Water permeability and related rock properties measured on core samples from the Yucca Mountain USW GU-3/G-3 and USW G-4 boreholes, Nevada Test Site, Nevada

Abstract

Core samples from the Yucca Mountain USW GU-3/G-3 and USW G-4 boreholes were measured for bulk density, grain density, porosity, resistivity, and water permeability as part of a comprehensive geologic investigation designed to determine the suitability of Yucca Mountain as a site for the containment of high-level radioactive waste products. The cores were selected at the drill sites so as to be representative of the major lithologic variations observed within stratigraphic units of the Paintbrush Tuff, Calico Hills Tuff, Crater Flat Tuff, Lithic Ridge Tuff, and Older Tuffs. USW GU-3/G-3 was drilled to a depth of 1533.8 meters and the USW G-4 borehole penetrated to the 914.7 meter level. Two hundred and twenty six samples were used in the laboratory study of which two hundred were sample pairs drilled from a common core. The paired samples were oriented axially and perpendicular to the alignment of the borehole.Dry and saturated bulk density, grain density, and porosity measurements were made on the core samples principally to establish that a reasonable uniformity exists in the textural and mineral character of the sample pairs. Where bulk densities are different, grain density data show that the disparities can usually be attributed to porosity variations rather than to inequalities in mineral content. Electrical resistivity measured on sample pairs tended to be lower along the plane transverse to the vertical axis of the drill core herein referred to as the horizontal plane. Permeability values, ranging from virtually 0 (<.02 microdarcies) to over 200 millidarcies, also indicate a preferential flow direction along the horizontal plane of the individual tuff units. Of the 67 sample pairs from the USW GU-3/G-3 borehole, 58 percent of the horizontally oriented core had a higher permeability and lower resistivity than their vertically oriented counterparts. Only in 10 percent of the 67 sample pairs did the vertical core demonstrate a similar permeability/resistivity relationship. In those sample pairs from the USW G-4 borehole, 65 percent of the horizontal plugs and 24 percent of the vertical plugs exhibited this same permeability/resistivity correspondence. Despite the non-bedded character of the ash-flow tuffs, the welding process possibly produced an interconnecting pore structure along the implied bedding plane so as to provide a continuous and less tortuous path for both current and water flow. Permeability decreases with flow duration in all but the non-welded tuffs as unconsolidated particles within the pore network are repositioned so as to impede the continued flow of water through the rock. Reversing flow direction initially restores the permeability of the rock to its original or maximum value.

Air-injection testing in vertical boreholes in welded and nonwelded Tuff, Yucca Mountain, Nevada

“…The welded Topopah Spring Tuff water-permeability values ranged from m2 with an average of approximately less than approximately 2.0 x by Anderson (1 99 1) were mostly from the Topopah Spring crystal-rich nonlithophysal zone, which is a zone of increased porosity (Buesch and others, 1996) and, therefore, apparently increased matrix permeability. The water-permeability values (Anderson,199 nonwelded tuff of the Paintbrush Group are as much as an order of magnitude greater than the Anderson (1994) and Flint (1990)…”

Three-dimensional numerical modeling of the influence of faults on groundwater flow at Yucca Mountain, Nevada

“…More than 90% of water pumped from the tuffs in the Crater Flat Group commonly emanates from the densely welded and fractured intervals, which are typically located in the central section of a particular unit. In addition, permeability measurements from pumping tests are several orders of magnitude larger than matrix permeability (Anderson, 1991;Anderson, 1994;Geldon, 1993;Lahoud et al, 1984;Rush et al, 1984), indicating that flow is primarily through fractures.…”

“…Sonnenthal et al (1997) analyzed fracture frequency and fracture lengths in the Exploratory Studies Facility (ESF) tunnel in the unsaturated zone to calculate fracture porosity. Their estimates range from 10-5 to 10- 4 . Matrix porosity beneath the water table was measured from core samples and generally ranges between 0.20 and 0.30 (Anderson, 1991;Anderson, 1994;Geldon, 1993;Lahoud et al, 1984;Rush et al, 1984;Thordarson, 1983).…”