Ground Water at Yucca Mountain How High Can It Rise? (1992) / Chapter Skim
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Can an Igneous Intrusion Raise the Water Table to the Proposed Repository Level?
Can an Igneous Intrusion Raise the Water Table to the Proposed Repository Level?
Pages 93-103
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From page 93... ...
Concurrent with the silicic episode, large-volume olivine basalt lava flows erupted between 11 and ~ Ma near and within some of the silicic centers in the Yucca Mountain region (Crowe et al., 1983a and 19861. In roughly the same time period (6-10 Ma)
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From page 94... ...
The current southernmost boundary of active subduction below western North America is the Mendocino triple junction at approximately 40°N, more than 300 km north of the latitucle of Yucca Mountain. The present day volcano-tectonic regime throughout the northern Basin and Range province, which includes most of Nevada and the western half of Utah, is extensional, characterizec]
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From page 95... ...
To place constraints on the size and geometry of such intrusions we can look at the occurrence and distribution of other Quaternary basaltic dikes, flows, and cinder cones throughout the northern Basin and Range province. Most of this young basaltic activity occurs within the moclern basins, with many of the eruptive centers occurring along basin margins, particularly localized near range-bounding faults (Smith et al., 1990~.
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From page 96... ...
A brittle crust 10-15 km thick should limit dike width in the upper crust to 2~ m (Carrigan et al., 1990~. Thus, theoretical considerations, as well as structural and geologic evidence from the Yucca Mountain region and the surrounding Basin and Range province, suggest a likely geometry for basaltic dike intrusion in the upper crust in the vicinity of the repository to be a 2 - m wide (or less)
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From page 97... ...
MODELS OF WATER TABLE RISE ACCOMPANYING DIKE INTRUSION Two independent types of models of water level changes associated with dike intrusion have been investigated for Yucca Mountain. Both models assume intrusion along a vertical zone beneath the repository with the top of the intrusion located 250-500 m below the water table.
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From page 98... ...
The three percent lower velocity anomaly identified under Crater Flat Is considerably smaller in magnitude than velocity anomalies associated with known large-scale high silica magma chambers, such as Long Valley, where velocity decreases of six to eight percent are typically found (Iyer, 1988~. The possibility that molten rock is present beneath Crater Flat deserves further study.
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From page 99... ...
All rely on diddling the probability assessment into two steps: (1) Pi, the probability of basaltic activity in the general region of the proposed repository, including Lathrop Wells and Crater Flat (called the Crater Flat Volcanic Zone by Crowe)
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From page 100... ...
Concentrating the water rise in a 100 m wide vertical highly permeable zone directly above the dike resulted in a maximum rise of 12-14 m when vertical permeabilities were enhanced 103-104 within the vertical zone. Thus, the panel concludes that the elastic effect of dike intrusion would result in raising the water table no more than a few tens of meters.
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From page 101... ...
While the thermal effects may dominate, the panel notes that this modeling could establish the maximum water table excursion due to dike intrusion. The study plan for the characterization of volcanic features calls for drilling core holes to investigate aeromagnetic anomalies that may represent either buried volcanic centers or intrusive rocks (probably basaltic in composition)
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From page 102... ...
1990. A scoping study of water table excursions induced by seismic and volcanic events.
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From page 103... ...
Implications for Volcanic Risk Assessment, In Proceedings, International High-Level Radioactive Waste Management Conference, Las Vegas, Nevada, ANS/ASCE 1:81-90. Sheridan, M
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