Geohydrology of Volcanic Tuff Penetrated by Test Well UE-25B No. 1, Yucca Mountain, Nye County, Nevada

摘要

Test well UE-25b No. 1, located on the east side of Yucca Mountain in the southwestern part of the Nevada Test Site, was drilled to a total of 1220 meters and hydraulically tested as part of a program to evaluate the suitability of Yucca Mountain as a nuclear-waste repository. The well penetrated almost 46 meters of alluvium and 1174 meters of Tertiary volcanic tuffs. The composite hydraulic head for aquifers penetrated by the well was 728.9 meters above sea level (471.4 meters below land surface), with a slight decrease in hydraulic head with depth. Average hydraulic conductivities for stratigraphic units determined from pumping tests, borehole-flow surveys, and packer-injection tests ranged from less than 0.001 meter per day for the Tram Member of the Crater Flat Tuff to 1.1 meters per day for the overlying Bullfrog Member of the Crater Flat Tuff. Small values for the Tram Member represented matrix permeability of unfractured rock; large values near the lower part of the Bullfrog Member were associated with the basal bedded or reworked tuffaceous beds, but probably resulted from fracture permeability. Large hydraulic conductivities of the rhyolitic tuffs of Calico Hills, the Prow Pass Member at the top of the Crater Flat Tuff, and the middle of the Bullfrog Member probably resulted from fracture permeability in the ash-flow tuffs. Chemical analyses indicated that the water is a soft sodium bicarbonate type, slightly alkaline, with large concentrations of dissolved silica and sulfate. Uncorrected carbon-14 age dates of the water were 14,100 and 13,400 years. 25 references, 26 figures, 5 tables. (ERA citation 10:007549)