The U.S. Department of Energy and the U.S. Nuclear Regulatory Commission are evaluating rock-armored, conventional covers and evapotranspiration (ET) covers for long-term containment of uranium mill tailings (UMT), and weighing the environmental trade-offs of converting existing conventional covers into ET covers. The Uranium Mill Tailings Radiation Control Act requires UMT covers to limit radon escape and protect groundwater for up to 1000 years. However, 20–30 years after construction, natural processes have already changed the as-built engineering properties of existing covers. Our interdisciplinary suite of studies is
- Evaluating effects of ecological and soil-forming processes on soil hydraulic properties, percolation flux, and radon diffusion and flux in conventional covers;
- Measuring concentrations of tailings elements in deep-rooted plants growing on conventional covers;
- Monitoring soil water balance and plant ecology in a 3 ha drainage lysimeter embedded in an operational ET cover;
- Testing soil manipulation and revegetation methods to enhance the natural transformation of conventional covers into ET covers; and,
- Comparing the water balance of conventional covers, one planted to perform as an ET cover and the other without plants, using test sections in large drainage lysimeters.
Results/Conclusions
- Soil-forming processes increased the saturated hydraulic conductivity by at least 2 orders of magnitude and increased percolation flux from near zero to 17% of precipitation within 5 years in a conventional cover without plants.
- Soil-forming and ecological processes increased radon flux; however, mean fluxes remain below the 20 pCi/m2s regulatory standard.
- At some sites, concentrations of uranium and other UMT elements are higher in woody plants growing on conventional covers than in nearby reference areas, but concentrations remain well below environmental risk thresholds.
- Annual percolation in an operational ET cover ranged from zero to 0.8% of precipitation over 18 years, compared to 0.2% to 17.3% for conventional cover test sections over 11 years.
- Transplanting native shrubs after manipulating the soil profile of a conventional cover accelerates natural revegetation.
- Enhancing plant succession increased ET and reduced percolation flux in conventional covers, from 12.0% to 0.1% of precipitation in a test section within 3 years.
Our results suggest that near-term ecological processes reduce percolation causing conventional UMT covers to function like ET covers, and that increases in radon flux and plant uptake of contaminants remain below regulatory and risk standards. Ongoing studies will continue to weigh the long-term detriments and benefits of ecological and soil-forming processes on the performance of UMT covers.