Geophysical well logs from over 150 oil test and water wells in Mississippi, Alabama, and Georgia were examined and calculations of the dissolved solids concentration in ground water were made using the spontaneous potential deflection as a measure of ionic activity. The values derived from these calculations were used to prepare a map showing the altitude relative to sea level at which the concentration of dissolved solids in the groundwater reached 10,000 mg/L within a regionally extensive aquifer in Mississippi, Alabama, and Georgia. A dissolved solids concentration of 10,000 mg/L is used to delineate the interface between moderately saline and saline water; 10,000 mg/L dissolved solids was designated to delineate the freshwater-saltwater interface. For a finite difference computer flow model being used this interface represents the location where horizontal flow of freshwater is greatly diminished. 10 ,000 mg/L also is the lower limit for dissolved solids concentrations for water in target zones for injection wells. This 10,000 mg/L dissolved solids line is of value as a water quality indicator; although water containing 3,000 to 10,000 mg/L or more of dissolved solids is too saline for agricultural use (upper limit approximately 3,000 mg/L), it may be useful for some industrial purposes. Waters containing dissolved solids concentrations > 10,000 mg/L have little potential for any use involving human activities. (Lantz-PTT)