Hydrologic Investigations of Deep Ground-Water Flow in the Chihuahuan Desert, Texas

Regional and local hydrologic investigations were conducted in Trans-Pecos Texas as part of a study to evaluate potential sites 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. Ground water 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 ground-water flow paths and velocities, and (3) testing hydrologic hypotheses using ground-water flow models.Development of ground-water resources in the vicinity of the principal study area is limited by (1) costs of drilling and completing wells and of producing water at depths typically greater than 400 ft (122 m) and (2) extremely low aquifer productivity. Transmissivities of aquifers in bolson and Cretaceous strata, as revealed by 12 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 available wells, assuming hydrologic interconnection of the Diablo Plateau aquifer, Hueco Bolson silt and sand aquifer, and Rio Grande alluvium aquifer, indicates that ground water is recharged on the Diablo Plateau and flows to the south and southwest toward the Rio Grande beneath the bolson pediment. Little, if any, groundwater recharge occurs through the bolson surficial sediments, because of a high degree of precipitation runoff and extremely high evapotranspiration rates. A planar ground-water flow model was constructed to investigate the dominant controls on the regional flow pattern. The inferred distribution of permeability zones focuses flow from the Diablo Plateau in the eastern part of the study area toward Cretaceous outcrops in the footwall of the Campo Grande fault, creating a ridge in the potentiometric surface. The relatively low hydraulic heads reflected in the valley in the potentiometric surface 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 data 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.