Regional Groundwater Flow in a Semi-arid Region

Regional Groundwater Flow in a Semi-Arid Region - Controls of Fracture Systems in Carbonate Units

John M. Sharp, Jr. and Matthew M. Uliana
Department of Geological Sciences, The University of Texas at Austin,
Austin, TX 78712

For presentation at GSA Symposium 27 in Toronto, 10/98

ABSTRACT

Trans-Pecos Texas is part of the Chihuahuan Desert and includes the southwesternmost part of the Basin and Range in the USA and part of the petroleum-rich Permian Basin. Individual groundwater basins of Trans-Pecos Texas are connected by regional flow systems through carbonate rocks and clastic basin fills.

Structural features in carbonate units have created the templates for fractures and subsequent karstification. In several cases, regional fracture trends directly connect the major recharge and discharge areas over great distances. We hypothesize that the fracture systems are responsible for the evolution of these areas into zones for concentrated recharge and discharge. A priori analysis of fracture systems is shown to provide critical details on interpretation of regional flow system properties. Permeability distributions are defined by fracture domains. Recharge is primarily from fractures in the highlands, losing streams on proximal portions of alluvial fans, and irrigation return flow along well casings. Natural discharge is to several large springs and (formerly) to the Pecos River. The major discharge at present is to irrigation wells. ⁸⁷Sr/⁸⁶Sr ratios (in conjunction with other chemical and isotopic data) are used to infer flow paths and residence times. A significant, but yet poorly constrained portion of discharge was recharged during wetter conditions in the Pleistocene and the groundwater system is evolving because of both climatic trends and the effects of man. Similar fracture-mapping techniques and ⁸⁷Sr/⁸⁶Sr analyses may be applicable in other semi-arid zones with extensive carbonate rocks.

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