Studies of infiltration and lead-soil interactions at the Radioactive Waste Management Site in Area 5 of the Nevada Test Site

Several studies were conducted to investigate the possibility of buried lead being transported by water in the unsaturated zone at the Area 5 Radioactive Waste Management Site (RWMS) on the Nevada Test Site. All involved soil from a 37-m soil core collected at the RWMS. The core consisted primarily of sand and small pebbles, with occasional layers of loose rocks. Few buried soil horizons were observed, and the core showed no evidence of a carbonate layer that would act as a barrier to infiltration. Samples chosen from various depths in the soil core were analyzed chemically. Calcium and sulfate occurred in a prominent layer about 5 m below the surface. The concentration of soluble carbonate increased gradually with depth, while chloride concentrations decreased. Lead concentrations ranged from 1 to 2 mg/kg. Additional data from the soil core were combined with results of earlier field infiltration studies at two sites near the RWMS to estimate flow velocities for water in the unsaturated zone. Under normal (dry) conditions, the degree of saturation is so small that gravity drainage does not occur; water moves by vapor transport and capillary action. Significant water movement occurs only if the soil is at or near saturation. The results suggest that even continuously ponded water at the RWMS would take several months to infiltrate to the water table. Seven samples from the soil core were tested for their ability to adsorb lead. All took up lead with about the same intensity and capacity. Adsorption of lead by insoluble carbonate minerals and precipitation of lead by soluble carbonate in the soil at the RWMS should provide a barrier to lead migration. Finally, measurements were made of the corrosion rates of lead and steel in contact with soil samples from the core. Corrosion rates generally increased with increasing soil saturation at all depths. Under ambient soil conditions at the RWMS, corrosion rates would be low