Uranium (im)mobilization driven by evaporite wet-dry cycling: impact on groundwater quality

Groundwater quality can be profoundly altered when flow coincides spatially or temporally with enhanced biogeochemical activity. Such conditions can be triggered by hydrological transitions (e.g., flooding or drought). The wet-dry cycling is a hot moment for such hydrological-biogeochemical coupling as it can successively precipitate and dissolve minerals, and consequently trap and release contaminants. As an example, evaporite mineralization are suspected to trap contaminants in dry conditions, but their dissolution during water infiltration could significantly contribute to contamination of the groundwater. In context of the impending climate crisis, leading to extreme weather events and droughts, the wet-dry cycling could be dramatically perturbed. However, contaminant (im)mobilization driven by evaporite wet-dry cycling has received relatively little research attention. We have investigated the impact of hydrological-biogeochemical coupling on U (im)mobilization in the contaminated floodplain aquifer at the Riverton, WY legacy U ore processing site. Our results show that seasonal changes in moisture content trigger strong downward and upward U transport as the depth of saturated-unsaturated interface varies. Under warm summer conditions, capillary rise driven by evapotranspiration rapidly transports U from the shallow aquifer up into the unsaturated zone and precipitate as U-bearing evaporites. Whereas under spring flooding, the U is rapidly flushed from soils due to evaporite dissolution, releasing U back to the groundwater. We however detected a second evaporite wet-dry mechanism localized in transiently reduced low permeability sediments. During spring flooding, reducing processes accumulate U(IV), while during drying season evapotranspiration promotes conversion of U(IV) to U(VI) trapped in evaporite mineral assemblages that resist dissolution during rewetting. The resistance to dissolution is likely due to slow exchange fluxes and higher saturation indexes of porewaters in low permeability sediments. This study underlines the need to better characterize the specific conditions promoting trapping/release of U to enable predictions of U (im)mobilization in response to climate change driven alterations of wet-dry cycles.