PROCESSES CONTROLLING THE NATURAL ATTENUATING CAPACITY OF AN AQUIFER DOWNGRADIENT FROM A URANIUM ROLL-FRONT DEPOSIT, WYOMING, USA

Some sandstone-hosted uranium (U) deposits are mined using in-situ recovery (ISR), a solution-mining method where U is oxidized and dissolved in the subsurface, then transported to the surface, and recovered. Uncertainty exists about how mineralogical heterogeneities within sandstone-hosted U deposits can affect the efficiency of U recovery during mining, post-mining aquifer remediation, and groundwater quality over time. Following ISR, groundwater undergoes a restoration process. However, restoration goals are not easily met and contaminant concentrations can rebound post-remediation. More information is needed regarding the long-term effects of ISR mining and groundwater restoration. It is generally thought that reducing conditions downgradient may favor natural attenuation of U and other elements of concern (Se, As, Mo, and S). Understanding how mineralogy contributes to an aquifer’s natural attenuation is important to help determine the extent of post-mining remediation necessary to protect groundwater quality.

This study used aquifer material collected downgradient from a roll-front ore zone at Smith Ranch-Highland, WY to evaluate the attenuating capacity of the aquifer. Multiple depth intervals were combined to make a composite sample, which was characterized for mineralogy, bulk chemistry, and iron and sulfur speciation. Batch uptake experiments were performed using the composite of aquifer sediment and an artificial groundwater having varying U(VI) concentrations (from 0-50 mM). Additional batch uptake experiments used artificial groundwater with 50 mM U(VI) in aquifer sediments amended with solid phase organic carbon and pyrite. Experiments were conducted in both oxic and anoxic conditions. Results show U attenuation greater than 50% in uptake experiments and that the addition of organic carbon increased attenuation in the presence and absence of oxygen. Pyrite amendments had a smaller effect on attenuation with oxic solutions showing no additional attenuation, but anoxic solutions showed a 5% (~4 mM) increase in U attenuation. The results suggest that aquifer sediment downgradient from mined zones has substantial natural attenuating capacity and that zones with higher concentrations of organic carbon and/or sulfide minerals can enhance attenuation.