Edwards Balcones Fault Zone Aquifer

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)

Endangered Barton Springs Salamander. The Barton Springs Salamander, Eurycea sosorum, is a federally listed endangered species. It has been found only in and around the major springs of the Barton Springs segment of the Edwards aquifer. (Photo courtesy of Lisa O'Donnell, 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)

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)

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)

The Edwards aquifer is the most transmissive of all the aquifers in Texas and Oklahoma. Large discharges from springs and from flowing and pumped wells attest to the highly permeable nature of the aquifer. The area underlain by the Edwards aquifer is a combination of agricultural and ranch land and areas of dense population, including the cities of Austin in Travis County and San Antonio in Bexar County. It has been designated a sole source aquifer by the U.S. Environmental Protection Agency and is the primary source of water for San Antonio, America's eighth largest city. The aquifer is separated into three parts by a groundwater divide and the Colorado River.

Following deposition of Cretaceous rocks, tectonic movement caused the relative uplift of the Edwards Plateau and subsidence of the Gulf of Mexico, and a number of en echelon, northeastward-trending faults formed along the Balcones Fault Zone. The Edwards aquifer is generally coincident with the fault zone. The downdip boundary of the aquifer is largely fault controlled. As a result of the faulting, the chemical quality of the water in the Edwards aquifer can change abruptly in a very short distance across a zone often referred to as the "saline-water line."

As streams cross the Balcones Fault Zone, water percolates downward along the faults where permeability might be greatly enhanced by partial dissolution of limestone. Secondary sources of recharge are direct infiltration of precipitation that falls on aquifer outcrop areas, internal flow of groundwater from the Trinity aquifer where the Edwards and the Trinity aquifers are juxtaposed, and upward leakage from the underlying Trinity aquifer where an upward vertical head gradient exists. Direct recharge to the aquifer can be quite rapid through sinkholes. Water levels in wells completed in the Edwards aquifer rise immediately and springflows increase quickly after major recharge events, thus attesting to a dynamic flow system and the rapid movement of large volumes of water.