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 105 km northwest of the city of Las Vegas in southern Nevada, has been used for the disposal of low level radioactive waste since 1961. 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. 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 meet