bioavailability, and in most Cr-contaminanted sites, Cr(VI) reduction is an important natural attenuation mechanism. Cr is also of interest in oceanographic and sedimentological studies, as both Cr(VI) and Cr(III) are present in ocean and estuarine water, and Cr concentration and speciation data hold information about redox conditions.Stable isotope ratios of nitrogen and sulfur have proven useful in tracking reduction of nitrate and sulfate to lower oxidation states, and in identifying sources of nitrate and sulfate in groundwater. This project is an initial step toward developing a similar capability for Cr isotopes. For nitrogen and sulfur, lighter isotopes are reduced preferentially, and thus isotope ratio shifts in nature provide evidence that reduction reactions are occurring, and indicate the degree of reduction. It is likely that in chromate reduction, 52Cr will be reduced preferentially relative to 53Cr, and observed shifts in 53Cr/52Cr ratio of the Cr in groundwater would indicate Cr(VI) reduction. Cr isotopes are thus promising as a new geochemical toll, but the isotopic fractionations must be observed in controlled experiments before extensive research can be undertaken.The proposed study will measure the size of this kinetic isotope effect and assess the potential for Cr isotope applications. Three experiments are planned, in which Cr(VI) reduction will occur in simple solutions, on mineral surfaces, and in sediment slurries. Also, Cr isotope ratio variations in groundwater from one field site will be examined in order to observe this kinetic isotope effect in nature and establish the practical measurement precision.This research has potential impacts in gaining an understanding of Cr isotope systematic that may then be used in further studies applied to groundwater, surface water, oceanographic, and sedimentological studies. Cr isotopes may ultimately be a practical tool for the use in understanding Cr sources and transport at contaminanted sites. This project is designed to be part of a Ph.D. dissertation for a graduate student, to be combined with similar work on Se isotopes that is in progress.