Geohydrology of the unsaturated zone at the burial site for low-level radioactive waste near Beatty, Nye County, Nevada

Low-level radioactive solid waste has been buried in trenches at a site near Beatty, Nev., since 1962. In 1976, as part of a national program, the U.S. Geological Survey began a study of the geohydrology of the waste-burial site to provide a basis for estimating the potential for radionuclide migration in the unsaturated zone beneath the waste-burial trenches. Data collected include meteorological information for calibration of a long-term water-budget analysis, soil-moisture profiles, soil-water potentials, and hydraulic properties of representative unsaturated sediment samples to a depth of about 10 meters (m).

The waste-burial facility is in the northern Amargosa Desert about 170 kilometers (km) northwest of Las Vegas, Nevo The region is arid; mean annual precipitation at Lathrop Wells, 30 km south of the site, is only 7.4 centimeters (cm). The mean daily maximum temperature at Lathrop Wells in July, the hottest month, is 37 °C. The site is underlain by poorly stratified deposits of gravelly or silty sand and sandy gravel, and thick beds of clayey sediments. The total thickness of valley-fill deposits beneath the site is about 175 m; the unsaturated zone is about 85 m thick. Volumetric soil moisture to depths of 4 m ranges from 4 to 10 percent but commonly is in the range of 6 to 8 percent. Soil-water potential, measured to depths of 3 to 10 m, ranged from -10 to -70 bars. Unsaturated hydraulic conductivity computed from laboratory analyses of representative samples ranges from 10^-13 to 10^-14 centimeters per day (cm/d).

Evaporation studies over a 2-year (yr) period were used to calibrate a numerical procedure for analyzing long-term precipitation data and estimating annual water budgets during the 15-yr period 1962-76. This analysis (1) demonstrated that a potential exists for deep percolation (greater than 2 m), despite high annual evaporation demands, and (2) provided predictions of the time of year and the antecedent conditions that enhance the probability of deep percolation. Soil-moisture profiles obtained monthly over an 18-month (mo) period demonstrate that deep percolation does occur. Soil-moisture conditions antecedent to an observed deep-percolation event, and the time of year when the percolation occurred, support the interpretations based on long-term meteorological records.

Calculation of downward moisture movement through the waste-trench backfill material, on the basis of simplified assumptions, suggests that moisture could have penetrated as much as 6 m below land surface from 1963, when the oldest trenches were closed, to 1980, but that the moisture requirement for such penetration far exceeded the amount of moisture actually available. Steady-state downward movement of moisture at depths greater than 10 m and beneath the waste-burial trenches would be on the order of 4 cm per 1,000 yr, assuming a steady flux rate of 1x10^-5 cm/d.