Influence of Deterministic Geologic Trends on Spatial Variability of Hydrologic Properties in Volcanic Tuff

by Christopher A. Rautman, Sandia Natl Lab, Albuquerque, United States,
Alan L. Flint, Sandia Natl Lab, Albuquerque, United States,
Jonathan D. Istok, Sandia Natl Lab, Albuquerque, United States,
Lorraine E. Flint, Sandia Natl Lab, Albuquerque, United States,
Michael P. Chornack, Sandia Natl Lab, Albuquerque, United States,

Document Type: Proceeding Paper

Part of: High Level Radioactive Waste Management 1993


Hydrologic properties have been measured on outcrop samples taken from a detailed, two-dimensional grid covering a 1.4 km outcrop exposure of the 10-m thick nonwelded-to-welded, shardy base microstratigraphic unit of the Tiva Canyon Member of the Painbrush Tuff of Miocene age at Yucca Mountain, Nevada. These data allow quantification of spatial trends in rock matrix properties that exist in this important hydrologic unit. Geologic investigation, combined with statistical and geostatistical analyses of the numerical data, indicates that spatial variability of matrix properties is related to deterministic geologic processes that operated throughout the region. Linear vertical trends in hydrologic properties are strongly developed in the shardy base microstratigraphic unit, and they are more accurately modeled using the concept of a thickness-normalized stratigraphic elevation within the unit, rather than absolute elevation. Hydrologic properties appear to be correlated over distances of 0.25 to 0.3 of the unit thickness after removing the deterministic vertical trend. The use of stratigraphic elevation allows scaling of identified trends by unit thickness, which may be of particular importance in a basal, topography-blanketing unit such as this one. Horizontal changes in hydrologic properties do not appear to form obvious trends within the limited lateral geographic extent of the ash-flow environment that was examined. Matrix properties appear to be correlated horizontally over distances between 100 and 400 m. The existence and quantitative description of these trends and patterns of vertical spatial continuity should increase confidence in models of hydrologic properties and ground-water flow in this area that may be constructed to support the design of a potential high-level nuclear waste repository at Yucca Mountain.

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