HYDROGEOLOGY AND GEOCHEMISTRY OF THE FRESHWATER/SALINE-WATER TRANSITION ZONE OF THE EDWARDS AQUIFER, SOUTH-CENTRAL TEXAS

The Edwards aquifer is a carbonate karst aquifer and is the primary water supply for nearly two million people. The transition zone is defined as the region in the aquifer containing water with dissolved-solids concentrations ranging from 1,000 to 10,000 milligrams per liter. Dissolved-solids concentrations gradually increase from the freshwater zone to the saline zone in the western part of the aquifer. Farther to the east, the change from freshwater to saline water is more abrupt, in part the result of faulting along the Balcones and Luling fault zones. Water-level data from monitor wells in transects through the transition zone show a similar response over time to changes in hydrologic conditions. This similarity in response indicates that the freshwater zone and the transition zone of the Edwards aquifer are closely interconnected and operate as a single hydrogeologic unit. A comparision of water levels from the transition zone to water levels from the freshwater zone and to springflow from Comal Springs, a major freshwater discharge point of the Edwards aquifer, also show strong relations over time. Borehole geophysical techniques were used to determine the spatial and temporal distribution of salinity in the transition zone and to quantify vertical flow within a borehole. Results from the fluid profiling and water-quality sampling indicate that in some areas the ground water in the Edwards aquifer is stratified and has a uniform temperature gradient, which indicates active flow in both the freshwater and the transition zones. Some vertical stratification of salinity occurs in the eastern transects; indicated flow between wells probably is controlled by structure to some degree. The westernmost transect has a progressive, more gradational increase in salinity and temperature down dip of the freshwater/saline-water interface with less structural influence. Isotopes indicate that a primary source of recharge to the transition zone is meteoric. Helium-4 concentrations increase with salinity and with depth. Methane concentrations vary widely. Noble gas and tritium data indicate that much of the water in the transition zone was recharged more than 60 years ago.