NELSON, Philip H., U.S. Geological Survey, Box 25046 Federal Center MS 939, Denver, CO 80225, pnelson@usgs.gov
As part of a site investigation by the U.S. Geological Survey (USGS) for the potential disposal of radioactive waste at Yucca Mountain, Nevada, numerous boreholes were drilled into a sequence of Miocene pyroclastic flows and related volcaniclastic deposits. Geophysical logs were obtained in 25 boreholes (designated as the G, H, and WT boreholes) drilled during 1979-1984. In addition to the geophysical data, other data collected from these boreholes included: hydrologic data (depth to water, transmissivity, and water chemistry), mineralogy, fracture density, physical properties, lithologic boundaries, and the degree of welding and zeolitization of the rock units. Porosity and water content were computed from the geophysical logs, and porosity results were combined with mineralogy from X-ray diffraction to provide whole-rock volume fractions. Compilations of these data were used by USGS project personnel during the 1990s, but many were unpublished until this year as improvements in scanning technology, computer storage, and delivery make it possible to publish the displays.
Excerpts from these integrated borehole displays highlight a number of features of the volcanic rocks at Yucca Mountain, including: (1) the relation of the mineralogy of altered tuffs to degree of welding, (2) the downward transition from unaltered (glass-bearing) to altered (zeolitic) tuff with respect to present-day water levels, (3) volumetric estimates and spatial distribution of lithophysal zones in the Topopah Spring Tuff, (4) estimates of air-filled pore space and water saturation in the unsaturated zone, and (5) a comparison of fracture and matrix permeability. In summary, the displays provide a view of the spatial distribution of parameters relating to mineralogy, porosity, water saturation, matrix flow, and fracture flow. These data were used extensively by hydrologists to help determine flow direction and probable fate of any potential radionuclide releases.
Hydrogeology
Preferred Presentation Type: Oral
