USGS Publications Warehouse

Abstract

Urban development over the Edwards aquifer in the Austin, Texas, area has caused concerns about the availability and quality of water in the aquifer. The study area, the Edwards aquifer that discharges to Barton Springs, includes parts of Travis and Hays Counties and extends from the city of Kyle to the Colorado River. The aquifer is a karst system composed of limestone and dolomite of Cretaceous age. The water occurs in solution channels in the Edwards and Georgetown Limestones. Yields of adjacent wells often differ by as much as four orders of magnitude. Storage within the aquifer is about 306,000 acre-ft, of which about 31,000 acre-ft is within the ' transient ' part of storage--the change in volume occurring between high flow and the lowest known flow of Barton Springs. The average specific yield of the aquifer is 0.017. Three geologic sections are presented in the report, as well as maps showing the altitudes of the base and the top of the Edwards aquifer. Recharge occurs predominantly along faults and fractures crossing six creeks in the recharge area, which covers the western-most 90 sq mi of the aquifer. Leakage probably occurs into the Edwards aquifer from the underlying upper Trinity aquifer. Water budget analyses show that surface recharge and groundwater discharge (springflow and pumpage) are reasonably balanced, suggesting that the groundwater system is in dynamic equilibrium. Based on 65 yr of measurements, Barton Springs has a long-term mean discharge of 50 cu ft/sec and a minimum and maximum discharge of 10 and 166 cu ft/sec. As of 1982, the estimated total groundwater pumpage of about 3,800 acre-ft/yr represented just over 10% of the average annual discharge of 36,000 acre-ft to Barton Springs. Increased pumpage associated with future groundwater development could reduce the discharge at Barton Springs and reduce groundwater availability. Water quality from Barton Springs and the wells is better than the creeks providing surface recharge, which have fecal bacteria values as high as 100,000 colonies/100 ml. Significant densities of fecal bacterial have been found in water from Barton Springs. (Author 's abstract)