Behavior and fate of geogenic uranium in a shallow groundwater system

To unveil behavior and fate of uranium (U) in the Quaternary aquifer system of Datong basin (China), we analyzed sediment and groundwater samples, and performed geochemical modeling. The analyses for sediments were implemented by a sequential extraction procedure and measurements including X-ray power diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Concentrations of main elements and U, and ²³⁴U/²³⁸U activity ratios for groundwater were determined. Results show that sediment U contents range from 1.93 to 8.80 (average 3.00 ± 1.69) mg/kg. In relation to the total U, average fractions of residual U (probably as betafite) and U(VI) bound to carbonates and FeMn oxides are 74.4 ± 18.7%, 17.2 ± 13.3%, and 4.3 ± 2.9%, respectively. Lower average fractions were determined for both organic matter- and sulfide-bound U (mainly as U(IV), e.g., brannerite) (2.0 ± 0.7%) and exchangeable U(VI) (2.0 ± 2.8%). For the groundwater (pH 7.36-8.86), Ca₂UO₂(CO₃)₃⁰, CaUO₂(CO₃)₃^2-, and UO₂(CO₃)₃^4- constitute >99.5% of the total dissolved U; and elevated U concentrations occur mainly in shallow aquifers (3-40 m deep below land surface) of the west flow-through and discharge areas, with 50% of the sampled points exceeding 30 μg/L. We argue that betafite and carbonate weathering and U(VI) desorption from ferrihydrite are the primary geochemical processes responsible for U mobilization, with a minor contrition from U(IV) oxidation. Abiotic U(IV) oxidation may be induced mainly by dissolved oxygen under oxic/suboxic conditions (e.g., in the recharge and flow-through areas), but significantly linked to amorphous ferrihydrite under Fe(III)- and sulfate-reducing conditions. Abiotic U(VI) reduction could be caused principally by siderite and mackinawite. Under alkaline conditions, higher HCO₃^- concentrations and lower Ca²⁺/HCO₃^- molar ratios (<~0.2) cause formation of CaUO₂(CO₃)₃^2- and UO₂(CO₃)₃^4-, and U(VI) desorption. With increases in concentrations of Ca²⁺ and Ca²⁺/HCO₃^- ratios (>~0.2), these anionic forms may shift to neutral Ca₂UO₂(CO₃)₃⁰, which can facilitate further desorption of U(VI). Our results improve the understanding of U environmental geochemistry and are important for groundwater resources management in this and similar other Quaternary aquifer systems.