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This report is the latest attempt to accurately describe the location of the freshwater/saline-water interface from Kyle to Uvalde, Texas. Multiple sources were used to obtain high quality measured specific conductance and total dissolved solids values. All comparisons relating specific conductance to total dissolved solids have shown a high degree of correlation in every area near the interface. Total dissolved solids estimates derived from quantitative geophysical log recordings agree well with actual measurements taken in the same boreholes. The practical use of merged, measured, and calculated data is demonstrated along the interface from Kyle to Lytle. where abundant measured data is observed to be sufficiently distributed among map values generated from geophysical logs. As a result, the revised interface closely parallels former estimates. The changes are dictated by added control and, to a minor degree, by the contour options on the part of the various investigators.

The area from Lytle to Uvalde is lacking an abundant supply of TDS measurements. Therefore, reliance upon TDS estimates obtained from quantitative geophysical log interpretations is necessary. This is especially true in southern Medina County. To verify the major difference between the interface location presented in EUWD Report 92-03, EUWD South Medina Observation Well #I was drilled. This well proved that freshwater was at least three miles south of the commonly accepted freshwater/saline-water interface. This test well data and other additional geophysical logs were used to enhance the TDS and specific conductance maps furnished in EUWD Report 92-03. The TDS map shown in EUWD Report 93-06 has been merged with the TDS map shown in this report (Lytle to Uvalde) to provide the most current interpretation of the freshwater/ saline-water interface position from Kyle to Uvalde, Texas. Accuracy of the interface position west of Lytle is not as sound as that shown for the area east extending to Kyle, Texas. To more precisely determine the freshwater/saline-water interface location between Lytle and Uvalde, acquisition of measured well data, which may have been overlooked in the past, and drilling of additional observation wells will be required. The position of these new wells will be dependent upon the area where the most critical information is required.