Quantification of Hexavalent Cr Reduction in Groundwater Using Cr Stable Isotopes

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0229079JohnsonThis project will develop chromium (Cr) stable isotope ratios as a new tool for understanding how this toxic element moves in contaminated groundwater. Cr contamination is widespread, because the electroplating, leather tanning, and wood treatment industries use large volumes of highly concentrated Cr(VI) (hexavalent Cr) solutions. Cr(VI) moves readily with groundwater as a highly soluble oxyanion, but in many groundwater systems, it undergoes chemical reduction to Cr(III), which is less soluble and less toxic. Preliminary work by this group has demonstrated that this reduction reaction involves a fractionation of the stable isotopes 53Cr and 52Cr. The products of the reduction reaction are enriched in 52Cr, the lighter isotope, relative to the reactants, and as reduction proceeds, the remaining unreduced Cr(VI) becomes progressively more enriched in the heavier isotope relative to its starting composition. The working hypothesis of this project is that the 53Cr/52Cr ratio of the Cr(VI) in groundwater provides a quantitative indication of the amount of Cr(VI) reduction that has occurred. This project addresses several basic questions raised in previous work that must be answered before this new approach can be used quantitatively and with confidence in practical applications. The primary objectives are 1) to assess variability in the size of the isotopic fractionation during Cr(VI) reduction, 2) determine the isotope fractionations, if any, that are induced by Cr(VI) sorption and Cr(III) oxidation, and 3) assess 53Cr/52Cr variability among contaminant sources. The work will involve several experiments in the laboratory, analysis of groundwater samples from Cr(VI) contamination plumes, and analysis of industrial and natural Cr. This laboratory-based study will be complemented by a field study to be funded from other sources. It will be carried out as three separate projects for graduate students, who will be trained in environmental geochemistry, hydrogeology, and isotope geochemistry. The focus is on groundwater contamination, but most of what is learned here may be applied to other areas of geochemistry and oceanography.