Lysimeter Tests for an ET Cover Design at Monticello, Utah

摘要

The U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA) collaborated on the design of an alternative cover for a uranium mill tailings disposal cell at Monticello, Utah. The Monticello design relies on the water storagecapacity of a thick, fine-textured soil layer overlying a sand-and-gravel capillary barrier to store precipitation while plants are dormant, and on evapotranspiration (ET) to remove stored water during the growing season. A series of field lysimeter studies were conducted to help design and then monitor the performance of the Monticello cover. Initially, small weighing lysimeters containing mature native vegetation growing in intact monoliths of undisturbed soil provided evidence that an ET cover was areasonable option even at Monticello, which has an annual average precipitation of 390 mm and a relatively short growing season. Favorable results led to installation of 15 additional small weighing lysimeters to compare water-storage capacities and drainage from different borrow soil types and layer thicknesses. Loam and clay loam soils had the highest water storage capacities. Drainage was less than 3 mm/yr for all treatments during an 8-yr period. Two caisson lysimeters (3-m diameter) were installedadjacent to the construction site to mimic and monitor the range of conditions as built in the actual cover. A loam topsoil compacted to 1.45 g/cm, installed in one lysimeter, had a 40% greater water storage capacity than a gravely clay loam subsoil compacted to 1.65 g/cm~2 installed in the other lysimeter. Only 0.1 mm of drainage occurred in both lysimeters, an amount well below the EPA target of <3.0 mm/yr.