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Mechanisms for the Formation of a Perched Water Zone inFractured Tuff: a Natural Analogue Study

Published online by Cambridge University Press:  03 September 2012

E. G. Woodhouse  and
R. L. Bassett
E. G. Woodhouse
Affiliation:
Department of Hydrology and Water Resources, P.O. Box 210011, University of Arizona, Tucson, AZ 85721–0011, betsy@hwr.arizona.edu
R. L. Bassett
Affiliation:
Department of Hydrology and Water Resources, P.O. Box 210011, University of Arizona, Tucson, AZ 85721–0011, betsy@hwr.arizona.edu
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Abstract

Perched water zones have been identified in the fractured, welded tuff inthe semi-arid to arid environments of Yucca Mountain, Nevada and nearSuperior, Arizona. An understanding of the formation of such zones isnecessary in order to predict where future perched water might form at YuccaMountain, the proposed site of a high-level nuclear waste repository. Theformation or growth of a perched zone near a repository is one of thefactors to be considered in the risk assessment of the Yucca Mountainsite.

The Apache Leap Research Site near Superior, Arizona is a natural analog tothe Yucca Mountain site in terms of geology, hydrology, and climate.Information used to study possible mechanisms for the formation of theperched zone included data regarding isotopie and geochemical properties ofthe waters in and above the perched water zone; measured hydrologieparameters of the perched zone; geophysical and measured parameters of thetuff; megascopic and microscopic observations of the tuff, includingmineralogical, alteration, and structural features; and the lateral andvertical extent of perched water in the region.

Aquifer test, geophysical, geochemical, and radioisotopic data show thatfractures are the means by which water is recharging the perched zone. Thereduced hydraulic conductivity of the formation in the perched zone appearsto result from both a severe reduction in matrix porosity and permeabilitycaused by welding, devitrification, and vapor phase crystallization; and byan increase in fracture filling which restricts the pathways for flow.

Information

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 465: Symposium II – Scienfific Basis for Nuclear Waste Management XX , 1996 , 1161
Copyright
Copyright © Materials Research Society 1997

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References

REFERENCES

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