Uranium (U) is a globally pervasive geogenic toxin with no known biological function. (1−3) The World Health Organization and the United States Environmental Protection Agency set a guideline of 30 micrograms L^-1 for drinking-water quality. (4,5) Canadian guidelines are 20 micrograms L^-1 for drinking water, and 15 micrograms L^-1 for the protection of aquatic life. (6,7) Dissolved U concentrations in groundwater and surface water are generally less than 5 micrograms L^-1, but can be 100–1000’s of times higher in regions impacted by geogenic or anthropogenic contamination. (1,3) Uranium mobility is closely tied to its oxidation state: U(VI) prevails under oxidizing conditions and has relatively higher mobility than U(IV), which forms poorly soluble phases. (8)

Organic-rich soils (ORS) are globally important U sinks, concentrating it to levels of ecotoxicological concern and economic interest. (9−15) While the average upper Earth crustal abundance is 2.7 micrograms^-1 U, organic-rich peatlands downgradient of uraniferous groundwater flowpaths can accumulate Greater than 5000 micrograms g^-1U. (16) This enrichment involves a combination of redox reactions and pH-dependent surface and aqueous complexation reactions that sequester U onto organic matter. (8,14−19) Reducing conditions that are characteristic of water-saturated ORS can favor U attenuation by forming insoluble noncrystalline U(IV) monomers and uraninite. (8,14,17,20,21) Several recent studies have also documented U(VI) enrichment in ORS. (14−16,18) Uranium in both its prevailing oxidation states adsorbs onto organic functional groups, which outcompete inorganic sorbents such as Fe-(oxyhydr)oxides and clays for U binding in ORS. (14,15,17,18) Uranium(VI) adsorption is typically greatest in the pH range of approximately 5 to 8, but is inhibited by U aqueous complexation with organic and inorganic ligands. (14,22) In most natural waters, dissolved organic matter (DOM) and calcium-uranyl-carbonate complexes [e.g., Ca(UO₂)(CO₃)₃^-2 Ca2(UO₂)(CO₃)₃^-2 dominate aqueous U(VI) speciation. The balance between these species is a function of pH and ligand availability (e.g., DOM, Ca²⁺, CO₃^2-), which also influence electrostatic attraction of U toward adsorption sites. (22−27) In addition to inhibiting U(VI) adsorption, these aqueous complexes can enhance U mobility by inhibiting U(VI) reduction kinetics. (14,22,28,29)

Permafrost soils are estimated to contain half of the global soil carbon (C) reservoir, and are thawing at an accelerated rate because of postindustrial global warming. (30,31) Permafrost thaw is accompanied by biodegradation of previously frozen organic matter, activation of flowpaths, and increased mineral-microbe interactions. (32,33) These processes collectively drive dynamic changes in the key variables governing metal mobility [e.g., pH, oxidation–reduction potential (EH), DOM, soil organic C, and major-ion chemistry]. Recent field observations indicate that shifting hydrological conditions and biogeochemical reactions resulting from permafrost thaw mobilize metal(loid)s such as arsenic, (34) iron, (35,36) mercury, (37,38) and selenium. (39,40) To our knowledge, no studies have yet examined U speciation, abundance, or mobility in thawing permafrost soils, despite its occurrence in subpermafrost groundwater (41) and its tendency to accumulate in ORS─which are ubiquitous in (sub)arctic soils. Uranium mobilization during thaw of permafrost organic matter therefore constitutes an unknown risk to water resources in cold regions. (41,42)

In the present study, we address this knowledge gap with an analysis of the chemical speciation and abundance of U in permafrost soils and thawed porewaters collected from a subarctic study area of thawing discontinuous permafrost...

Figure 1. Concentrations of U in (A) soil porewater; and (B) bedrock (lighter shades) and soil (darker shades), grouped by catchment. MAC = maximum allowable U concentration in Canadian drinking water; (64) AUC = average upper Earth crustal U abundance. (48) Bedrock data from Skierszkan et al. (42) and Yukon Geological Survey. (65)

Figure 4. Compilation of dissolved U in ∼34,000 stream water samples over western North American subarctic from Canadian (89) and American (90) federal data sets. Panel (A) shows U concentrations over the regional permafrost distribution (from Obu et al. (91)) Panel (B) shows U concentrations over various Geochemical Provinces of Yukon. (92) Numbers delineate clusters of U-rich streams over various geological terranes including: (1) and (2) Cretaceous granitic and Paleozoic metamorphic rocks of the Yukon-Tanana Terrane; (3) and (4) interbedded Paleozoic marine siliciclastic and carbonate rocks; and (5) Paleozoic marine black shales and limestones intruded by Cretaceous granites.

Our results reveal that permafrost ORS can be enriched in U by 10–100 s of times relative to local bedrock. This U pool is dominated by its mobile hexavalent state and is complexed onto soil organic matter. Incipient thaw of this material can produce porewater U concentrations that exceed safe thresholds (e.g., for drinking water and for the protection of aquatic life), especially downgrading of U-rich bedrock. Thermokarst processes can also export this material to surface water bodies. The fate of U after permafrost thaw is uncertain as it depends on complex and coupled interactions between fluid transport, microbial activity, and transient changes in groundwater pH, E_H, major-ion chemistry, and DOM composition. The uncertain outcomes of these processes on U mobility, bioavailability, and toxicity warrant further investigation, particularly given the occurrence of U-rich geological terranes and aquifers in permafrost-dominated subarctic regions worldwide (e.g., Figure 4; Alaska, (93) Yukon, (41) Northern Canada’s U-rich Athabasca Basin, (94) Scandinavia (3,95)).