Geophysical Delineation of the Freshwater/Saline-Water Transition Zone In the Barton Springs Segment of the Edwards Aquifer, Travis and Hays Counties, Texas, September 2006

The U.S. Geological Survey, in cooperation with the Barton Springs/Edwards Aquifer Conservation District, conducted a geophysical pilot study to determine whether time-domain electromagnetic (TDEM) sounding could be used to delineate the freshwater/saline-water transition zone in the Barton Springs segment of the Edwards aquifer in Travis and Hays Counties, Texas. There was uncertainty regarding the application of TDEM sounding for this purpose because of the depth of the aquifer (200–500 feet) and the relatively low-resistivity clayey upper confining unit. Twenty-five TDEM soundings were made along four 2–3-mile-long profiles in a study area overlying the transition zone near the Travis-Hays County boundary. The soundings yield measurements of subsurface electrical resistivity, the variations in which were correlated with hydrogeologic units, and then with dissolved solids concentrations in the aquifer. Geonics Protem 47 and 57 systems were used to collect the TDEM soundings. A smooth model (vertical delineation of calculated apparent resistivity that represents an estimate of the true resistivity) for each sounding site was created using an iterative software program for inverse modeling. The effectiveness of using TDEM soundings to delineate the transition zone was indicated by comparing the distribution of resistivity in the aquifer with the distribution of dissolved solids concentrations in the aquifer along the profiles. In general, the Edwards aquifer in the study area is characterized by a sharp change in resistivity from west to east. The higher resistivity regions correspond to lower dissolved solids concentrations (freshwater), and the lower resistivity regions correspond to higher dissolved solids concentrations (saline water). On the basis of reasonably close matches between the inferred locations of the freshwater/saline-water transition zone in the Edwards aquifer in the study area from resistivities and from dissolved solids concentrations in three of the four profiles, TDEM sounding appears to be a suitable tool for delineating the transition zone.