Sediment properties and water movement through shallow unsaturated alluvium at an arid site for disposal of low-level radioactive waste near Beatty, Nye County, Nevada

A commercial disposal facility for low-level radioactive waste has been in operation near Beatty, Nevada, since 1962. The facility is in the arid Amargosa Desert where wastes are buried in trenches excavated into unsaturated alluvial sediments. Thick unsaturated zones in arid environments offer many potential advantages for disposal of radioactive wastes, but little is known about the natural movement of water near such facilities. Thus, a study was begun in 1982 to better define the direction and rates of water movement through the unsaturated zone in undisturbed sediments near the disposal facility. This report discusses the analyses of data collected between 1983 and 1988. Precipitation during the period October 1984 through February 1988 was 326 millimeters, and measurements of water content of the sediments indicate that the precipitation did not penetrate a depth of more than 1 meter. Surficial deposits at the study site are a sandy silt with a low hydraulic conductivity. This deposit acts as a barrier to infiltration of precipitation. In addition, a clean gravel layer beneath the surficial deposits acts as a capillary barrier and further impedes downward percolation of water. Thermocouple psychrometers were used to measure water (pressure) potentials and temperatures in the dry alluvial sediments. The psychrometers were installed through 2.5-centimeter-diameter holes drilled a minimum of 3 meters horizontally outward from a 1.5-meter-diameter vertical shaft. Psychrometers were installed between depths of 3 and 13 meters below land surface. Thermocouple psychrometers were also installed in a borehole between depths of 1 and 12 meters below land surface and near-surface psychrometers were installed in shallow excavations. Water potentials measured during the study period generally ranged from -5.5 megapascals at a depth of 1.6 meters below land surface to -3.2 megapascals at a depth of 13 meters. Seasonal variations in water potentials and temperatures were observed to a depth of about 9 meters. Below a depth of 9 meters, observed water potentials and temperatures did not vary seasonally. Calculations of water flux indicate little water movement through interconnected pores, either as liquid or vapor. Depending on the season, water movement in the upper 9 meters is either upward toward the surface or downward. Between the depths of 9 and 13 meters, water movement was consistently upward. The rate of liquid water movement through interconnected pores was estimated to range from 1x10 to 1x10 centimeter per day. This range reflects the large uncertainty in the estimate of unsaturated hydraulic conductivity. The rate of water movement through vapor diffusion generally ranged from 1x10"* to 1x10 centimeter per day. The upper 13 meters of sediments at the study site can be divided into three zones on the basis of water potential, water content, temperature, and sodium-chloride content. The upper zone, from land surface to a depth of about 2 meters, is subject to rapid changes in water potentials, water contents, and temperatures in response to precipitation and air-temperature fluctuations. A middle zone, from a depth of about 2 meters to 9 meters, is characterized by unchanging water content and seasonal trends in water potential as vapor moves upward and downward in response to changes in soil temperature. This zone is also characterized by an accumulation of sodium chloride that may be related to the maximum depth of water percolation into the sediments. The lower zone, from a depth of about 9 meters to at least 13 meters, is characterized by relatively unchanging water potentials, water contents, and temperatures. Gradients in the lower zone indicate that water movement, both as liquid and vapor, is upward.