ABSTRACTPerched 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.
Yucca Mountain Site Characterization Project: Technical Data Catalog quarterly supplement