Estimates of percolation rates and ages of water in unsaturated sediments at two Mojave Desert sites, California-Nevada

Thick unsaturated zones in arid regions increasingly are being sought for the burial of radioactive and other hazardous wastes. Estimating percolation rates of water from precipitation at proposed burial sites is important for site assessment. Chloride profiles in unsaturated sediments are used to show differences and similarities in the rates of percolation at two sites in the Mojave Desert of southern Nevada and southeastern California; the first is an existing burial site for low-level radioactive waste in the Amargosa Desert, and the second is a proposed waste-burial site in Ward Valley. The Mojave Desert is one of the most arid regions of the United States.

Chloride concentrations in pore water of unsaturated sediments peak between depths of 2 and 3 meters at both the Amargosa Desert and Ward Valley, sites; maximum chloride concentration is 9,000 milligrams per liter at the Amargosa Desert site and 15,000 milligrams per liter at the Ward Valley, site. Below a depth of 10 meters, however, chloride concentrations at the Amargosa Desert site decrease to less than 50 milligrams per liter, whereas concentrations at the Ward Valley site are greater than 2,300 milligrams per liter. Estimated age of water at a depth of 10 meters at both sites is between 16,000 and 33,000 years. Below a depth of 10 meters, estimated age of water in the sediments at the Ward Valley site is considerably older than at the Amargosa Desert site because of the greater chloride concentrations at the Ward Valley site.

The dilute chloride concentrations in the pore water below a depth of 10 meters at the Amargosa Desert site could indicate that the sediments were flushed with water in the past. The climate in the region was wetter and cooler from about 30,000 to 18,000 years ago. Perhaps increased precipitation or more frequent flooding of the Amargosa River resulted in deep percolation at the site. Downward percolation of water since that time seems limited to the upper 10 meters.

The downward percolation rate of water below a depth of 10 meters at the Amargosa Desert site, as estimated from chloride concentrations, is 0.2 centimeters per year; however, the rate is not representative of present conditions as measured water-potential and vapor-density, gradients indicate upward water flow. The rate may be representative of a period during the Pleistocene when the climate was wetter and cooler. Estimated percolation rates below a depth of 10 meters at the Ward Valley site are on the order of 3 to 5 centimeters per 1,000 years, also reflecting flow conditions during the Pleistocene.