Se Isotopes as Groundwater Redox Indicators: Detecting Natural Attenuation of Se at an in Situ Recovery U Mine

Anirban Basu, Kathrin Schilling, Shaun T. Brown, Thomas M. Johnson, John N. Christensen, Matt Hartmann, Paul W. Reimus, Jeffrey M. Heikoop, Giday WoldeGabriel, Donald J. DePaolo

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One of the major ecological concerns associated with the in situ recovery (ISR) of uranium (U) is the environmental release of soluble, toxic selenium (Se) oxyanions generated by mining. Post-mining natural attenuation by the residual reductants in the ore body and reduced down-gradient sediments should mitigate the risk of Se contamination in groundwater. In this work, we investigate the Se concentrations and Se isotope systematics of groundwater and of U ore bearing sediments from an ISR site at Rosita, TX, USA. Our results show that selenate (Se(VI)) is the dominant Se species in Rosita groundwater, and while several up-gradient wells have elevated Se(VI), the majority of the ore zone and down-gradient wells have little or no Se oxyanions. In addition, the δ82SeVI of Rosita groundwater is generally elevated relative to the U ore up to +6.14‰, with the most enriched values observed in the ore-zone wells. Increasing δ82Se with decreasing Se(VI) conforms to a Rayleigh type distillation model with an ε of −2.25‰ ± 0.61‰, suggesting natural Se(VI) reduction occurring along the hydraulic gradient at the Rosita ISR site. Furthermore, our results show that Se isotopes are excellent sensors for detecting and monitoring post-mining natural attenuation of Se oxyanions at ISR sites.
Original languageEnglish
Pages (from-to)10833-10842
Number of pages10
JournalEnvironmental Science and Technology
Issue number20
Early online date22 Aug 2016
Publication statusPublished - 18 Oct 2016

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