Using Groundwater and Solid-Phase Geochemistry for Reactive Transport Modeling at the Proposed Dewey Burdock Uranium In-Situ Recovery Site, Edgemont, South Dakota

Uranium deposits at the Dewey Burdock site near Edgemont, South Dakota are being proposed for in-situ recovery mining, which extracts uranium by enhanced dissolution and mobilization of solid-phase uranium in sandstone aquifers. Understanding the geochemical changes that occur with the in-situ recovery process can assist local groundwater users, regulatory agencies, and other stakeholders in evaluating the potential effects on groundwater quality during and after mining.

Paleogroundwater flowing down dip from the Black Hills uplift contained uranium and oxygen in solution that formed uranium ore deposits at oxidation/reduction fronts due to the presence of organic carbon and/or pyrite. On the up gradient side of the front, the groundwater remained oxidizing and contained dissolved uranium, since the solid phase did not contain pyrite or organic carbon. On the down gradient side of the front, groundwater was reduced and uranium precipitated due to the presence of solid-phase organic carbon and/or pyrite. However, groundwater flow and geochemistry are currently different from the initial depositional conditions and no additional uranium ore is currently forming. Two general conditions exist: 1) up gradient groundwater entering the existing uranium ore deposit does not contain dissolved oxygen or uranium, and the down gradient groundwater enters a reducing zone, and 2) the up gradient groundwater contains dissolved oxygen but no dissolved uranium, crosses a reducing zone with uranium ore, and the down gradient groundwater enters a zone of previous oxidization (no organic carbon or pyrite in the solid phase).

Solid-phase geochemistry (mineralogy and reducing capacities) provides semi-quantitative information on geochemical conditions that post-mining groundwater might encounter. Using the solid-phase information, the two general conditions described above, and several post-mining groundwater quality scenarios, long-term transport of dissolved uranium in groundwater away from the post-mining zone is modeled using reactive transport simulations.