EFFECTS OF WIND-BLOWN DUSTS- URANIUM AND ARSENIC IN SHALLOW GROUNDWATER

High uranium (30-175 ppb), arsenic (10-33 ppb), and other trace elements in domestic and monitoring wells near a trash dump in northeastern Arizona initially led U.S. Geological Survey investigators to conclude that uranium-bearing waste may be present in the dump. However, affected wells occur down-gradient, cross-gradient, and up-gradient from the dump with no evidence of significant sources in the dump or proximal watershed. U-234/U-238 isotopic activity ratios in groundwater are 1.3 or greater suggesting a natural origin for the uranium. The site vicinity is characterized by Navajo Sandstone bedrock covered by sheet sands and low sand dunes that vary in thickness up to 20 ft; the water table is <30 ft deep. Shallow, high total dissolved solid (TDS to 6400 ppm) sodium-sulfate-chloride groundwater with the variably elevated uranium, arsenic and trace element concentrations is underlain by a deeper, low TDS (100-150 ppm), calcium-bicarbonate water low in uranium (≤ 2ppb) and all trace elements. Highest values (several hundred ppb uranium) occur at the down-gradient edge of the dump. The surficial sands contain caliche zones with elevated uranium concentrations greater than the bedrock and minerals not found in the bedrock (barite and strontianite). Estimated pore water uranium concentrations based on results of leaching the labile fraction from site sediment samples are similar to the maximum uranium concentrations observed at the site. Dust storms blow across the region from the southwest several times a year. Upwind sources of dust include the Chinle Formation, a potential source for uranium. High uranium and arsenic in groundwater in and near the dump may have been mobilized by rain and snowmelt recharge through the dusts deposited at the site. Uranium and arsenic move downward with sodium, sulfate and chloride as the dusts are leached, ultimately reaching the shallow groundwater. Evapotranspiration, suggested by stable isotope data, may further concentrate solutes in the shallow groundwaters. Aeolian dusts are being investigated in the region as potential sources of widespread, naturally-occurring uranium and arsenic groundwater concentrations which could have significant implications for shallow, domestic drinking water sources in the rural areas of the arid southwest United States.