FLOW PATHS IN THE EDWARDS AQUIFER, NORTHERN MEDINA AND NORTHEASTERN UVALDE COUNTIES, TEXAS, BASED ON HYDROLOGIC IDENTIFICATION AND GEOCHEMICAL CHARACTERIZATION AND SIMULATION

The U.S. Geological Survey, in cooperation with the San Antonio Water System, conducted a 4-year study during 2001–04 to identify major ground-water flow paths in the Edwards aquifer in northern Medina and northeastern Uvalde Counties, Texas. The study involved use of geologic structure, surface-water and ground-water data, and geochemistry to identify ground-water flow paths. Relay ramps and associated faulting in northern Medina County appear to channel ground-water flow along four distinct flow paths that move water toward the southwest. Statistical correlations between water-level departures for 11 continuously monitored wells provide additional evidence for the hypothesized flow paths. Of the 55 combinations of departure-dataset pairs, the stronger correlations (those greater than .6) are all among wells in the same flow path, with one exception. Simulations of compositional differences in water chemistry along a hypothesized flow path in the Edwards aquifer and between ground-water and surface-water systems near Medina Lake were developed using the geochemical model PHREEQC. The plausible models demonstrate that four hypothesized flow paths can be partially supported geochemically.