HYDROGEOLOGIC AND GEOCHEMICAL IDENTIFICATION OF GROUND-WATER FLOW PATHS IN THE EDWARDS AQUIFER, NORTHEASTERN UVALDE AND NORTHERN MEDINA COUNTY, TEXAS

The Edwards aquifer is the major source of water for more than 1.5 million people in the San Antonio area and provides nearly all of the water used for industrial, military, irrigation, and public supplies. The Edwards aquifer is composed of several geologic formations consisting primarily of limestone and dolostone, depending on location and depositional province. Most of the recharge to the San Antonio segment of the Edwards aquifer occurs in Medina and Uvalde Counties from direct infiltration of precipitation and streamflow loss in the recharge zone. The Balcones fault zone is characterized by an en echelon network of mostly down-to-the-southeast normal faults where the Cretaceous strata is displaced vertically, fractured intensively, and rotated differentially within a series of southwest-to-northeast trending fault blocks. These faults are responsible for the complexity of study area's ground-water-flow system. Ground-water flow paths within the Edwards aquifer recharge zone are complicated by geology, structure and influences of surface water recharge. Ground-water flow paths in the Edwards aquifer recharge zone of Medina County are, generally to the southwest parallel to Balcones faults. Ground-water flow paths in northeastern Uvalde County are to the southeast. These oppositely moving flow path coalesce in an area locally known as the Knippa Gap, where they generally turn to the south and then east.

Extension within the Balcones fault zone is a major controlling factor in ground-water flow paths within the study area. Primary extension is down to the coast; however secondary extension parallel to the Balconies fault zone has resulted in recharge along fractures which are perpendicular to the major faults. Results from this study indicates five major flow paths within the study area; four of these flow paths move from the northeast to the southwest parallel to the Balcones fault zone. The flow paths are from north to south; the Northwestern Medina flow path, the Northcentral Medina flow path (northern portion), the Northcentral Medina flow path (southern portion), the Southcentral Medina flow path and to the west the Northeastern Uvalde flow path.