Abstract

Regional and local hydrologic and hydrochemical investigations of the saturated zone were conducted in Trans-Pecos Texas at the site of the proposed low-level radioactive waste repository in Hudspeth County, Texas. 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 occurs in Hueco Bolson silts and sands at depths of 361 ft (110 m) and 478 ft (146 m), respectively, and at a depth 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.

Transmissivities of aquifers in bolson and Cretaceous strata range from approximately 0.19 to 290.0 ft²/d (0.018 to 26.9 m²/d). A composite potentiometric-surface map based on water levels measured in all available wells (assuming hydrologic communication between the Diablo Plateau, Hueco Bolson silt and sand, and Rio Grande alluvium aquifers) suggests that ground water 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 directs flow from the Diablo Plateau in the eastern study area toward Cretaceous outcrops along the Campo Grande fault trend. The low level of hydraulic heads near the principal study area is caused by preferential drainage toward the Rio Grande along permeable bolson deposits in the west and southwest.

Water chemistry data, particularly on tritium, carbon-14, and total dissolved solids, generally supports this interpreted flow pattern; some discrepancies may be related to paleohydrologic effects associated with the incision of the Rio Grande during Quaternary time.

Predominant cations and anions in ground waters, whether in Hueco Bolson silts and sands or Cretaceous limestones in the principal study area, are sodium and sulfate.

Development of ground-water resources near the principal study area was found to be limited by two key factors: (1) costs of drilling and completing wells and producing water from depths typically greater than 400 ft (122 m) and (2) the low productivity of aquifers.