The principal aquifer in Uvalde County is the Edwards and associated limestones of Cretaceous age. The aquifer underlies an extensive area in south-central Texas extending along the Balcones fault zone from Kinney County eastward to San Antonio, and thence northeastward to Hays County. The hydrologic unit making up the Edwards and associated limestones consists of the Comanche Peak limestone, the Edwards limestone, the Kiamichi formation, and the Georgetown limestone. Other less important aquifers in Uvalde County include the Glen Rose limestone, the Buda limestone, the Austin chalk, and the Leona formation.

Uvalde County occupies parts of two physiographic provinces, the Edwards Plateau on the north and the Coastal Plain on the south; the two provinces are separated by the Balcones fault zone. In the Edwards Plateau the formations of the Edwards and associated limestones crop out on the plateau surface; in the Coastal Plain where the formations have been down-faulted, they underlie younger rocks. In both provinces the formations dip gently toward the south and southeast; the dips are greatest in the Coastal Plain. Igneous intrusions have penetrated the formations in many places in the Coastal Plain, and locally have formed barriers to ground-water movement.

The aquifers in Uvalde County are recharged by precipitation within the county and in the drainage areas of streams entering the county from the north and west. In the Edwards Plateau, the Edwards and associated limestones are recharged by precipitation that falls on the outcrop. The aquifer is drained by springs at the contact with the underlying Glen Rose limestone where streams have cut through the Edwards at the edge of the plateau. These springs maintain the base flow of the streams that drain the plateau. Most of the base flow and much of flood flow of the streams is lost to the Edwards and associated limestones where they crop out in streambeds in the Balcones fault zone. This stream loss to the aquifer constitutes the greatest part of the recharge in the county, although some of the recharge from the West Nueces River enters the county as underflow from Kinney County. The normal annual recharge to the Edwards in Uvalde County is estimated to be about 200,000 acre-feet.

Discharge from the Edwards and associated limestones in Uvalde County can be divided into two .major segments: discharge by underflow from the county to the east and south and discharge to the surface through wells and springs. Discharge by underflow during the period 1934-47, when changes in storage were small, is estimated to have been about 190,000 acre-feet per year. During the drought years 1947-56 the underflow was somewhat less. The discharge to the surface during 1934-47 averaged about 17,000 acre-feet per year. During the 1947-56 drought the rate of discharge to the surface increased principally because of increased use of water for irrigation, reaching a maximum of 58,000 acre-feet in 1956.

The chemical quality. of the ground water in Uvalde County ranges between wide limits. Except in the extreme southern part of the county where the water is saline, the water in the Edwards and associated limestones is of good chemical quality except that it is hard. The water in the Glen Rose limestone is saline in many places; the principal objectionable constituents are high concentrations of calcium and magnesium sulfate. The water in the Leona formation is generally of good chemical quality. The water from the other formations varies widely in quality from place to place and no generalizations can be made.

Ground-water withdrawals from the Edwards and associated limestones in Uvalde County probably could be maintained indefinitely at a rate of about 200,000 acre-feet per year, provided that withdrawals north and west of the county were not increased. However, continued withdrawals at this rate-would cause wells in structurally high areas to go dry, and underflow into Medina County would cease. Furthermore, saline water might invade the fresh-water part of the aquifer from the south, and perennial spring flow in the Leona River valley would cease.