Evaluation of the Ground-Water Resources of the Paleozoic and Cretaceous Aquifers in the Hill Conntry of Central Texas

Robert L. Bluntzer

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The evaluation of the ground-water resources of a part of the Hill Country area of central Texas includes all or part of Bandera, Blanco, Comal,Gillespie, Hays, Kendall, Kerr, Medina, and Travis Counties. This report wasprepared in response to the Sixty-ninth Texas Legislature's passage of House Bill 2 which stipulated the identification and study of areas within the State that are experiencing or expected to experience within the next 20 years critical ground-water problems. The relatively extensive study area of all or parts of nine counties has a subhumid to semiarid climate that has low to moderate rainfall and high rates of evaporation. The economy of the area is dominated by agribusiness related to the raising of livestock and exotic game animals, tourism, andhunting, and is significantly influenced by the population and economic growth conditions associated with the metropolitan centers at San Antonio, New Braunfels, San Marcos, and Austin.In 1985, about 62 percent of the water supplies in the area were obtained from the Paleozoic and Cretaceous aquifers. The Paleozoic aquifers includethe Hickory and Mid-Cambrian aquifers of Cambrian age, the Ellenburger San Saba aquifer of Cambrian and Ordovician age, and the Marble Falls aquifer of Pennsylvanian age. The aquifers of Cretaceous age include the Lower Trinity, Middle Trinity, Upper Trinity, and Edwards Plateau aquifers. The average annual recharge to the Paleozoic and Cretaceous aquifers was estimated to be about 450,000 acre-feet. However, because of the erratic occurrence of ground waters within these aquifers and their inherently low to extremely low coefficients of transmi~sihility and storage, only about 46,000 acre-feet of ground water has been estimated as the annual sustained yield or these aquifers in the study area. Of the 18,739 acre-feet of ground water used in 1985, approximately 74 percent was used for drinking and household purposes (public and domestic uses). Historical development of ground water in areas of concentrated withdrawals for pu