WATER-QUALITY ISSUES RELATED TO URANIUM IN SITU RECOVERY SITES

In the United States, uranium extraction by in situ recovery (ISR) techniques can be done in productive aquifers with surrounding groundwater that could have other uses, such as agriculture irrigation, livestock watering, or drinking water. For uranium ISR to proceed, the U.S. Environmental Protection Agency (EPA) must approve an aquifer exemption request, which allows for injection of the in situ recovery fluids. This exemption recognizes that the presence of uranium ore makes the groundwater unfit as drinking water and possibly for other uses. Current EPA regulations indicate that no impact on groundwater quality can occur beyond the aquifer exemption boundary.

After uranium ISR is completed, restoration requirements usually specify that the water quality in the ore-bearing aquifer must be restored to pre-ISR conditions. In Texas and Wyoming, some past closures of uranium ISR sites have only required limited stability monitoring. In addition, sites were often closed based on a ‘‘class of use’’ for groundwater, which meant that the groundwater within the restored zone was not always restored to pre-ISR conditions for all constituents. For many of these sites, long-term impacts on downgradient groundwater quality were not thoroughly evaluated, and long-term monitoring was not always required.

Uranium ISR results in significant geochemical changes in the solid and water phases. The long-term impacts of these changes are not easy to determine or predict; however, nearby stakeholders (including the U.S. Department of Energy Office of Legacy Management) are interested in strong evidence that downgradient groundwater quality is protected. An evaluation of future groundwater quality requires an understanding of the surrounding hydrogeology, knowledge of post-ISR and post-restoration geochemistry, including both water and solids, and predictions of future rock–water interactions. These interactions can be tested using batch, column, and field-scale experiments, and they should include testing with actual restored groundwater. Reactive transport modeling using data from these experiments is likely the best approach to demonstrate future downgradient groundwater quality, albeit having a relatively large range of uncertainty. Current research on these various techniques will be summarized.