Brian J. Andraski C.A. Cooper S.W. Wheatcraft David A. Stonestrom R. L. Michel C.J. Mayers 2005 <p><span>Understanding transport of tritium (</span>³<span>H) in unsaturated zones is critical to evaluating options for waste isolation. Tritium typically is a large component of low-level radioactive waste (LLRW). Studies at the U.S. Geological Survey's Amargosa Desert Research Site (ADRS) in Nevada investigate </span>³<span>H transport from a closed LLRW facility. Two boreholes are 100 and 160 m from the nearest waste trench and extend to the water table at 110 m. Soil-water vapor samples from the deep boreholes show elevated levels of </span>³<span>H at all depths. The objectives of this study were to (i) test source thermal and gas-advection mechanisms driving </span>³<span>H transport and (ii) evaluate model sensitivity to these mechanisms and to selected physical and hydraulic properties including porosity, tortuosity, and anisotropy. A two-dimensional numerical model incorporated a non-isothermal, heterogeneous domain of the unsaturated zone and instantaneous isotopic equilibrium. The TOUGH2 code was used; however, it required modification to account for temperature dependence of both the Henry's law equilibrium constant and isotopic fractionation with respect to tritiated water. Increases in source temperature, pressure, and porosity enhanced </span>³<span>H migration, but failed to match measured </span>³<span>H distributions. All anisotropic simulations with a source pressure component resembled, in shape, the upper portion of the </span>³<span>H distribution of the nearest borehole. Isotopic equilibrium limited migration of </span>³<span>H, while effects of radioactive decay were negligible. A 500 Pa pressure increase above ambient pressure in conjunction with a high degree of anisotropy (1:100) was necessary for simulated </span>³<span>H transport to reach the nearest borehole.</span></p> application/pdf 10.2136/vzj2004.0179 en Soil Science Society of America Modeling tritium transport through a deep unsaturated zone in an arid environment article