Karst Features of the Edwards Balcones Fault Zone aquifer

Main Barton Spring, Austin, Texas. The fourth largest spring in Texas, this is a karst spring that discharges an average of 50 cubic feet per second (about 32 million gallons per day). The spring supplies water to a swimming pool enjoyed by over 300,000 people per year. (Photo by Brad Garner)

Upper Barton Spring, Austin, Texas. Located in the creekbed of Barton Creek, this karst spring has been monitored by the USGS periodically. The results of those monitoring efforts have indicated that, despite being less than 1 kilometer from the much larger Main Barton Spring, its water has a substantially different major-ion and contaminant geochemistry. (Photo by Brad Garner)

Barton Spring Pool, Austin, Texas. A public swimming pool visited by over 300,000 people annually. The pool is filled by discharge from Main Barton Spring, and is a centerpiece of political and environmental dialog. (Photo courtesy of the City of Austin)

Main Barton Spring, with Barton Springs Pool drained. USGS Research Hydrologist Barbara Mahler makes water-quality measurements of Main Barton Spring. The spring normally discharges under the presssure of 12 feet of overlying water, but here that water had been lowered temporarily, to quantify the water-quality changes it causes. (Photo by Marcus Gary)