Hydrogeochemical Characteristics and Formation Mechanisms of Uranium-rich Groundwater in the Bayanwula Area, Inner Mongolia, China.

Sandstone-hosted uranium (U) deposits are genetically linked to hydrogeochemical processes, with groundwater serving as the principal medium for uranium activation, migration, and immobilization. To elucidate the evolutionary characteristics and primary factors of aqueous uranium, this study jointly employs hydrochemical characteristics, ion ratio analysis and geochemical modeling to characterize the hydrogeochemical evolution, the spatial distribution and speciation of aqueous uranium. The results indicate that groundwater in the study area is weakly alkaline brackish water of Cl·SO4-Na type. Its hydrochemical evolution is primarily controlled by silicate mineral dissolution coupled with intense direct cation exchange. Dissolved uranium concentrations in groundwater vary from 0.26 to 114.00 μg/L, with a mean value of 28.55 μg/L. Uranium anomalies in groundwater display pronounced stratigraphic and spatial compartmentalization: elevated uranium concentrations are predominantly observed in the K1S confined aquifer, whereas the C-P aquifer consistently exhibits low background levels. Uranium enrichment is co-driven by oxidative conditions and Ca2+-mediated complexation. Oxidants (e.g., NO3 -) promote oxidative dissolution of tetravalent uranium minerals into soluble uranyl ions. In addition, the dissolution of carbonate minerals supplied sufficient Ca2+ and HCO3 - for uranyl complexation. Abundant Ca2+ further stabilizes dissolved uranium as neutral Ca2UO2(CO3)3 complexes, which markedly enhance uranium enrichment and mobility in groundwater. The positive correlation between carbonate saturation indices and uranium concentrations provides further thermodynamic evidence for this complexation-driven enrichment pattern. These findings decipher the hydrogeochemical driving mechanisms of uranium migration and provide vital scientific guidance for uranium exploration in sandstone aquifers of the Erlian Basin and analogous basins.