Geology and Hydrology of the Edwards Aquifer in the San Antonio Area, Texas

The Edwards aquifer, which is the sole source of water for the city of San Antonio, is one of the most permeable and productive carbon- ate aquifers in the United States. The aquifer is composed of extensively faulted, fractured, and cavernous limestone and dolomite of Early Cretaceous age lying within the Balcones fault zone a seriesofnormalenechelonstrikefaults that separate the Edwards Plateau from the Gulf Coastal Plain in south Texas. Along segments of some faults, the entire thickness of the aquifer is displaced vertically, and these faults then act as barriers to downdip ground-water flow. The large porosity and exceptional perme- ability of the unconfined part of the Edwards aqui- fer result from the dissolution of limestone by circulating ground water and development of a cavernous network along fractures. The large porosity and permeability of the freshwater part of the confined Edwards aquifer result primarily from dedolomitization. The small permeability of the saline-water part of the confined aquifer is caused by the limited interconnection between the pores in the rock matrix and by the lack of substan- tial dissolution along fractures. The large transmissivity of the Edwards aquifer is indicated by the hundreds of high- yielding wells, small hydraulic gradients, and large spring discharges. The determined transmis- sivity throughout most of the confined freshwater aquifer ranges from 430,000 to 2,200,000 feet squared per day; the determined transmissivity of the unconfined aquifer generally is less than 430,000 feet squared per day. Faulting causes the aquifer to be highly anisotropic, and simula- tion indicates anisotropy ratios ranging from 0.0:1 to 1:1. The ground-water-flow system of the Edwards aquifer includes several components. These include a catchment area on the Edwards Plateau where the unconfined aquifer receives direct recharge, an area of confining beds crossed by streams draining the Edwards Plateau, a major recharge area within the Balcones fault zone where streams lose flow dire