USGS Publications Warehouse

Abstract

A digital model of two-dimensional groundwater flow was used to estimate the hydraulic properties of the Edwards Aquifer in a 151 sq mi area near Austin, Texas. The transmissivity, hydraulic conductivity, and specific yield were estimated for the part of the aquifer that discharges at Barton Springs in Austin. The aquifer is composed of the Edwards and overlying Georgetown Limestones of Cretaceous age and ranges in thickness from about 100 ft to about 450 ft. More than 60 years of discharge measurements and 5 years of gaged discharge for Barton Springs were used to adjust springflow for the simulations. Barton Springs accounts for about 96% of springflow from the study area and 90% of the total discharge. The remaining discharge was pumpage from wells which was entered in the model. Four years of gaged recharge were used in the simulations. The potentiometric surfaces used by the models were constructed from water level measurements in as many as 75 wells. The transmissivity was calibrated through steady-state simulations that used the mean value of recharge and mean potentiometric surface to represent average conditions for the aquifer. The transmissivities vary from about 100 sq ft/day in the western part of the aquifer to > 1 million sq ft/day near Barton Springs. Specific yield was calibrated through transient-state simulations for 5 consecutive months using time-dependent data for recharge, discharge, and water levels. The mean specific yield for the aquifer is 0.014 and ranges from 0.008 to 0.064. Additional aquifer properties used in the simulations include storage coefficient, altitudes of the base and top of the aquifer, and hydraulic conductivity. A simulation for the year 2000 using projected pumping rates for municipal, industrial, agricultural, and domestic supplies indicates that the aquifer would be dewatered in the southwestern part of the study area and have large declines in the southeastern part of the study area. (Author 's abstract)