Structure Map of the San Antonio Segment of the Edwards Aquifer

EW Collins
SD Hovorka

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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 of 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 inBexar, 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 endangeredspecies. 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. This report summarizes key elements of the aquifer's structural framework and describes structural attributes that affect aquifer recharge, ground-water flow, areal extent, and depth (Hovorka and others, 1995). Faults and large relay ramps control the structural position of the 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 of greater stratal continuity and numerous smaller displacement faults, they can be preferential pathways of ground-water flow.