Release of geogenic uranium and arsenic results in water-quality impacts in a subarctic permafrost region of granitic and metamorphic geology

This study investigates geogenic U and As mobilization in relation to bedrock geology, groundwater geochemistry, and the presence of mineral deposits in the Dawson Range, Yukon, Canada, a remote subarctic region that has drawn recent interest from mining industry. For this study we compiled and interpreted a geochemical dataset from the region that includes 1075 rock samples, 365 sediment samples, 3189 surface water samples, and 384 groundwater samples. Elevated concentrations of U and As occur through natural weathering processes. Median U concentrations exceed the Canadian guideline for the protection of aquatic life of 15 mu g/L (long-term exposure) at 8% of 547 surface-water monitoring locations and the maximum observed concentration is 337 mu g/ L. 39% of monitoring wells have median groundwater U concentrations above this guideline and the maximum observed concentration is 589 mu g/L. Uranium mobilization is driven by weathering of granitic and metamorphic bedrock that has contents near or slightly above the average upper crustal abundance of 2.7 mu g/g and by formation of soluble calcium-carbonato-uranyl complexes in groundwater. Arsenic is more heterogeneously distributed in rock than U, with localized enrichment occurring near sulfide-mineral bearing ore deposits. While 2000 mu g/L As is observed present in chemically reduced groundwater, As attenuation is also observed through sorption onto Fe-(oxyhydr)oxides. Despite the occurrence of As-rich groundwater, surface-water As concentrations are consistently low (< 1 mu g/L), likely due to As attenuation in groundwater discharge zones. This work provides understanding of processes controlling U and As at a regional scale and presents baseline against which possible water-quality changes induced by climate change or industrial activity can be evaluated.