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Reactive transport of uranium in a groundwater bioreduction study: Insights from high-temporal resolution ²³⁸U/²³⁵U data

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  • We conducted a detailed investigation of U isotopes in conjunction with a broad geochemical investigation during field-scale biostimulation and desorption experiments. This investigation was carried out in the uranium-contaminated alluvial aquifer of the Rifle field research site. In this well-characterized setting, a more comprehensive understanding of U isotope geochemistry is possible. Our results indicate that U isotope fractionation is consistently observed across multiple experiments at the Rifle site. Microbially-mediated reduction is suggested to account for most or all of the observed fractionation as abiotic reduction has been demonstrated to impart much smaller, often near-zero, isotopic fractionation or isotopic fractionation in the opposite direction. Data from some time intervals are consistent with a simple model for transport and U(VI) reduction, where the fractionation factor (ε = +0.65‰ to +0.85‰) is consistent with experimental studies. However, during other time intervals the observed patterns in our data indicate the importance of other processes in governing U concentrations and ²³⁸U/²³⁵U ratios. For instance, we demonstrate that departures from Rayleigh behavior in groundwater systems arise from the presence of adsorbed species. We also show that isotope data are sensitive to the onset of oxidation after biostimulation ends, even in the case where reduction continues to remove contaminant uranium downstream. Our study and the described conceptual model support the use of ²³⁸U/²³⁵U ratios as a tool for evaluating the efficacy of biostimulation and potentially other remedial strategies employed at Rifle and other uranium-contaminated sites.
  • Keywords: Bioremediation, Uranium isotopes, Uranium reduction, MC-ICP-MS, Isotope fractionation
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  • Shiel, A. E., Johnson, T. M., Lundstrom, C. C., Laubach, P. G., Long, P. E., & Williams, K. H. (2016). Reactive transport of uranium in a groundwater bioreduction study: Insights from high-temporal resolution ²³⁸U/²³⁵U data. Geochimica et Cosmochimica Acta, 187, 218-236. doi:10.1016/j.gca.2016.05.020
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  • 187
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  • Funding was provided through the U.S. Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research under contracts DE-SC0006755 (University of Illinois at Urbana-Champaign) and DE-AC02-05CH11231 (Lawrence Berkeley National Laboratory; operated by the University of California). This material is based upon work equally supported through the Integrated Field Research Challenge Site (IFRC) at Rifle, Colorado and the Lawrence Berkeley National Laboratory's Sustainable Systems Scientific Focus Area.
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