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: Kinney, Uvalde, Medina, Bexar, Comal, and Hays Counties

E. W. Collins
D. Hovorka. S.


Normal faults of the regional Balcones Fault Zone are the principal structural control on the Edwards limestone aquifer and recharge zone, currently the sole source aquifer o f San Antonio, Texas. The San Antonio segment of the aquifer, extending over an area of -3,000 mi2 in Kinney, Uvalde, Medina, Bexar, Comal, and Hays Counties (fig. 1 and map), is the main water resource for residential, agricultural, and industrial use in Bexar, Medina, and Uvalde Counties. Discharge from the aquifer in Comal and Hays Counties feeds springs that are tourist attractions and that have been shown to be critical habitats of endangered species. The map of the aquifer depicts the subsurface structure of the base of the Del Rio Formation (approximate top o f the confined part o f the aqui- fer), the Edwards Group outcrop belt (aquifer recharge area and unconfined part ofthe aquifer), faults, large relay ramps, and the approximate interface between fresh and saline water. This report summarizes key elements of the aquifer's structural framework and describes structural attri- butes that affect aquifer recharge, ground-water flow, areal extent, and depth (Hovorka and others, 1995). Faults and large relay ramps control the structural position ofthe aquifer strata. Some large faults may act as barriers or partial barriers to ground-water flow. Smaller faults and associated joints form local and regional ground-water conduits. Because relay ramps are areas o f greater stratal continuity and numerous smaller displace- ment faults, they can be preferential pathways of ground-water flow.