Persistence of Uranium in Old and Cold Subpermafrost Groundwater Indicated by Linking 234 U- 235 U- 238 U, Groundwater Ages, and Hydrogeochemistry

• Main Authors: Elliott K. Skierszkan (6681497), John W. Dockrey (6681506), Jordi Helsen (11778998), Laura-Lee Findlater (11779001), Clément P. Bataille (11779004), Ghislain de Laplante (11779007), Joyce M. McBeth (2328241), K. Ulrich Mayer (2237767), Roger D. Beckie (6681512)
• Format: Other Non-Article Part of Journal/Newspaper
• Language: unknown
• Published: 2021
• Subjects: Biophysics; Biochemistry; Genetics; Ecology; Space Science; Environmental Sciences not elsewhere classified; Chemical Sciences not elsewhere classified; study provides insight; sorption – desorption; ray absorption spectroscopy; mineralized structures around; median 38 μg; hydrogeochemical changes may; 3 </ sup; 238 </ sup; 235 </ sup; 234 </ sup; 14 </ sup; fe ­( iii; u isotope fractionation; phase u present; geogenic u release; elucidate u sources; u /< sup; u -< sup; rich groundwater produced; cold groundwater conditions; groundwater u concentrations; groundwater ages inferred; u ­( vi; ­( vi; groundwater ages; u transport; u mobilization; subarctic groundwater; groundwater indicates; time scales; surface water; sequential extractions; range 1; pivotal time; oxidative weathering; multifaceted isotopic; mobile form; main driver; geochemical approach; fractured bedrock; Yukon; Canada; permafrost; Subarctic
• Online Access: https://doi.org/10.1021/acsearthspacechem.1c00307.s001

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 238 U/ 235 U 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 238 U/ 235 U 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 3 H and 14 C 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.