Structure Map of the San Antonio Segment of the Edwards Aquifer and Balcones Fault Zone, South-Central Texas: Structural Framework of a Major Limestone Aquifer

Collins, E. W., and Hovorka, S. D.

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Normal faults of the Balcones Fault Zone are the principal structural control on the important Edwards limestone aquifer and recharge zone, currently the sole source aquifer of San Antonio, Texas, and many other municipalities in Central Texas. The San Antonio segment of the aquifer, extending over an area of approximately 3,000 mi2 in Kinney, Uvalde, Medina, Bexar, Comal, and Hays Counties, is the main water resource for residential, agricultural, and industrial use in Bexar, Medina, and Uvalde Counties. The map of the aquifer depicts the subsurface structure of the base of the Del Rio Formation (approximate top of the confined part of the aquifer), the Edwards Group outcrop belt (aquifer recharge area and unconfined part of the aquifer), faults, large relay ramps, and the approximate interface between fresh and saline water. The map builds on many previous studies of the structure and stratigraphy of the Edwards aquifer. The authors used an updated subsurface data base and recent comprehensive surface geologic mapping conducted by the senior author, however, to illustrate the structure at a scale that provides more detail than that of any map previously available. The authors are thus able to make new interpretations on fault occurrence, throw, and geometry. The accompanying booklet summarizes key elements of the Edwards aquifer's structural framework and describes structural attributes that affect aquifer recharge, ground-water flow, areal extent, and depth.