H31I-1540
Field Testing of Downgradient Uranium Mobility at an In-Situ Recovery Uranium Mine

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Paul W. Reimus1, James T Clay2, Michael Rearick3, George Perkins1, Shaun T Brown4, Anirban Basu5 and Kevin Chamberlain6, (1)Los Alamos National Laboratory, Los Alamos, NM, United States, (2)Cameco Resources, Casper, WY, United States, (3)Los Alamos Natl Lab, Los Alamos, NM, United States, (4)Lawrence Berkeley National Laboratory, Berkeley, CA, United States, (5)University of California Berkeley, Berkeley, CA, United States, (6)University of Wyoming, Laramie, WY, United States
Abstract:
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.

Three single-well push-pull tests and one cross-well test 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 tests, non-reactive tracers were injected with the previously-mined ore zone water to allow the transport of uranium and other constituents to be compared to that of the nonreactive species. In the single-well tests, it was shown that the recovery of uranium relative to the nonreactive tracers ranged from 12-25%, suggesting significant attenuation capacity of the aquifer. In the cross-well test, selenate, molybdate and metavanadate were injected with the unrestored water to provide information on the transport of these potentially-problematic anionic constituents. In addition to the species-specific transport information, this test provided valuable constraints on redox conditions within the system, as redox couples involving these species collectively bracket the predicted transition redox potential for the U(VI)/U(IV) couple. Reduction should provide much longer-lasting immobilization of constituents than adsorption, especially given the inherent reducing characteristics of roll-front systems.

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