Browsing by Subject "Eagle Flat Basin"
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Item Delineation of the ground-water flow systems of the Eagle Flat and Red Light Basins of Trans-Pecos Texas(1997) Darling, Bruce Kelley, 1951-; Sharp, John Malcolm, Jr., 1944-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 SurveyItem Unsaturated Zone Studies at the Proposed Low-Level Radioactive Waste Disposal Facility, Eagle Flat Basin, Texas(1993) Scanlon, Bridget R.; Xiang, Jiannan; Goldsmith, Richard S.Hydrologic studies were conducted to characterize unsaturated zone processes at the proposed low-level radioactive waste disposal site and surrounding area in southern Hudspeth County, Texas. The study area is in northwest Eagle Flat basin, which is within the Basin and Range Physiographic Province. Fractured Cretaceous bedrock crops out to the southeast of the site. The thickness of the basin-fill sediments at the proposed site ranges from 164 feet (50 m) to approximately 656 feet (200 m). Northwest Eagle Flat basin is an internally drained basin that drains through the ephemeral Blanca Draw into Grayton Lake playa. The climate in the study area is subtropical arid and the long-term average annual rainfall is 12.6 inches (320 mm). Unsaturated zone studies were conducted in ephemeral stream and interstream geomorphic settings. In addition to studies of areas typical of these settings, the impact of pseudofissures, an earth fissure, and borrow pits on shallow zone unsaturated processes was also investigated. To evaluate unsaturated zone processes, 57 boreholes were drilled in the various geomorphic settings for collection of soil samples and installation of monitoring equipment. Soil samples were analyzed in the laboratory for particle size, water content, water potential, and chloride concentration. Water potential data are used to evaluate the direction of the driving force for water movement. Chloride concentration data provide information on water fluxes because chloride concentrations are inversely proportional to water flux; low chloride concentrations indicate high water fluxes because chloride is flushed through the soil, whereas high chloride concentrations indicate low water fluxes because chloride is concentrated by evapotranspiration. In addition to laboratory analyses, a monitoring program was initiated by installing neutron probe access tubes in the different geomorphic settings to monitor water content. Field psychrometers were installed to a depth of 60.7 feet (18.5 m) in the interstream setting to monitor water potential and temperature. Hydraulic conductivity was also measured in the field using permeameter tests, constant-head borehole infiltration tests, and multistep constant-head borehole infiltration tests.