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Description
Since the 1940s, this nation has been accumulating low level radioactive waste (LLRW). Proper disposal of this growing hazard is essential to minimizing risks to humans and other environmental receptors. Arid environments of the Western United States have increasingly been sought for the safe disposal of LLRW. These areas receive little precipitation, have high evaporation rates, and their thick unsaturated zones provide large separating distances between subsurface wastes and groundwater. One such disposal facility where these and other assumptions were made is located approximately 100 miles northwest of Las Vegas, near Beatty, Nevada. The Beatty facility operated from the early 1960s through the mid 1990s. Although the facility's license allowed for disposal of solid LLRW to unlined trenches, illegal disposal of liquid LLRW to the trenches occurred until approximately 1975. The United States Geological Survey (USGS) began investigations in the 1980s to evaluate environmental impacts from the facility's disposal practices. These investigations suggested that groundwater, soil vapor, and soil moisture were impacted with tritium, an unstable isotope of hydrogen and a common component of LLRW. Chloride mass balance studies conducted adjacent to the facility suggest the maximum infiltration depth of rainwater is approximately 20 meters below ground surface (bgs). This suggests regional groundwater has not received rainwater recharge within the past 10,000 years, a time consistent with the arid conditions which dominate at the facility. However, tritiated soil moisture has been detected at depths of approximately 60 meters bgs at these same locations. This indicates that within several decades, waste disposed at the facility's trenches migrated to vertical and lateral distances greater than originally expected. The purpose of this thesis was to determine how the facility failed to ensure containment of LLRW within several decades of operational startup. Given known meteorological, hydrogeological, and waste disposal data, this study used the numerical model Hydrus2D to predict arrival of tritiated soil moisture at a time consistent with observed concentrations. Two natural phenomena, geologic heterogeneities and immobile pore water, were evaluated as potential factors in influencing the transport of tritium in soil moisture. In general, Hydurs2D predicted infiltration of tritiated soil moisture to depths consistent with observed peak chloride accumulation. However, whereas tritium has been observed in soil moisture to depths of 60 meters bgs, Hydrus2D did not predict tritium in soil moisture beyond depths of 25 meters bgs, even under "worst case" waste loading scenarios. The introduction of geologic heterogeneities did not have a noticeable effect on tritium transport; however, immobile residual water did result in marked increases in tritium transport depths. Proper understanding of unsaturated zone water and solute movement is key to minimizing potential threats to larger water sources in the vicinity of arid disposal sites. The model, as used in this study, did not describe or predict processes at the scale needed to generally duplicate observed distributions of tritium. Although tritium is generally not considered an environmental problem, it is the fact that tritium is being used as an analogue for more problematic radionuclides, which makes the ability of the model to duplicate observed tritium distributions so important.
