Preliminary hydrogeologic assessment of boreholes UE-25c #1, UE-25c #2, and UE-25c #3, Yucca Mountain, Nye County, Nevada

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Authors:Geldon, Arthur L.
Author Affiliations:Primary:
U. S. Geological Survey, Denver, CO, United States
Source:Reston, VA [Water-Resources Investigations - U. S. Geological Survey, No.WRI92-4016, 85p. Publisher: U. S. Geological Survey,], United States. ISSN: 0092-332X
Publication Date:1993
Note:In English. 90 refs.Prepared in cooperation with the U.S. Department of Energy
Summary:Boreholes UE-25c #1, UE-25c #2, and UE-25c #3 (collectively called the C-holes) each were drilled to a depth of 914.4 meters at Yucca Mountain, on the Nevada Test Site, in 1983 and 1984 for the purpose of conducting aquifer and tracer tests. Each of the boreholes penetrated the Paintbrush Tuff and the tuffs and lavas of Calico Hills and bottomed in the Crater Flat Tuff. The geologic units penetrated consist of devitrified to vitrophyric, nonwelded to densely welded, ash-flow tuff, tuff breccia, ash-fall tuff, and bedded tuff. Below the water table, which is at an average depth of 401.6 meters below land surface, the rocks are argillic and zeolitic. The geologic units at the C-hole complex strike N. 2p W. and dip 15p to 21p NE. They are cut by several faults, including the Paintbrush Canyon Fault, a prominent normal fault oriented S. 9p W., 52.2p NW. The rocks at the C-hole complex are fractured extensively, with most fractures oriented approximately perpendicular to the direction of regional least horizontal principal stress. In the Crater Flat Tuff and the tuffs and lavas of Calico Hills, fractures strike predominantly between S. 20p E. and S. 20p W. and secondarily between S. 20p E. and S. 60p E. In the Topopah Spring Member of the Paintbrush Tuff, however, southeasterly striking fractures predominate. Most fractures are steeply dipping, although shallowly dipping fractures occur in nonwelded and reworked tuff intervals of the Crater Flat Tuff. Mineral-filled fractures are common in the tuff breccia zone of the Tram Member of the Crater Flat Tuff, and, also, in the welded tuff zone of the Bullfrog Member of the Crater Flat Tuff. The fracture density of geologic units in the C-holes was estimated to range from 1.3 to 7.6 fractures per cubic meter. Most of these estimates appear to be the correct order of magnitude when compared to transect measurements and core data from other boreholes 1.3 orders of magnitude too low. Geophysical data and laboratory analyses were used to determine matrix hydrologic properties of the tuffs and lavas of Calico Hills and the Crater Flat Tuff in the C-holes. The porosity ranged from 12 to 43 percent and, on the average, was larger in nonwelded to partially welded, ash-flow tuff, ashfall tuff, and reworked tuff than in moderately to densely welded ash-flow tuff. The pore-scale horizontal permeability of nine samples ranged from 5.7x10'3 to 2.9 millidarcies, and the pore-scale vertical permeability of these samples ranged from 3.7x10'* to 1.5 millidarcies. Ratios of pore-scale horizontal to vertical permeability generally ranged from 0.7 to 2. Although the number of samples was small, values of pore-scale permeability determined were consistent with samples from other boreholes at Yucca Mountain. The specific storage of nonwelded to partially welded ash-flow tuff, ash-fall tuff, and reworked tuff was estimated from porosity and elasticity to' be 2xlO'6 per meter, twice the specific storage of moderately to densely welded ash-flow tuff and tuff breccia. The storativity of geologic units, based on their average thickness (corrected for bedding dip) and specific storage, was estimated to range from 1xlO's to 2xlO'4. Ground-water flow in the Tertiary rocks of the Yucca Mountain area is not confined by strata but appears to result from the random intersection of water-bearing fractures and faults. Even at the C-hole complex, an area of only 1,027 square meters, water-producing zones during pumping tests vary from borehole to borehole. In borehole UE-25c #1, water is produced mainly from the lower, nonwelded to welded zone of the Bullfrog Member of the Crater Flat Tuff and secondarily from the tuff-breccia zone of the Tram Member of the Crater Flat Tuff. In borehole UE-25c #3, water is produced in nearly equal proportions from these two intervals and the central, moderately to densely welded zone of the Bullfrog Member. In borehole UE-25c #2, almost all production comes from the moderately to densely welded zone of the Bullfrog Member.Intraborehole temperature surveys during pumping tests at the C-hole complex indicate that water apparently is transmitted from the faults that transect the site via interconnected south-southeasterly to south-southwesterly, southwesterly, and northeasterly trending fractures.There appears to be no relation between water production from boreholes and either fracture orientation, lithology, or the degree of welding of ash-flow tuff. In support of this interpretation, pumping tests in the C-holes produce curves of drawdown as a function of time that are characteristic of a fracture-dominated ground-water flow system. These curves also indicate that borehole storage commonly affects the drawdown in tested intervals during the first 1 to 15 minutes of pumping. Hydraulic-head data for the Nevada Test Site and vicinity indicate that ground water in the area flows locally from block-faulted mountains to intermontane basins and regionally from basin to basin toward Death Valley and Alkali Flat.Hydraulic heads in the C-holes and in other boreholes indicate that an upward flux from the regional to the local flow system exists at Yucca Mountain. Hydrochemical data support this interpretation. These data include the predominance of sodium and bicarbonate ions in the water, small (pre-1952) tritium concentrations in the water, the "lightness" of the water in deuterium and oxygen-l8 with respect to modern meteoric water, an apparent carbon-14 age of about 15,000 years before present for the water, and elevated ground-water temperatures.
Subjects:Aquifers; Ash-flow tuff; Boreholes; Fractured materials; Geochemistry; Geologic hazards; Ground water; High-level waste; Hydraulic conductivity; Hydrochemistry; Igneous rocks; Movement; Pump tests; Pyroclastics; Radioactive waste; Site exploration; Tuff; Underground storage; USGS; Volcanic rocks; Waste disposal; Welded tuff; Well-logging; Nevada; Nevada Test Site; Nye County Nevada; United States; Yucca Mountain; Southern Nevada
Coordinates:N363700 N372300 W1155200 W1163730
Record ID:1994039392
Copyright Information:GeoRef, Copyright 2018 American Geosciences Institute. Reference includes data from USGS product, Selected Water Resources Abstracts, Reston, VA, United States
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