Occurrence, Availability, and Chemical Quality of Ground Water in the Edwards Plateau Region of Texas

The Edwards Plateau is located in southwest Texas and lies between 98 o and 103o west longitude and 29 o and 32o north latitude. The area composes approximately 23,000 square miles and includes all or parts of 28 counties. The region is bounded on the west by th e Pecos River ; on the north, northwest, and northeast by the physical limit of the Cretaceous rocks; on the east by the Llano uplift; on the south and southeast by the Balcones fault system; and on the southwest by the Rio Grande. The agricultural economy of the region is based primarily on ranching. Some of the leading sheep- and goat-producing counties in the State are in the Edwards Plateau region . In 1972, the agricultural income was over $135 million. The production of oil and gas is the principal industry, especially in the northwestern part of the region. More than three billion barrels of oil have been produced in the study area since oil was first discovered in the area in 1925. Rocks of sedimentary origin overlie the Precambrian granites beneath the Edwards Plateau. These sedimentary rocks range in thickness from a few hundred feet along the eastern part of the Plateau to about 15,000 feet in the western part. The principal aquifers, or water-bearing units, in order of importance and development are the Edwards-Trinity (Plateau), composed of the Antlers Formation and the Edwards and associated limestones; the alluvium; the lower Cretaceous, composed of the Hosston, Sligo, Pearsall, and Glen Rose Formations; the Hickory; and the Ellenburger-San Saba. Other units that yield fresh to slightly saline water in limited areas on or near the Edwards Plateau are the Ogallala aquifer and rocks of Pennsylvanian, Permian, and Triassic age. The total amount of fresh to slightly saline ground water available from all aquifers on the Edwards Plateau is more than 450,000 acre-feet per year. Of this amount, approximately 308,000 acre-feet is available annually from the Edwards-Trinity (Plateau) and the alluvium aquifers. In 1972 approximately 86,000 acre-fee