A MODELING STUDY OF PERCHE WATER PHENOMENA IN THE UNSATURATED ZONE AT YUCCA MOUNTAIN Page: 3 of 35
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The presence of perched water bodies in the vicinity of the potential repository at Yucca
Mountain has many implications, and at the same time it may provide invaluable insights
into water movement, flow pathways, or surface infiltration history of the mountain.
First it implies that water particles may not travel vertically through the unsaturated zone
to the water table directly, but has somehow been trapped or diverted laterally. As a
result, non-uniform recharge rates are expected at the water table. Another concern is that
perched zones may divert water around low-permeability zeolitic layers, a lower
formation unit, underlying the potential repository horizon. By-passing of these units,
which are thought to have substantial capacity to retard radionuclide transport, could have
important implications for the capability of the geologic system to mitigate radionuclide
releases to the environment. In addition, a perched body, if close to the repository, may
affect thermo-hydrological conditions at the repository during thermal loading from
decayed heat of high level radioactive nuclear wastes.
In this study, we have conducted a series of 3-D modeling simulations using the
TOUGH2 code (Pruess, 1991) and available perched water data from six boreholes (UZ-
14, SD-9, NRG-7/7a, G-2, SD-9, and SD-12). These modeling studies are designed to
investigate the perched water occurrences at the Yucca Mountain site. In these
simulations, both the generalized effective continuum method (ECM) and the more
rigorous dual-permeability conceptualization have been used to account for fracture and
matrix interactions (Pruess et al., 1990a and 1990b; Wu et al., 1996a). As indicated by
residence times derived from geochemical data (Yang et al., 1996), perched water bodies
at Yucca Mountain may have existed for thousands of years, and are currently present
under steady-state or quasi steady-state conditions. Under such conditions, the ECM will
provide a reasonable approximation for fracture/matrix interactions (Wu et al., 1996b;
Doughty and Bodvarsson, 1996; 1997 and this issue).
A spatially varying surface infiltration map (Flint et al., 1996) is used to describe areally
distributed net infiltration at the land surface. Perched water data observed in the field
were used to calibrate the model in terms of matrix and fracture permeabilities, capillary
functions, and relative permeabilities within the perched zones. Calibrated parameter
values were within the range of field and laboratory measurements. The steady-state
simulation results are in agreement with the observed perched water data in terms of
water saturation and perched water locations. Furthermore, the simulation results of a
transient numerical pumping test using a 3-D submodel matched water level data
observed during field pumping tests.
2. Perched Water Data
Perched-water has been intersected in a number of boreholes (UZ-14, NRG-7a, SD-9,
and G-2), in the UZ at Yucca Mountain (Striffler et al.; 1996). Perched water was also
found at borehole SD-7 (Czarnecki; 1995) and (Soeder; USGS, 1995), and wet core was
recovered from borehole SD-12 (Patterson; 1996). In addition to the perched water3
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Y.S. WU, A.C. RITEEY AND G.S. BODVARSSON. A MODELING STUDY OF PERCHE WATER PHENOMENA IN THE UNSATURATED ZONE AT YUCCA MOUNTAIN, report, January 12, 1999; Las Vegas, Nevada. (https://digital.library.unt.edu/ark:/67531/metadc723840/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.