TESTING OF DOWNGRADIENT URANIUM MOBILITY AT AN IN-SITU RECOVERY MINING SITE

In-situ recovery (ISR) mining of uranium involves the injection of O₂ and CO₂ (or NaHCO₃) into saturated roll-front deposits to oxidize and solubilize the uranium, which is then removed by ion exchange at the surface and processed into U₃O₈. While ISR is economical and environmentally-friendly relative to conventional mining, one of the challenges of extracting uranium by this process is that it leaves behind a geochemically-altered aquifer that is exceedingly difficult to restore to pre-mining geochemical conditions, a regulatory objective.

In this research, we evaluated the ability of the aquifer downgradient of an ISR mining area to attenuate the transport of uranium and other problem constituents that are mobilized by the mining process. Such an evaluation can help inform both regulators and the mining industry as to how much restoration of the mined ore zone is necessary to achieve regulatory compliance at various distances downgradient of the mining zone even if complete restoration of the ore zone proves to be difficult or impossible. Any reduction in the amount of active restoration necessary will translate into water and cost savings.

Three single-well push-pull tests were conducted in which water from an unrestored, previously-mined ore zone was injected into an unmined ore zone that served as a geochemical proxy for the downgradient aquifer. In all the tests, non-reactive tracers were injected with the previously-mined ore zone water to allow the transport of uranium to be compared to that of the nonreactive species, and it was shown that the recovery of uranium relative to the nonreactive tracers ranged from 12-25%. This result suggests significant attenuation capacity of the aquifer, especially considering the rather limited interrogation volume of the single-well test method. Uranium isotope analyses were inconclusive regarding the fate of the unrecovered uranium (i.e., adsorption or reduction). Reduction would, in principle, provide much longer-lasting immobilization than adsorption, especially given the inherent reducing characteristics of roll-front systems. A cross-well tracer test is planned to further evaluate uranium attenuation mechanisms in the downgradient aquifer at the site.