Modeling tritium transport through a deep unsaturated zone, Amargosa Desert Research Site, Nye County, Nevada: Reno, University of Nevada, M.S. thesis.

C.J. Mayers

ABSTRACT

Studies at the U. S. Geological Survey’s Amargosa Desert Research Site (ADRS) in Nevada are investigating tritium transport away from a closed low-level radioactive waste (LLRW) facility. Soil-gas samples collected from deep boreholes 100 m and 160 m from the nearest waste-burial trench show elevated levels of tritium in water vapor throughout the two unsaturated zone profiles. Field data indicated that tritium movement primarily occurs in the gas phase with preferential transport through coarse-textured sediment layers.

This study used a two-dimensional numerical model that incorporates a non-isothermal, heterogeneous domain of the deep unsaturated zone at the ADRS to test possible mechanisms driving tritium transport. The mechanisms tested were temperature pressure, and a combination of both. In addition, physical properties including porosity, tortuosity, and intrinsic permeabilities were adjusted in an effort to enhance the movement of tritiated water vapor (HTOg).
Simulation results indicated that a 500 Pa source-pressure-difference in the LLRW trench in conjunction with a high degree of anisotropy was necessary for HTOg to reach the closer borehole. The HTOg field generated resembled, in shape only, the upper portion of the HTOg profile of the closer borehole. Simulations also indicated that migration of HTOg may have been limited by the isotopic equilibrium assumption in which liquid water in the soil matrix acted as a large sink for the HTOg.