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Modeling tritium transport through a deep unsaturated zone in an arid environment

C.J. Mayers·Brian J. Andraski·C.A. Cooper·S.W. Wheatcraft·David A. Stonestrom·R. L. Michel
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Abstract

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

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C.J. Mayers, Brian J. Andraski, C.A. Cooper, S.W. Wheatcraft, David A. Stonestrom, R. L. Michel. 2005. Modeling tritium transport through a deep unsaturated zone in an arid environment. https://doi.org/10.2136/vzj2004.0179

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