Ground-Water Hydrology and Hydrochemistry of Eagle Flat and Surrounding Area

This report presents the results of the Bureau of Economic Geology's investigation of the saturated-zone hydrology and hydrochemistry of Eagle Flat, Red Light Draw, and surrounding areas of southern Hudspeth County, Texas. Boundaries of the study area are the Rio Grande and Quitman Mountains to the south and west, the Van Horn and Carrizo Mountains to the east, and the Diablo Plateau to the north. Total land area is about 1,200 mi2 (3,110 km2). The Bureau of Economic Geology initiated this study in July 1991 at the request of the Texas Low-Level Radioactive Waste Disposal Authority (TLLRWDA). The objective of the investigation was to ascertain whether northwest Eagle Flat may be considered a suitable location for disposal of low-level radioactive waste.Bureau of Economic Geolog

Eagle Flat Project, Hudspeth County, Texas

The geomorphic, stratigraphic, and structural geologic studies are designed to characterize the geologic setting of the site and region. The results of these studies are required for the license application, to support design and geotechnical studies, and to construct the hydrogeologic framework. Various elements of the regional stratigraphy and structural setting have been described by many authors. Our studies will synthesize relevant portions of the previous work but focus on those issues that require more detailed characterization and analysis to evaluate the proposed site. Exposed bedrock geology, for example, was generally well mapped in the vicinity of the Faskin Ranch-Eagle Flat region by Underwood (1963), Albritton and Smith (1965), and King (1965). We have compiled the previous mapping on the six topographic maps (1:24,000) that cover the area designated by the Texas Legislature (fig. 1). Drilling, mapping, and geophysical studies associated with this project will add new information on the bedrock units beneath the basin-fill sediments and on the character of the basin-fill sediments themselves, particularly in the siting area.Bureau of Economic Geolog

Ground-Water Hydrology and Hydrochemistry of Eagle Flat and Surrounding Area

This report presents the results of the Bureau of Economic Geology's investigation of the saturated-zone hydrology and hydrochemistry of Eagle Flat, Red Light Draw, and surrounding areas of southern Hudspeth County, Texas. Boundaries of the study area are the Rio Grande and Quitman Mountains to the south and west, the Van Horn and Carrizo Mountains to the east, and the Diablo Plateau to the north. Total land area is about 1,200 mi^2 (3,110 km^2). The Bureau of Economic Geology initiated this study in July 1991 at the request of the Texas Low-Level Radioactive Waste Disposal Authority (TLLRWDA). The objective of the investigation was to ascertain whether northwest Eagle Flat may be considered a suitable location for disposal of low-level radioactive waste.Bureau of Economic Geolog

Preliminary Evaluation of the Eagle Flat Region Hudspeth County, Texas

Preliminary evaluation of the Eagle Flat region, as designated by the Texas Legislature, indicates several areas that may have geologic and hydrologic conditions favorable for further evaluation as potential siting areas for the Texas low-level radioactive waste repository. This determination is based on a review of available data regarding surface drainage, thickness and character of alluvial fill, depth to groundwater, and apparent presence or absence of such features as late Cenozoic faults, fissures, known natural resources, and evidence of erosion. Some general siting areas have been identified that contain several sections that appear to have favorable characteristics. Examples of apparently favorable general siting areas are east of Yucca Mesa, south of Eagle Flat Mountain, and north of Little Hills. Our preliminary assessment is that, of the three areas cited above, the Yucca Mesa location should be given priority consideration. Initial flooding and drainage analysis indicates that the Yucca Mesa site includes sufficient surface areas unaffected by flooding. There are no known fissures or late Cenozoic faults. Depth to groundwater may be in excess of 500 ft, and the water quality may be poor. Shallow alluvium may be somewhat finer grained than at other settings, and the surface appears to be relatively stable and devoid of major incision by existing drainages. Although each of these characteristics needs to be investigated by further work, the most critical unanswered question is the thickness of alluvial fill. Available gravity data, including recent work by The University of Texas at El Paso, indicates that 100 ft or more of alluvial fill may be present in the area. Other areas may be present that are of equal or similar merit, but the preliminary analysis and available information suggest that the Yucca Mesa location apparently has the most favorable characteristics of those general siting areas identified currently. Smaller potential siting areas, generally about 400 to 800 acres in size, also may exist locally throughout the region where alluvial fill may be of sufficient thickness and drainage characteristics may be appropriate. These smaller potential siting areas would require additional site-specific evaluation of surface drainage characteristics before drill testing could be recommended. Any potential siting area is unique and would require site-specific evaluation activities to assess its suitability for characterization.Bureau of Economic Geolog

Delineation of the ground-water flow systems of the Eagle Flat and Red Light Basins of Trans-Pecos Texas

The hydrogeologic systems of the Eagle Flat and Red Light basins of Trans-Pecos are characterized by three-dimensional flow components, fracture and double-porosity flow, hydrochemically zoned water in alternating permeable and low-permeability rocks and poorly consolidated strata, and areal transitions between unconfined, confined, and leaky-confined aquifers. Ground-water depths greater than 900 ft (275 m) beneath the topographically lowest areas of the Eagle Flat Basin, along with closed potentiometric contours suggest drainage to a regional flow system that transports ground water beyond basin boundaries, probably beneath the Devil Ridge ground-water divide and the thick Tertiary basin fill deposits of the Red Light Basin, where hydraulic head along the Rio Grande is as much as 400 ft (122 m) below the lowest point of the potentiometric surface in the Eagle Flat Basin. Radiocarbon is unreliable as estimator of absolute age. This is attributable to contributions of dead carbon by the dissolution of carbonate rocks and/or to isotope exchange. Mixing with younger (tritiated) ground waters adds to the complexity. δ¹⁸O values of the apparently oldest ground waters, however, are about 3%₀ (SMOW) lower than those of demonstrably modem ground waters. This pattern is thought to be consistent with the influence of precipitation under an older and probably cooler climatic regime. This suggests that the oldest ground waters may have late Pleistocene recharge dates (e.g., 10,000 to 20,000 years). In addition, the temperature-fractionation gradient for δ¹⁸O suggests precipitation under climatic conditions that were from 5.5 to 7.3°C below those of average Holocene values. Isotopic data indicate that major recharge areas are focused in the upper elevations of the mountains and along mountain fronts. Recharge attributable to precipitation does not occur in the flats and draws, except where shallow water-table conditions exist in alluvial deposits adjacent to the Rio Grande. Low-permeability late-stage calcic soils limit the potential for precipitation recharge across the broad alluvial fans of the Eagle Mountains and the Indio Mountains. Estimates of precipitation recharge derived from a numerical flow model are about 0.07 in per year, or about 14% of estimates by the U.S. Geological SurveyGeological Science