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

doi:10.1021/acs.est.6b01464

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

Environ Sci Technol. 2016 Oct 18;50(20):10833-10842. doi: 10.1021/acs.est.6b01464. Epub 2016 Oct 4.

Authors

Affiliations

¹ Department of Earth and Planetary Science, University of California , 307 McCone Hall, Berkeley, California 94720, United States.
² Department of Environmental Science, Policy and Management, University of California , 130 Mulford Hall, Berkeley, California 94720, United States.
³ Lawrence Berkeley National Laboratory , 1 Cyclotron Road, Berkeley, California 94720, United States.
⁴ Department of Geology, University of Illinois at Urbana-Champaign , 605 East Springfield Avenue, Champaign, Illinois 61820, United States.
⁵ Uranium Resources, Inc. , 6950 South Potomac Street, Suite 300, Centennial, Colorado 80112, United States.
⁶ Earth and Environmental Sciences Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.

PMID: 27547844
DOI: 10.1021/acs.est.6b01464

Abstract

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 δ⁸²Se_VI 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.