Site characterization and performance assessment for a low-level radioactive waste management site in the American Southwest

The Area 5 Radioactive Waste Management Site located in southern Nevada, has been used for the disposal of low-level radioactive waste since 1961. The site is located in the Mohave Desert of the American Southwest, an extremely arid region receiving as little as 0.1 m/yr of precipitation. Site characterization studies have measured the physical, hydrologic, and geochemical properties of core samples collected from 10 shallow boreholes and 3 deep boreholes that extend through the unsaturated zone to the uppermost aquifer. Results indicate that the unsaturated zone consists of 240 m of dry alluvial sediments and is remarkably uniform with respect to most physical parameters. Measurements of saturated hydraulic conductivity with depth showed no evidence of trends, layering, or anisotropy. Parameters for hydraulic functions were not highly variable and exhibited little trend with depth. Water potential profiles indicate that water movement in the upper alluvium is upward, except immediately following a precipitation event. Below the evaporative zone, the liquid flux was downward and of the same order of magnitude as the upward thermal vapor flux induced by the geothermal gradient. The extreme climatic conditions at the site reduce or eliminate many radionuclide release and transport mechanisms. Downward transport of radionuclides to the uppermost aquifer appears unlikely under current climatic conditions. Important radionuclide transport pathways appear to be limited to upward diffusion and advection of gases and biologically-mediated transport. Conceptual models of disposal site performance have been developed based on site characterization studies. The limited transport pathways and limited land use potential of the site provide reasonable assurance that regulatory performance objectives can be met