GROUNDWATER FLOW VELOCITIES FOR THE DEEP ARTESIAN PORTION OF THE EDWARDS AQUIFER, NEAR COMAL SPRINGS, TEXAS

Two tracer tests assessed groundwater flow velocities and flowpath relationships between the deep artesian and recharge zones of the Edwards Aquifer at Comal Springs, Texas. Results of the tracer tests will help calibrate flow models of the Edwards, design an improved ambient water quality monitoring program, and improve response to hazardous material spills. Comal Springs, the largest in the southwestern United States with an average discharge of 8.5 m3/s, emerge from hundreds of individual locations and form the 1.2 km long Landa Lake in New Braunfels, Texas. The Comal Springs “complex” is located on the 250 m displacement Comal Springs Fault, which juxtaposes the recharge and deep artesian zones of the Edwards Aquifer.

Over 80 participants, representing four countries and more than 15 public and private entities monitored thirty surface sites and three municipal wells, and collected and analyzed over 1,600 water samples. A trace from the 200 m deep flowing artesian well (with a head about 15 feet above ground level) yielded flow velocities of 290 m/d to springs 350 – 750 m away, and demonstrated conduit flow in the deep artesian zone. The dye emerged from rise points in a 400 m stretch of the middle of Landa Lake as described by Martel’s (1896) “underground delta” model. Dye was also detected in a municipal well 300 m from the dye injection point. A second dye, simultaneously injected into a 25 m deep well on the up thrown side of the fault, moved at 1400 m/d to a separate set of rise points at the springs.