Tracing U Mobility in Deep Groundwater Using Ra Isotopes

Abstract The mobility of natural U is compared among four boreholes in a fractured granite using Ra isotopes and geochemical modelling. 222 Rn/ 226 Ra activity ratios (ARs) spanning an order of magnitude underline differences in reactive surface area. ( 224 Ra/ 228 Ra) ARs up to 9 indicate recent changes in hydrogeochemistry, and ( 226 Ra/ 228 Ra) ARs 0.6-30 indicate variable deposition of U. Dissolved U is related to dissolution of a solid U(VI) phase by groundwater with HCO 3 − >20 mg·L -1 . U reduction is hindered by Ca 2 UO 2 (CO 3 ) 3 0 .