Variations of uranium concentrations in a multi-aquifer system under the impact of surface water-groundwater interaction

Abstract

Understanding uranium (U) mobility is vital to minimizing its concentrations in potential drinking water sources. In this study, we report spatial-seasonal variations in U speciation and concentrations in a multi-aquifer system under the impact of Sanggan River in Datong basin, northern China. Hydrochemical and H, O, Sr isotopic data, thermodynamic calculations, and geochemical modeling are used to investigate the mechanisms of surface water-groundwater mixing-induced mobilization and natural attenuation of U. In the study site, groundwater U concentrations are up to 30.2 μg/L, and exhibit strong spatial-seasonal variations that are related to pH and Eh values, as well as dissolved Ca²⁺, HCO₃^-, and Fe(III) concentrations. For the alkaline aquifers of this site (pH 7.02-8.44), U mobilization is due to the formation and desorption of Ca₂UO₂(CO₃)₃⁰ and CaUO₂(CO₃)₃^2- caused by groundwater Ca²⁺ elevation via mineral weathering and Na-Ca exchange, incorporated U(VI) release from calcite, and U(IV) oxidation by Fe(OH)₃. U immobilization is linked to the adsorption of CaUO₂(CO₃)₃^2- and UO₂(CO₃)₃^4- shifted from Ca₂UO₂(CO₃)₃⁰ because of HCO₃^- elevation and Ca²⁺ depletion, U(VI) co-precipitation with calcite, and U(VI) reduction by adsorbed Fe²⁺ and FeS. Those results are of great significance for the groundwater resource management of this and similar other surface water-groundwater interaction zones.

Keywords: Surface water-groundwater interaction; U mobility; U speciation.