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ORIGINAL RESEARCH

Modeling Tritium Transport Through a Deep Unsaturated Zone in an Arid Environment

^a U.S. Geological Survey, WRD, 333 West Nye Lane, Carson City, NV 89706
^b Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512
^c Dep. of Geological Sciences, Univ. of Nevada, Reno, NV 89557
^d U.S. Geological Survey, WRD, NRP, 345 Middlefield Road, Menlo Park, CA 94025
^* Corresponding author (cjmayers{at}usgs.gov)

Received 17 December 2004.

Understanding transport of tritium (³H) 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 ³H 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 ³H at all depths. The objectives of this study were to (i) test source thermal and gas-advection mechanisms driving ³H 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 ³H migration, but failed to match measured ³H distributions. All anisotropic simulations with a source pressure component resembled, in shape, the upper portion of the ³H distribution of the nearest borehole. Isotopic equilibrium limited migration of ³H, 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 ³H transport to reach the nearest borehole.

Abbreviations: ADRS, Amargosa Desert Research Site • EOS, equation of state • ³H, tritium • ³HHO_g, tritiated water vapor • ³HHO_l, tritiated liquid water • LLRW, low-level radioactive waste • MHE, maximum horizontal extent • MVE, maximum vertical extent • P_S, source-pressure difference above ambient • T_S, source temperature