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

Geochemical Controls on Uranium Release from Neutral-pH Rock Drainage Produced by Weathering of Granite, Gneiss, and Schist

“…97 million years ago through interaction of Au-As-Sb-S bearing fluid with metamorphic and granitic country rock along structurally controlled faults and fractures [49,50]. The deposit is located in the Dawson Range, wherein plutonic and metamorphic rocks are regionally enriched in U relative to typical crustal rocks [51]. Weathering of these rocks produces baseline U concentrations that can exceed 500 µg/L in groundwater and 300 µg/L in surface water, i.e., an order of magnitude above water-quality guidelines, making U an element of regional environmental concern [51].…”

“…The deposit is located in the Dawson Range, wherein plutonic and metamorphic rocks are regionally enriched in U relative to typical crustal rocks [51]. Weathering of these rocks produces baseline U concentrations that can exceed 500 µg/L in groundwater and 300 µg/L in surface water, i.e., an order of magnitude above water-quality guidelines, making U an element of regional environmental concern [51]. The dominant local geological units are the Permian-aged Sulphur Creek orthogneiss, Klondike schist, and Snowcap schist, which were intruded by the Cretaceous-aged Coffee Creek biotite granite [49].…”

“…Rocks at the Coffee deposit are modestly enriched in U relative to typical crustal rocks, with a median abundance of 3.7 µg/g and a maximum of 144 µg/g (Table 1), both above the 2.7 µg/g average of Earth's upper crust [59]. The deposit is also enriched in U in comparison with regional country rocks: Sulphur Creek suite orthogneiss and Whitehorse suite granite samples collected outside the Coffee deposit have median abundances of 3.3 and 3.2 µg/g, respectively [51]. The lack of clear relationships between U and hydrothermally sourced elements As, S, and Sb in fresh-facies rocks in the deposit suggests that its U was not sourced from the hydrothermal system (Supplementary Figure S3).…”

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

“…Uranium is a potentially hazardous contaminant in groundwater that is commonly derived from geogenic sources. − Elevated exposure to U causes tissue damage in humans and aquatic organisms. − In Canada, water-quality guidelines are 15 μg/L U for the protection of aquatic life (chronic exposure) and 20 μg/L U for drinking water. , Groundwater unaffected by anthropogenic activity frequently exceeds these concentrations, especially in aquifers hosted in granitic rocks (or their metamorphosed derivatives) which tend to be enriched in U through magmatic differentiation. ,,,,,− …”

“…238 U/ 235 U is therefore an effective tracer of U mobility in groundwater at U-ore deposits and sites contaminated by anthropogenic activity. ,− , However, few high-precision 238 U/ 235 U measurements of groundwater are available at sites unaffected by anthropogenic activity despite the frequent occurrence of this element as a geogenic contaminant . No studies of 238 U/ 235 U exist in subarctic groundwater, despite the commonality of elevated geogenic U [e.g., Canada, , Scandinavia, ,− ]. Unique hydrogeochemical conditions in cold climates include low temperatures that significantly inhibit U­(VI) reduction kinetics and may therefore exacerbate U mobility. , Furthermore, these regions are dominated by permafrost that is thawing due to climate change and leading to alteration in physical and chemical hydrogeology with unknown impacts on geogenic U release. , …”