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

Authors

Yichuan Zhang, Shiping Xing, Chenbo Lei, Huaming Guo

Published in

Water research. Volume 306. Pages 126623. Aug 04, 2026. Epub Aug 04, 2026.

Abstract

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)₃^2- 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.

PMID:
42574793
Bibliographic data and abstract were imported from PubMed on 11 Aug 2026.

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