Groundwater Quality at In Situ Leach Uranium Mines

In situ leach (ISL) uranium mining extracts uranium from the subsurface using chemically amended groundwater to dissolve uranium minerals in an aquifer so that the dissolved metal can be pumped to the surface and recovered in an on-site plant. Uranium ore amenable to ISL mining occurs in permeable sands and sandstone aquifers. The uranium is concentrated in roll-front deposits by long-term, natural, oxidation-reduction processes in which dissolved U(VI) is reduced to U(IV) and precipitated as uraninite (UO₂) at a redox interface along the groundwater flowpath. Other elements commonly concentrated at the redox interface include arsenic, selenium, molybdenum, and vanadium. In the United States, the leaching solution used to dissolve uranium is groundwater amended with molecular oxygen and carbonate to convert uranium to soluble U(VI) and form a carbonate complex to further increase uranium solubility. The oxidizing subsurface geochemical environment commonly mobilizes the other trace elements concentrated in the ore zone, and dissolved concentrations may be greater than water quality criteria during the leaching phase of mining.

The next phase of the mining operation is restoration of the aquifer so that water quality is returned to an acceptable condition. This is commonly done by a process called groundwater sweeping whereby clean groundwater is circulated through the mined area to remove residual, soluble contaminants. Because of the presence of elevated uranium and other redox-sensitive contaminants in the groundwater, the recirculated groundwater is often amended with a reducing agent, such as sulfide compounds, to help create the original reducing condition of the aquifer in which the contaminants were less soluble. Aquifer restoration is one of the biggest challenges in ISL uranium mining and is often a major cost of decommissioning a site after mining. This presentation covers the basic geochemistry of the leaching and restoration processes.