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This article in VZJ

  1. Vol. 4 No. 4, p. 967-976
     
    Received: Dec 17, 2004
    Published: Nov, 2005


    * Corresponding author(s): cjmayers@usgs.gov
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doi:10.2136/vzj2004.0179

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

  1. C. J. Mayers *a,
  2. B. J. Andraskia,
  3. C. A. Cooperb,
  4. S. W. Wheatcraftc,
  5. D. A. Stonestromd and
  6. R. L. Micheld
  1. 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

Abstract

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.

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