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Note:  Large-format versions of the plates for this report are available in a companion document.

Over one hundred geophysical logs from locations near the freshwater/saline-water interface of the Edwards aquifer between Uvalde and San Antonio, Texas, were acquired and analyzed. In conjunction with these logs, test data from several wells along the interface in San Antonio and to the northeast were used to verify a high correlation between estimated and measured water quality parameters. Geochemical conditions such as specific conductance and dissolved solids in the freshwater/saline-water transition zone in the Edwards aquifer can be interpreted from geophysical logs.

Results of the estimated water quality data determined in this study indicate that the freshwater/saline-water interface is irregular, both vertically and horizontally, and extends downdip into Frio County in the area of the Frio-Zavala County line, contrary to the location previously estimated. Concurrently, a similar condition exists west of Devine, Texas, where the estimated position of the freshwater/saline-water interface is again downdip and farther south than has been shown in earlier studies.

Data such as rock color, information indicating lost circulation and/or caverns, and reports of “freshwater” on well tests supplements geophysical log-derived water quality parameter maps in supporting the concept of the occurrence of freshwater between the previous freshwater/saline-water interface and the position of the estimated freshwater/saline-water interface determined by this report.