Formatted Title

Secondary Uranium Sources Remain in the Unsaturated Zone after Tailings Removal: Now What?

Background/Objectives

The U.S. Department of Energy Office of Legacy Management (LM) oversees sites where uranium mill tailings have been removed and now have monitored natural attenuation (MNA) strategies. Uranium concentrations in groundwater under the MNA (or natural flushing in LM terminology) approach are greater than prior model predictions at some sites, and new estimates of flushing times are greatly exceeding predictions. Thus, we are evaluating (1) the occurrence of residual secondary sources of uranium, (2) secondary source release rates to update estimates of MNA time frames, and (3) the need for updated remedial or compliance strategy options. At the Riverton, Wyoming, Processing Site, secondary uranium sources occur in the unsaturated zone downgradient from the former mill tailings area, which contribute to increased uranium concentrations in the groundwater after large recharge or flooding events. At the Monticello, Utah, Processing Site, secondary uranium sources occur in unsaturated zone sediments that were below the former tailings impoundments. At the Monticello site, some seasonal increases in groundwater uranium concentrations are seen after spring snowmelt. At both sites, these secondary uranium sources create long-term source zones that greatly delay natural flushing.

Approach/Activities

Evaluating point No. 1 above for both sites, secondary sources of uranium were identified by core collection and direct solid-phase extractions for uranium content. Release rates (point No. 2) were evaluated at both sites using column tests on the collected unsaturated zone sediment. With deionized water as an influent, effluent data provide information on uranium concentrations versus pore volume, which was then upscaled to approximate field conditions. At the Monticello site, the upscaled column data were used as input into a contaminant transport model to evaluate MNA time frames in comparison to other remedial strategies (point No. 3). For the Riverton site, the upscaled column data were used to estimate MNA time frames based on flooding frequency and length of flooding (resulting in different amounts of flow through the unsaturated zone). Direct infiltration experiments with traced river water were also completed at the Riverton site. These experiments quantified the amount of uranium released from the unsaturated zone during a simulated flooding event.   Transport modeling and updating the remedial strategy (point No. 3) for the Riverton site are planned, but have not been completed yet.

Results/Lessons Learned

Much of the uranium in these unsaturated zone secondary sources is mobile, which creates a long-term source of uranium that can keep the groundwater concentrations above standards for much longer than the maximum allowance of 100 years for some LM sites. The predicted time for MNA is more than 1500 years for the Monticello site and more than 300 years for the Riverton site. Dealing with these time frames and the difficulty of residual contamination in the unsaturated zone are complex problems. Remedial options range from secondary source removal, to enhanced flushing, to fixing the uranium in place using chemically engineered remedial fluids. Full removal is an expensive option that requires long-term containment at a new location, and getting fluid movement to contact all the uranium in the unsaturated zone is very challenging. Discussions are ongoing about what to do next. Questions are focused on expectations for a reasonable time frame for MNA.