In the San Antonio area, the Edwards aquifer is an extensively faulted, fractured, and cavernous limestone and dolomite of Early Cretaceous age lying within the Balcones fault zone, and consisting of rocks of the Edwards Group and its stratigraphic equivalents. The thickness of the Edwards aquifer ranges from 400 to 800 feet and increases toward the west and south. The porosity and permeability of the unconfined part of the aquifer result from the dissolution of limestone by circulating ground water that developed a cavernous network along vertical or inclined fractures. The large transmissivity of the confined part of the aquifer is indicated by the hundreds of large-capacity wells, small hydraulic gradients, similarity of water levels among widely-spaced wells, and large spring discharges. Estimated transmissivity throughout most of the confined freshwater aquifer ranges from 430,000 to 2,200,000 feet squared per day; transmissivity of the unconfined aquifer generally is less than 430,000 feet squared per day. The total volume of freshwater in the Edwards aquifer is about 45 million acre-feet. Long-term hydrographs showed no net decline in ground-water levels during the 76-year period from 1911 to 1987; thus, there was no net loss of water from storage in the freshwater zone of the Edwards aquifer during that long-term period. However, short-term changes in water levels largely resulted from the variability of precipitation. This is readily perceived by the severe declines during the drought of the late 1940's to middle 1950's, and by the rises to record highs during the abnormally wet years in the 1970's and 1980's. The average annual recharge of 635,500 acre-feet is about equal to the sum of the average annual springflow of 359,500 acre-feet and annual average pumpage of 273,000 acre-feet for 1934-88.
Additional Publication Details
USGS Numbered Series
Geology and hydrology of the Edwards Aquifer in the San Antonio area, Texas
Water-Resources Investigations Report
U.S. Geological Survey ;
Earth Science Information Center, Open-File Reports Section [distributor],
v, 64 p. :ill., maps (some col.) ;28 cm. [PGS - 69 p.]