IN-SITU REMEDIATION OF URANIUM FOLLOWING ISR MINING (Invited Presentation)

In-situ recovery (ISR) mining is a common method of extracting uranium (U) through injection of an oxidant to oxidize mineralized U(IV) to soluble U(VI) and a complexing agent to produce mobile U(VI) species. This process often results in elevated uranium concentrations in groundwater post-mining. A promising strategy for remediating groundwater following ISR mining is through in-situ remediation where a reductant is injected into the aquifer to reduce U(VI) back to relatively insoluble U(IV). In this study, we performed several experiments utilizing the chemical reductant dithionite and/or the microbial stimulant acetate to induce U(VI) reducing conditions as a part of the restoration process following ISR mining at the Smith Ranch-Highland Mine in WY. Two push-pull experiments using dithionite demonstrated U(VI) reduction and removal as observed by decreasing U(VI) concentrations and δ²³⁸U. However, during injection of dithionite and acetate in larger multi-well experiments, U(VI) concentrations increased significantly due to desorption from aquifer sediments and the reduction/dissolution of ferric solids containing adsorbed U. Thermodynamic considerations imply that ferric solids were preferentially reduced before large-scale U(VI) reduction could begin, and this caused the reductant/stimulant to be consumed before large-scale U(VI) reduction could occur during the cross-hole test. These experiments provide insight into how better understanding restoration redox chemistry and adsorption/desorption are critical for engineering effective remediation strategies.