USGS abstract

Modeling Tritium Transport Through a Deep Unsaturated Zone, Amargosa Desert Research Site, Nye County, Nevada

C. J. Mayers^1,2, B. J. Andraski¹, S. W. Wheatcraft², D. E. Prudic¹, and D. A. Stonestrom³

Contaminant-transport processes in deep unsaturated zones of arid environments are poorly understood. Studies at the Amargosa Desert Research Site (ADRS; http://nevada.usgs.gov/adrs/) are investigating tritium transport away from a closed low-level radioactive waste facility. The unsaturated zone at the ADRS is 110 m thick; annual precipitation averages 0.11 m. Soil-gas samples collected from deep boreholes that are as far as 100 m from the waste facility and 160 m from the nearest waste-burial trench show elevated levels of tritium in water vapor throughout the unsaturated zone. Field data indicate that tritium movement primarily occurs in the gas phase with preferential transport through coarse-textured sediment layers. Preliminary models for diffusive movement of tritiated water vapor from the waste source in an isothermal, homogeneous material failed to predict the lateral extent of the transport indicated by field measurements. Homogeneous and heterogeneous models of the unsaturated zone system were used to test possible mechanisms driving tritium transport. Material properties assigned to the model domain were based on drillers’ and gamma logs, and on physical- and hydraulic-property data. Diffusive and advective processes were simulated with the TOUGH2 numerical code. Comparative results illustrate possible lithological controls on tritium transport. Discrepancies between measured and modeled results are used to highlight uncertainties in the characterization and understanding of arid-site transport processes.

¹U.S. Geologic Survey, Carson City, NV (cjmayers@usgs.gov, andraski@usgs.gov, deprudic@usgs.gov)

²Dept.of Geological Sciences, University of Nevada, Reno, NV (wheatcraft@unr.edu)

³U.S. Geologic Survey, Menlo Park, CA (dastones@usgs.gov)