WRIR 01-4210: Hydraulic-Property Estimates for Use With a Transient Ground-Water Flow Model of the Death Valley Regional Ground-Water Flow System, Nevada and California
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The U.S. Department of Energy (DOE) and the U.S. Department of Defense (DOD) conducted various types of underground nuclear tests at the Nevada Test Site (NTS) in southern Nevada (fig. 1) between 1951 and 1992. Those tests produced radionuclides that contaminated ground water beneath portions of the NTS. In 1972, DOE established a long-term monitoring program to detect the presence of any radioactivity that may have been related to nuclear testing activities. Currently, DOE is evaluating contaminated areas as part of the Environmental Restoration program. The U.S. Geological Survey (USGS), in cooperation with the DOE, is evaluating the geologic and hydrologic characteristics of an area near Yucca Mountain, adjacent to the NTS, which is being considered for construction of an underground high-level nuclear waste repository. As part of these programs, the USGS is evaluating the regional ground-water flow system in the Death Valley region.
USGS evaluations include a detailed characterization of the ground-water flow system including development of a regional three-dimensional (3-D) conceptual and numerical ground-water flow model to help: (1) characterize regional 3-D ground-water flow paths, (2) define boundaries of the subregional and local flow systems, (3) define locations of regional ground-water discharges, (4) estimate magnitudes and rates of regional subsurface flux, (5) evaluate existing and potential anthropogenic effects on ground-water flow, (6) characterize potential impacts of the regional carbonate aquifer on subregional and local flow components, (7) determine potential effects of regional geologic structure on the flow system, (8) establish regional hydrologic boundaries of ground-water resources that may be unsafe for domestic or municipal use, and (9) prioritize ongoing local investigations.
Steady-state and time-dependent (transient) numerical ground-water flow models are being developed by the USGS to integrate and expand upon the existing ground-water models (Frank D'Agnese, U.S. Geological Survey, written commun., 2001). The USGS has compiled, analyzed, and synthesized hydraulic-property estimates for rocks and sediments within the Death Valley region for the basis of assigning hydraulic-property values to the various hydrogeologic units within the study area.
The Death Valley regional ground-water flow system (DVRFS) is located within the Great Basin section of the Basin and Range physiographic province in southeastern California and southern Nevada between latitudes 35° and 38° 15' north and longitudes 115° and 117° 45' west (fig. 1). The topography typically consists of northerly and northwesterly trending mountain ranges separated by broad sediment-filled basins. The Spring Mountains, the highest topographic feature in the area, rise to about 3,600 m above mean sea level. Other prominent topographic features within the region include the Sheep Range, Pahute Mesa, the Funeral Mountains, and the Panamint Range. The inter-mountain basins generally decrease in altitude from north to south. The lowest altitude in the study area (86 m below sea level) is in Death Valley National Park. Other areas of national importance within the study area include the Nevada Test Site (NTS), Yucca Mountain, the Ash Meadows National Wildlife Refuge, the Desert National Wildlife Refuge, and several military installations. Pahrump, Nevada, is the largest of several towns in the study area.
The DVRFS model area encompasses about 45,000 km2. The area of the current study is significantly larger than the DVRFS model area to permit an adequate characterization of areas that contain sites important for defining hydraulic characteristics of hydrogeologic units (HGUs; fig. 2).
The purpose of this report is to compile and statistically summarize published and unpublished hydraulic-property estimates (such as transmissivity, hydraulic conductivity, storativity, and specific storage) and to provide a statistical range of quality-assured hydraulic-property estimates for use in on-going DVRFS simulation activities. The estimates are presented by proposed HGU's for use in a transient numerical ground-water flow model of the Death Valley region. Descriptive statistics of the estimates provide ranges and trends of the parameters for use in the model.
Ground-water flow in the Death Valley region was discussed and simulated independently by D'Agnese and others (1997) and IT Corporation (1996a). The two steady-state numerical models resulting from these investigations, the Yucca Mountain Project-Hydrologic Resources Management Program (YMP-HRMP) flow model (D'Agnese and others, 1997) and the Underground Test Area (UGTA) Phase I flow model (IT Corporation, 1996a), respectively, have overlapping domains (fig. 1). Both models were based on digital 3-D geologic framework models and both used 3-D finite-difference codes to simulate ground-water flow. The two models differ in the numerical codes used, the number of model layers, and the distribution of hydraulic properties within discrete layers.
Hydraulic-property estimates were compiled for use in the YMP-HRMP and UGTA ground-water flow models. Estimated values for the YMP-HRMP flow model (D'Agnese and others, 1997), however, were not developed from a hydraulic-properties database compiled as part of the simulation effort. Instead, model-layer properties were estimated from a plot of statistically distributed hydraulic properties for rock types in the Basin and Range province (Bedinger and others, 1989) as part of a study of the geology and hydrology of the province. Data compiled for their report consisted of published field and laboratory tests within the Basin and Range province, as well as general studies from rocks with similar characteristics from outside the province. Individual aquifer tests used to develop the statistical plot presented in Bedinger and others (1989) were not discussed, and no hydraulic data were evaluated.
IT Corporation (1996b) compiled a database that contains 731 analyses of transmissivity and hydraulic conductivity for the UGTA Phase I ground-water flow model. UGTA evaluated hydraulic properties from the literature (including re-interpretation of published data) and from UGTA-specific tests. Because the flow model was time independent, no values of storativity or specific yield were compiled. Databases from those simulation efforts have been expanded upon in this study.
We thank Dr. Keith Halford and Mr. David Prudic (both with the U.S. Geological Survey, Carson City, Nev.), Dr. Richard Waddell (GeoTrans, Denver, Colo.), and Mr. William Fryer (IT Corporation, Las Vegas, Nev.) for their thorough and critical reviews of the initial manuscript. Their efforts have greatly enhanced the quality of this work. We also acknowledge the efforts of Mr. Robert Bangerter (U.S. Department of Energy, Environmental Restoration Program, Nevada Operations) for providing the funding to complete this work.
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