Modeling tritium transport through a deep unsaturated zone, Amargosa Desert Research Site, Nye County, Nevada

Studies at the U.S. Geological Survey's Amargosa Desert Research Site in Nevada are investigating tritium transport away from a closed low-level radioactive waste facility. Soil-gas samples collected from deep boreholes 100 m and 160 m from the nearest waste trench show elevated levels of tritiated water vapor (HTO_g) throughout the unsaturated zone.; This study used a two-dimensional numerical model to test possible mechanisms driving tritium transport. The mechanisms tested were temperature and pressure. Simulation results indicated that a 500 Pa source-pressure-difference in conjunction with a high degree of anisotropy was necessary for HTO_g to reach the closer borehole. The HTO_g field generated resembled, in shape only, the upper portion of the HTO_g profile of the closer borehole. Simulations also indicated that migration of HTO_g may have been limited by the isotopic equilibrium assumption in which liquid water in the soil matrix acted as a large sink for the HTO_g.