TRACER TESTS IN THE EDWARDS AQUIFER RECHARGE ZONE

The Edwards Aquifer Authority conducted a series of tracer tests in the Edwards Aquifer Recharge Zone in northern Bexar County in the vicinity of San Antonio, Texas. The purpose of the tests was to measure the groundwater velocity, investigate groundwater flowpaths, and evaluate the hydraulic connectivity between the Edwards and the Trinity aquifers in the Recharge Zone beneath the Panther Springs Creek basin. The tests consisted of injecting invisible, harmless fluorescent dyes through caves into the aquifer and then collecting groundwater samples from wells in the area to determine dye arrival. The monitoring system consisted of more than 20 public and private wells completed in either the Edwards or the Trinity aquifers.

The recharge zone is defined by the outcrop of the Edwards Limestone, which is underlain by the Trinity Aquifer; both are karstified Cretaceous marine limestone formations. The area is located in the Balcones Fault Zone, which has faulted the Panther Springs Creek basin into several northeast-trending fault blocks. In addition, units of the Edwards Limestone are juxtaposed against the uppermost units of the Trinity Aquifer (Upper and Lower Glen Rose limestones) due to displacements up to 350 ft on the faults.

The results of the tracer tests revealed discrete groundwater flowpaths beneath Panther Springs Creek. Dyes were detected primarily in a single monitoring well (68-28-608), which is completed in the Edwards Aquifer. Dye was also detected in two of the Edwards Aquifer wells at the Club at Sonterra. Apparent groundwater velocities ranged from 3,000 to 12,000 feet per day. The results demonstrate the high groundwater velocities that are characteristic of karst aquifers.

The results also indicate that groundwater flows freely between the upper member of the Glen Rose Formation, which is the uppermost layer in the Trinity Aquifer, and the Edwards Aquifer. Dyes traveling along the flowpaths between the caves and Well 68-28-608 crossed several northeast-southwest trending faults in which members of the Edwards and Glen Rose formations are juxtaposed.