Hydrologic Investigations of the Saturated Zone in South Central Hudspeth County, Texas

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Mullican, William F.  
Senger, Rainer K.

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Hydrologic investigations are in progress in Trans-Pecos Texas at the proposed site of a low-level radioactive waste repository. The site 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 area of the proposed site is found at depths of 478 ft (146 m) and 592 ft (180 m) in bolson silt and sand and Cretaceous limestone, respectively. The unsaturated zone consists of approximately 50 ft (15 m) of alluvial silt, sand, and gravel underlain by 450 ft (137 m) of lacustrine and fluvial clay, silt, and fine sand.

High-priority tasks for characterizing the groundwater regime include (1) evaluating groundwater resources in the area, (2) determining groundwater flow paths and velocities, (3) testing hydrologic hypotheses using groundwater flow models, and (4) determining groundwater hydrochemistry. The objective of this report is to evaluate the groundwater resources and to discuss our current understanding of groundwater flow paths and velocities.

Groundwater resources in the vicinity of the site are limited by two key factors: (1) costs of drilling and completing wells and of producing water at depths greater than 400 ft (122 m) and (2) very low productivity of aquifers. Typical transmissivities of aquifers producing from bolson and Cretaceous strata, based on four tests, range from approximately 4.3 x 10^-2 ft²/day (4.0 x 10^-3 m²/day) to 2.9 x 10^2 ft²/day (2.7 x 10^1 m²/day). A composite potentiometric surface has been mapped on the basis of static water levels measured in all available wells. Regional groundwater flow is interpreted from the potentiometric surface to be generally south-southwest toward the Rio Grande.


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