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Mechanisms for the Formation of a Perched Water Zone in Fractured 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 in the semi-arid to arid environments of Yucca Mountain, Nevada and near Superior, Arizona. An understanding of the formation of such zones is necessary in order to predict where future perched water might form at Yucca Mountain, the proposed site of a high-level nuclear waste repository. The formation or growth of a perched zone near a repository is one of the factors to be considered in the risk assessment of the Yucca Mountain site.

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

Aquifer test, geophysical, geochemical, and radioisotopic data show that fractures are the means by which water is recharging the perched zone. The reduced hydraulic conductivity of the formation in the perched zone appears to result from both a severe reduction in matrix porosity and permeability caused by welding, devitrification, and vapor phase crystallization; and by an increase in fracture filling which restricts the pathways for flow.

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

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