No abstract

No abstract

No abstract

No abstract

Numerical simulation was used to study steady liquid water movement in a 5-m by 5-m vertical section containing a hypothetical fracture network under conditions of variable saturation. The fracture network was assumed to be embedded within an impermeable rock matrix. Three variations of a network were considered. The "mixed" network consisted of two fracture sets, a subvertical set containing five 125 /am average aperture fractures and a subhorizontal set containing four 25 /am average aperture fractures. The other two networks had identical fracture orientation and contained either all 125 •m or all 25/am average aperture fractures. The TOUGH simulator was used to calculate the total steady liquid water flux through the network, the flux through individual fracture segments, and the pressure head at each fracture segment. A unit hydraulic gradient was imposed on the network by applying fixed pressure head boundaries (ranging from -0.25 to 0.0 m of water) of equal value to the top and bottom. Saturation and permeability versus pressure head relations for the two sets of fractures were determined with the VSFRAC model, which assumed that aperture was variable within an individual fracture. Results showed that the spatial distributions of pressure head and flux within the network, as well as the location of the dominant pathways, depended strongly on the prescribed boundary pressure head. For the mixed network, both pressure head and flux tended to become more spatially uniform when the boundary pressure head approached the pressure head at which the permeability thickness products of the large-and small-aperture fractures are equal (the crossover pressure head). These results imply that for systems similar to the one considered here, interpretation of actual measurements of pressure head and flux may be quite complex, and that representation of variably saturated fracture networks as an equivalent continuum may be more valid for some ranges in pressure head than for others. Equivalent permeability as a function of pressure head was calculated for the fracture network, illustrating how information collected on individual fractures may be used to estimate the flow properties of rock at larger scales. 4091 4092 KWICKLIS AND HEALY: NUMERICAL iNVESTIGATION OF STEADY LIQUID WATER F• Ow From these studies it is clear that heterogeneity within fractured rock exists over a variety of scales. Within single fractures, spatial variations inaperture will control the locations of the principal pathways for liquid flow within the fracture. At larger scales, individual fracture transmissivities and network interconnectedness dictate the locations of the dominant pathways for flow. The scale-dependent nature of heterogeneities within saturated fracture systems was reflected in the work of Smith et al. [1987], who demonstrated with a three-dimensional fracture network model that estimates of hydraulic or tracer aperture based on borehole measurements were sensitive to a number of geometric factors, including aperture variation ...

On average, winter precipitation represents the greater part of total annual precipitation in the Mojave Desert [Hevesi and Flint, 1998]. In the southwestern United States (southern Great Basin and Mojave Deserts), winter precipitation, which often is in the form of snow, especially at higher altitudes (>2000 rn above sea level), tends to be lower in intensity and longer in duration than summer precipitation and tends to cover larger areas. The position of the jet stream determines the seasonal precipitation frequency for this area; the jet stream, in turn, depends strongly on global circulation patterns such as the E1 Nifio-Southern Oscillation [French, 1983]. In contrast, summer precipitation is controlled primarily by the southwest summer monsoonal storms [Houghton, 1969;Pyke, 1972], which tend to be higher in intensity and shorter in duration (1-2 hours) and cover more localized areas than winter precipitation. Orographic influences usually cause an increase in the frequency and amount of precipitation with an increase in altitude. During the summer, precipitation that develops at higher altitudes often evaporates as it passes through hotter and drier atmospheric conditions at lower altitudes and fails to reach the ground surface of the deeper valleys and basins. This phenomenon, known as virga, occurs frequently in the Yucca Mountain region.Precipitation data for selected measurement sites were collected, each contributing to the understanding of a particular component of precipitation characterization, such as timing, spatial distribution, or intensity [Ambos et al., 1995; Flint and Davies, 1997]. These data were used to statistically characterize precipitation for the Yucca Mountain site and regional study areas. Probability distribution functions for precipitation intensity and frequency were used to quantify and develop sto- Site Geomorphology The hydrogeologic setting of the Yucca Mountain site area is a direct result of its location in the northernMojave Desert with its associated climate and, more specifically, its geologic history and resultant physiography. The outcome is a hydrologic system with a 500-to 750-m-thick unsaturated zone over a saturated system with a relatively small gradient The Yucca Mountain site study area can be divided into five physiographic elements: (1) ridges and valleys, Side slopes are steep and commonly have thin to no soil cover that has developed in welded, fractured tuff. The steepness of the slopes creates conditions conducive to rapid runoff. The low storage capacity of the thin soil cover and the exposure of fractures at the surface may enable small volumes of water to infiltrate to greater depths, more so on slopes with north facing exposures that have low evapotranspiration demands. Shallow alluvium at the bases of the slopes can easily become saturated and initiate flow into the underlying fractures.Alluvial terraces are flat, broad deposits of layered rock fragments and fine soil with a large storage capacity. Little runoff is generated on the terraces, and pre...

Approximate solutions are presented for absorption of water into porous spherical, cylindrical, and slablike blocks whose characteristic curves are of the van Genuchten‐Mualem type. The solutions are compared to numerical simulations of absorption into blocks of the Topopah Spring member (Paintbrush tuff) from the site of the proposed nuclear waste repository at Yucca Mountain, Nevada. Guided by these results, a scaling law, based on the ratio of surface area to volume, is then proposed for predicting the rate of absorption into irregularly shaped blocks. This scaling law is tested against a numerical simulation of absorption into an irregularly shaped, 2‐dimensional polygonal block and is shown to be a good approximation.

DISCLAIMERT h e views and opinions of authors expressad herein do not n a n u n l y m t e or reflect t h e of the United States Government or any agency rherwf.