Water Balance Covers and Conventional Cover Enhancements for Sustainable Hydraulic Isolation of Uranium Mill Tailings.

The U.S. Department of Energy Office of Legacy Management (DOE) is investigating the performance of engineered covers for uranium mill tailings.  At some sites, soil development processes and ecological succession may be effectively converting existing conventional covers, which rely on low-conductivity compacted soil, into water balance covers, that store soil water and release it as evapotranspiration.  DOE is using lysimetry to evaluate the performance of a water balance cover, and methods to accelerate and enhance the natural conversion of conventional covers.  

In 1999, Dr. Glendon Gee organized and then guided the design and construction of a 3-hectare drainage lysimeter embedded within the final cover at the Monticello, Utah uranium mill tailings disposal site.  The Monticello cover relies on a 160-cm layer of sandy clay loam soil overlying a 40-cm sand capillary barrier for water storage, and a planting of native sagebrush steppe vegetation to seasonally release soil water through evapotranspiration.  Water balance monitoring within the lysimeter provided convincing evidence that the cover has performed well over a 10-year period.  The average percolation flux, approximately 0.6 mm yr^-1, satisfied a regulatory goal of <3.0 mm yr^-1.  Most percolation can be attributed to the very wet winter and spring of 2004–2005, when soil water content slightly exceeded the storage capacity.  Diversity, percent cover, and leaf area of vegetation increased over the monitoring period.

With favorable results at Monticello, DOE installed a test facility near Grand Junction, Colorado in 2008 to evaluate sustainable enhancements of conventional covers.  Identical low-conductivity covers were constructed in two large drainage lysimeters.  In one, soil will be ripped, blended, and planted in 2011; the other is a control.  Three years of baseline monitoring indicate that percolation has been less than 4% of precipitation, but follows a stair-step pattern which is commonly associated with formation of preferential flow paths.