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Determination of Cr(III) solids formed by reduction of Cr(VI) in a contaminated fractured bedrock aquifer: evidence for natural attenuation of Cr(VI)
Citation:
Zhao, J., T. Al, S. Chapman, B. Parker, K. Mishkin, D. Cutt, AND R. Wilkin. Determination of Cr(III) solids formed by reduction of Cr(VI) in a contaminated fractured bedrock aquifer: evidence for natural attenuation of Cr(VI). CHEMICAL GEOLOGY. Elsevier Science Ltd, New York, NY, 474:1-8, (2017).
Impact/Purpose:
The objective of this study was to develop a method to quantify Cr(III)-hydroxides formed in-situ by reduction of Cr(VI) in aquifer matrices. Cr(VI) is soluble, mobile, and toxic; whereas, Cr(III) is generally immobile and less toxic. The method provides a basis for estimating the amount of natural attenuation of hexavalent chromium that occurs in aquifers. Understanding attenuation of hexavalent chromium and having tools to access the amount of attenuation is of interest to EPA Regions and the states.
Description:
Hexavalent chromium Cr(VI) is toxic and can be highly mobile in many aquifer systems. Redox reactions with naturally occurring minerals and organic compounds can reduce Cr(VI) to Cr(III), forming labile Cr(III) oxyhydroxide precipitates, which is a natural attenuation process. In fractured bedrock aquifers, reduction of Cr(VI) in the rock matrix can enhance attenuation beyond that from matrix diffusion only, and potentially reduce back diffusion if concentrations in fractures decline following source reduction via natural processes or engineered remediation. In this study, we develop an extraction method for labile Cr(III) precipitates from Cr(VI) reduction using 5% hydrogen peroxide (H2O2). Combining Cr(III) extractions with an established sodium hydroxide (NaOH) method for determination of Cr(VI) concentrations in rock porewater, a measure of the labile Cr(III) and Cr(VI) fractions in geologic samples is achieved. The methods were applied to cores from a contaminated groundwater system in fractured porous bedrock in order to assess the effectiveness of natural attenuation and whether Cr(VI) mass that diffused into the bedrock matrix was undergoing reduction. Detailed vertical distributions display two depth intervals with corresponding elevated concentrations of Cr(VI) in the porewater and extractable Cr(III). The correspondence of Cr(VI) and Cr(III) provides evidence for reduction of Cr(VI) contamination in the bedrock matrix. This study provides evidence for the natural attenuation of anthropogenic Cr(VI) contamination in the porewater of a fractured bedrock aquifer, and it outlines a quantitative method for evaluating the effectiveness of natural attenuation in groundwater systems.
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Determination of Cr(III) solids formed by reduction of Cr(VI) in a contaminated fractured bedrock aquifer: evidence for natural attenuation of Cr(VI)