STRUCTURAL INFLUENCE ON GROUNDWATER FLOW AS EVIDENCED BY GROUNDWATER DYE TRACING IN THE BARTON SPRINGS SEGMENT OF THE EDWARDS AQUIFER, CENTRAL TEXAS: IMPLICATIONS FOR MODELING CONDUITS

Major groundwater flow routes in the Barton Springs segment of the Edwards Aquifer closely follow the structural trends of the Balcones Fault Zone. Locations of these groundwater flow routes and velocities of groundwater flowing along the routes were determined by injecting fluorescent, non-toxic, organic dyes (Fluorescein and Eosine) into two caves within Onion Creek, the largest capacity losing stream in the Barton Springs segment of the Edwards Aquifer. Cripple Crawfish and Antioch caves are located about 17.5 mi and 14.0 mi south of Barton Springs, the primary discharge point from the aquifer, respectively. First arrival of dyes from each of these caves to Barton Springs occurred after about 3.5 and 7.0 days, indicating minimum groundwater velocities under high spring flow and water-level conditions of 5.0 and 2.0 mi/day, respectively. Dye detections at water-supply wells indicate a karst system composed of multiple diverging flow routes from these caves, which appear to re-converge as groundwater flows northeast discharging at Barton Springs. Interpreted flow paths are oriented normal or parallel to potentiometric contours, indicating a highly anisotropic flow system consisting of conduits. Conduit flow within the aquifer appears to be strongly influenced by the bimodal fault and fracture system with trends of N40E (dominant) and N45W (secondary). Groundwater flow was traced in wells along paths that are parallel to the dominant fault trend presented on geologic maps. Groundwater flow was also traced parallel to the secondary structural trend, at a high angle to the dominant mapped fault trend. Results indicate that conduits are an important component of flow, and that the bimodal structural grain has influenced conduit flow in this aquifer. Future numerical groundwater models incorporating conduit flow should consider, among other data, structural grain for modeling the distribution and orientation of conduits.