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

Abstract

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.

Arsenic Mobilization from Thawing Permafrost

“… ,,, The study area features discontinuous permafrost and is contained within traditional territory of Tr’ondëk Hwëch’in and Selkirk First Nation. Its physiography, geology and permafrost coverage have been described elsewhere. ,,, Briefly, permafrost covers ∼60% of land area, with thicknesses ranging from >150 m near ridgetops to <30 m in valley bottoms . The dominant lithologies include Palaeozoic metamorphic rocks and Cretaceous intrusions locally associated with ore deposits (e.g., Coffee Au, Casino Cu–Au–Mo).…”

“…They were also selected to aid in the understanding of As speciation and mineralogy in the rock upslope of the permafrost soil core locations. These samples were previously characterized for their bulk geochemical properties. , …”

“…This study builds upon several recent investigations on metal­(loid) hydrogeochemistry in the Dawson Range, west-central Yukon, Canada (Figure ). ,,, The study area features discontinuous permafrost and is contained within traditional territory of Tr’ondëk Hwëch’in and Selkirk First Nation. Its physiography, geology and permafrost coverage have been described elsewhere. ,,, Briefly, permafrost covers ∼60% of land area, with thicknesses ranging from >150 m near ridgetops to <30 m in valley bottoms .…”

“…This area was unglaciated during the Pleistocene, preserving sequences of oxidized bedrock 10s to 100s of m in depth. 51,53 The lack of glaciation also means that soils in depositional environments are locally derived, with the exception of a minor veneer of allochthonous glacial loess. 55,57 Subsurface stratigraphy on ridgetops is composed of coarsegrained soils <1 m thick that overlie weathered rock.…”

“…These samples were previously characterized for their bulk geochemical properties. 29,51 An anoxic groundwater spring discharging onto alluvial permafrost near the Coffee deposit with precipitating Fe-(oxyhydr)oxide was also uncovered and sampled during field investigations, as it provided an opportunity to examine As attenuation in discharge zones (Figure S2). Fe-(oxyhydr)oxide precipitates were collected at the spring's point of discharge and placed in a clean HDPE bottle that was frozen until further processing in the laboratory.…”