Field Hydrology of Armored Earthen Final Covers with and without Vegetation

An instrumented test section was constructed and monitored for 14 years to characterize the hydrology of rock-armored earthen final covers used for uranium mill tailings disposal facilities in semiarid and arid environments. The test section included two different test areas constructed with the same earthen materials and methods employed at an adjacent disposal facility. Both test areas contained a large ($10\times 20\mathrm{m}$) instrumented drainage lysimeter to monitor the water balance at near-field scale. Herbicide was applied to the surface of both test areas to preclude vegetation for the first 7 years, simulating conventional practices used by the DOE. After 7 years, herbicide application ceased on one test area (“naturalized” test area), and vegetation was allowed to establish. Treatment of the other (“conventional”) test area continued as before. Percolation rates increased in both test areas during the initial 7 years while herbicide was being applied. Percolation rates ranging from 10 to $47\mathrm{mm}/\mathrm{year}$ were measured in wetter years, which is considerably higher than anticipated ($3\mathrm{mm}/\mathrm{year}$). Increases in saturated hydraulic conductivity of the protection layer and the radon (Rn) barrier contributed to the higher percolation rates. Ceasing herbicide application in the naturalized test area had a profound effect on the hydrology. As vegetation established, evapotranspiration increased, volumetric water contents in the protection layer and Rn barrier decreased, and the percolation rate decreased significantly. In contrast, the conventional test area continued to perform as before, with elevated volumetric water contents and higher percolation rates in wet years.