Uranium isotopes reveal the enrichment and attenuation processes of groundwater uranium in floodplain aquifers.

Although uranium (U) enrichment and attenuation in natural groundwater have been widely studied, geochemical controls of U behavior along redox gradients remain poorly constrained. This study investigated U isotopes and geochemical characteristics of groundwater along the redox gradients from the alluvial fan (AF) through the transition area (TA) to the flat plain (FP) of the Hetao Basin. Groundwater U concentrations markedly decreased along the flow path, accompanied by the decline in δ238U values and the increase in 234U/238U activity ratios (AR). High U concentrations and high Eh values in AF groundwater resulted from oxidative dissolution of U-bearing minerals promoting U enrichment. The positive correlations between U and Ca2+, Mg2+, and HCO3- and the predominance of CaUO2(CO3)32- and Ca2UO2(CO3)3 in U species indicated that U complexation enhanced U mobilization. Lower Eh, U concentrations, and δ238U values in FP groundwater suggested that U attenuation was induced by the reduction of U(VI) to U(IV) and precipitation. U attenuation was coupled with sulfate reduction and Fe(III) reduction. A significant negative correlation between U concentrations and δ238U values in TA and FP demonstrated the contribution of adsorption to U attenuation. Rayleigh fractionation modeling and isotope mass-balance calculations revealed that reductive precipitation accounted for 75.2 to 98.4 % of U attenuation, whereas adsorption contributed to 1.56 to 24.8 % in TA and FP groundwaters. This study highlights the different U enrichment and attenuation processes along the redox gradients, and predominant contributions of U reduction to U attenuation in reducing groundwaters.