CHARACTERIZATION OF KARST AQUIFERS USING INTEGRATED HYDROLOGICAL, GEOCHEMICAL, AND GEOPHYSICAL TECHNIQUES TO ADDRESS WATER-SUSTAINABILITY ISSUES IN SOUTH-CENTRAL TEXAS

The San Antonio segment of the Edwards aquifer is one of the most productive karst aquifers in the United States and the primary source of water for the city of San Antonio and numerous smaller municipalities in south-central Texas. The aquifer also supplies water to downstream users in the form of streamflow issuing from springs which support several threatened and endangered species. Numerous historical studies have been done primarily to characterize the hydrogeology of the aquifer. More recent studies have been designed using an integrated hydrological, geochemical, and geophysical approach to synthesize pre-existing and newly collected data with regards to both seasonal and event-related variability, including a study to evaluate the source of water to Comal Springs and San Marcos Springs, the two largest springs in Texas and the main discharge points for the San Antonio segment of the Edwards aquifer. Statistical correlation of water-level data and recession curve analysis were used to help determine flow regimes and connectivity of springs. Geochemical modeling provided insight into interactions between surface water and groundwater so that possible sources for spring flow could be traced. In another example of the use of integrated techniques, geochemical, hydrological, and geophysical techniques were used to document the physiochemical properties and hydraulics of flow in and near the freshwater/saline-water interface of the Edwards aquifer to assess the potential for movement of saline water into the freshwater zone of the aquifer. Geochemical techniques included statistical analysis of long-term major ion data and analysis of environmental isotopes and helium data. Continuous water-level data were used to calculate equivalent freshwater heads and lateral head gradients. Borehole geophysical logs were analyzed for changes in fluid conductivity and temperature associated with vertical flow in the boreholes. Integrated studies such as these allow for a more complete understanding of the aquifer and greater effectiveness in managing water resources and addressing water sustainability issues in south-central Texas, which is rapidly-growing and prone to droughts.