Site Characterization Data From The U3ax Bl Exploratory Boreholes At The Nevada Test Site PDF Download

Three exploratory boreholes were drilled and completed to the uppermost alluvial aquifer in Area 5 of the Nevada Test Site, Nye County, Nevada, in 1992. The boreholes and associated investigations were part of the Area 5 Site Characterization Program developed to meet data needs associated with regulatory requirements applicable to the disposal of low-level, mixed, and high-specific-activity waste at this site. This series of boreholes was specifically designed to characterize the hydrogeology of the thick vadose zone and to help define the water quality and hydraulic properties of the uppermost aquifer. Wells UE5PW-1, UE5PW-2, and UE5PW-3 are located in a triangular array near the southeast, northeast, and northwest corners, respectively, of the approximately 2.6-square-kilometer Area 5 Radioactive Waste Management Site to give reasonable spatial coverage for sampling and characterization, and to help define the nearly horizontal water table. Two of the wells, UE5PW-1 and UE5PW-2, penetrated only unconsolidated alluvial materials. The third well, located closer to the margin of the basin, penetrated both alluvium and underlying ash-flow and bedded tuff units. The watertable was encountered at the elevation of approximately 734 meters. The results of laboratory testing of core and drill cuttings samples indicate that the mineralogical, material, and hydrologic properties of the alluvium are very similar within and between boreholes. Additional tests on the same core and drill cuttings samples indicate that hydrologic conditions within the alluvium are also similar between pilot wells. Both core and drill cuttings samples are dry (less than 10 percent water content by weight) throughout the entire unsaturated section of alluvium, and water content increases slightly with depth in each borehole. Water potential measurements on core samples show a large positive potential gradient (water tends to move upward, rather than downward) to a depth of approximately 30.5 meters in each borehole, and a nearly zero potential gradient throughout the remaining portion of the vadose zone. These hydrologic condition data and hydrologic property data indicate that little net downward liquid flow is occurring (if any) through the thick vadose zone. Conversely, gas flow by diffusion, and possibly by advection, may be an important transport mechanism. Environmental tracer measurements made on water extracted from geologic samples suggest that water vapor in the upper portion of the vadose zone is moving upward in response to evaporative demand of the present arid climate. Preliminary water quality data indicate that the key hazardous and radioactive constituents do not exceed appropriate standards. Monitoring instruments and equipment were installed in each pilot well for making in-situ measurements of key hydrologic and pneumatic parameters and to monitor change in these parameters over time.