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 GSA Today, v. 9, no. 5, May 1999


Table of Contents

Science Article: (View Abstract)
Stressed Rock Strains Groundwater at Yucca Mountain, Nevada
by David A. Ferrill, James Winterle, Gordon Wittmeyer, Darrell Sims, Shannon Colton, Amit Armstrong, Alan P. Morris

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In Memoriam 2
Penrose Conference: Volcanic Rifted Margins 3
About People 7
GSA Internet Symposium: An Experiment in Scientific Communication 9
SAGE Remarks: Promoting Geoscience Education 11
1999 GSA Section Meeting 12
Washington Report — A Speech and a Legacy 14
GSAF Update 16
Letter 17
GSA Divisions — Do You Belong? 18
1999–2000 Section Officers and Past Chairs 19
Committees 20
New Headquarters Department Focuses on Development 24
Book Reviews 24
Field Forums 24
Call for Short Course Proposals 25
2000 Annual Meeting 25
Bulletin and Geology Contents 27
Calendar 30
Classifieds 31

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Stressed Rock Strains Groundwater at Yucca Mountain, Nevada
David A. Ferrill, James Winterle, Gordon Wittmeyer, Darrell Sims, Shannon Colton, Amit Armstrong, Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238-5166
Alan P. Morris, Division of Earth and Physical Sciences, University of Texas, San Antonio, TX 78249

Groundwater flow at Yucca Mountain, Nevada, must be understood to determine transport and dilution of contaminants released from the proposed high-level waste repository. The nature of such flow in a complexly faulted and fractured tuff aquifer is not easily characterized. Conceptual models of regional groundwater flow often assume isotropic transmissivity, resulting in flow being interpreted as parallel to the gradient of the potentiometric surface. However, fractured aquifers are typically anisotropic, their transmissivity controlled by the conductive properties of faults and fractures, which are partially controlled by the in situ stress field. Faults at Yucca Mountain predominantly strike approximately north (azimuth ~005). Slip and dilation tendency analysis of the region indicates that those faults and fractures ideally oriented for slip in the current stress field strike north-northeast (azimuth ~025–030) and dip moderately to steeply, whereas those ideally oriented for dilation strike north-northeast (azimuth ~025–030) and are vertical. Faults with favorable orientations for slip or dilation present potential fluid flow pathways. These observations imply anisotropic transmissivity at Yucca Mountain with an azimuthal direction of maximum transmissivity between 005 (based on dominant fault trend) and 030 (based on slip- and dilation-tendency constraints). Reinterpretation of data from a long-term aquifer pumping test on the eastern flank of Yucca Mountain indicates anisotropic transmissivity with a maximum principal transmissivity direction of approximately 030. This is consistent with anisotropic transmissivity controlled by faults and fractures active in the present-day in situ stress field.

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