Hydrologic Characterization of Ground Water Resources in South Central Hudspeth County, Texas

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Regional and local hydrologic investigations have been conducted in Trans-Pecos Texas at the principal study area for a low-level radioactive waste repository. The area is approximately 40 mi (65 km) southeast of El Paso in the Hueco Bolson, a fault-bounded desert basin that developed in the late Tertiary. Groundwater in the principal study area is found in Hueco Bolson silts and sands at depths of 361 ft (110 m) and 478 ft (146 m), and at depths of 592 ft (180 m) in Cretaceous limestones. The unsaturated zone consists of approximately 50 ft (15 m) of alluvial silt, sand, and gravel underlain by 300 to 500 ft (91 to 152 m) of lacustrine and fluvial clay, silt, and fine sand. The scope of this investigation included (1) evaluating groundwater resources in the area, (2) determining groundwater flow paths and velocities, and (3) testing hydrologic hypotheses using groundwater flow models. Development of groundwater resources in the vicinity of the principal study area is limited by two key factors: (1) costs of drilling and completing wells and of producing water at depths typically greater than 400 ft (122 m), and (2) the very low productivity of aquifers. Transmissivities of aquifers in bolson and Cretaceous strata, as revealed by 11 aquifer tests, range from approximately 0.19 to 290.0 ft²/d (0.018 to 26.9 m²/d); corresponding permeabilities range from 0.0015 to 2.82 ft/d (0.0005 to 0.861 m/d). A composite potentiometric surface based on water levels measured in all available wells and on the hydrologic interconnection of the Diablo Plateau aquifer, Hueco Bolson silt and sand aquifer, and Rio Grande alluvium aquifer indicates that groundwater is recharged on the Diablo Plateau and flows to the south and southwest toward the Rio Grande beneath the bolson pediment. The inferred distribution of permeability zones focuses flow from the eastern Diablo Plateau toward Cretaceous outcrops along the Campo Grande fault, creating an observed potentiometric high. The relatively low hydraulic heads near the principal study area are caused by preferential drainage along relatively permeable bolson deposits to the west and southwest toward the Rio Grande. Water chemistry data, particularly on tritium, carbon-14, and total dissolved solids, generally support the interpreted flow pattern; some discrepancies can be related to paleohydrologic effects associated with the incision of the Rio Grande during Quaternary time.


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