https://doi.org/10.1021/acsearthspacechem.1c00307

Skierszkan, E.K., Dockrey, J.W., Helsen, J., Findlater, L.-L., Bataille, C.P., de Laplante, G., McBeth, J.M., Mayer, K.U., Beckie, R.D. (2021) Persistence of subpermafrost groundwater uranium indicated by linking 234U-235U-238U, groundwater ages, and hydrogeochemistry. Earth & Space Chem. 5(12), 3474-3487. https://doi.org/10.1021/acsearthspacechem.1c00307

Uranium (U) contamination in groundwater from geogenic sources affects water quality globally. Here, we use a multifaceted isotopic and geochemical approach to elucidate U sources and controls on geogenic U release to groundwater and surface water at a prospective subarctic gold deposit in Yukon, Canada, that is characterized by permafrost, fractured bedrock, and cold (<2 °C) groundwater. X-ray absorption spectroscopy, sequential extractions, and micro X-ray fluorescence mapping show extensive subsurface oxidation and solid-phase U present in its hexavalent and mobile form. Limited 238U/235U isotope fractionation and predominance of U(VI) in rocks suggest U(VI) sorption–desorption is the main driver of U mobilization. Groundwater U concentrations are appreciable (median 38 μg/L, range 1.2–535 μg/L) and are explained by high-alkalinity, Ca-rich groundwater produced from oxidative weathering of sulfide and carbonate-mineralized structures around the deposit. Minor 238U/235U isotope fractionation in groundwater indicates that limited U(VI) reduction occurs beneath permafrost despite groundwater redox conditions below Fe(III) and S(VI) reduction, and groundwater ages inferred from 3H and 14C to be on the order of thousands of years. The complexation of U as uranyl–calcium–carbonate complexes and the resilience of these complexes to U(VI) reduction contributes to high U(VI) mobility under cold groundwater conditions. This study provides insight into processes and time scales of U transport in subarctic groundwater at a pivotal time when hydrogeochemical changes may be anticipated in cold regions worldwide due to permafrost degradation.

Geogenic Contamination of Groundwater Resources in Subarctic Regions, Refereed Publications

https://doi.org/10.1021/acsearthspacechem.1c00307