GSA Connects 2022 meeting in Denver, Colorado

Paper No. 50-8
Presentation Time: 3:25 PM

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


JEMISON, Noah, University of New Mexico, Center for Micro-Engineered Materials, Albuquerque, NM 87106; University of New Mexico, Center for Water and the Environment, Albuquerque, NM 87131, REIMUS, Paul W., Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545 and CLAY, James, Cameco Resources, Douglas, WY 82633

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.