Uranium\nSpeciation\nand Mobilization in Thawing Permafrost

Uranium is a toxic and pervasive\ngeogenic contaminant\noften associated\nwith organic matter. Its abundance and speciation in organic-rich\npermafrost soils are unknown, thereby limiting our ability to assess\nrisks associated with uranium mobilization during permafrost thaw.\nIn this study, we assessed uranium speciation in permafrost soil and\nporewater liberated during thaw using active-layer and permafrost\nsamples from a study area in Yukon, Canada where elevated uranium\nconcentrations occur in bedrock and groundwater. Permafrost contained\n1.1–28 wt % organic carbon and elevated uranium (range 7.6–1040\nμg g<sup>–1</sup>, median 25 μg g<sup>–1</sup>) relative to local bedrock. The highest soil uranium concentrations\nwere encountered in catchments hosting uranium-enriched bedrock and\ncorrelated positively with soil organic carbon. X-ray absorption spectroscopy,\nmicro-X-ray fluorescence, and electron microscopy analyses revealed\nthat solid-phase uranium predominantly occurs as uranium(VI) associated\nwith soil organic matter. Extended X-ray absorption fine structure\n(EXAFS) analyses suggested the presence of uranium(VI) coordinated\nwith carbon, consistent with bidentate-mononuclear uranyl complexation\non carboxyl groups. Permafrost thaw produced circumneutral pH porewater\n(pH 6.2–7.5) with elevated dissolved uranium (0.5–203\nμg L<sup>–1</sup>). Geochemical modeling indicated that\ncalcium-uranyl-carbonate complexes dominated the dissolved uranium\nspeciation. This study highlights that permafrost soil can mobilize\nuranium upon thaw and that uranium fate is linked to dynamic biogeochemical\nreactions involving organic carbon and groundwater chemistry.