Karst Aquifers: Edwards Balcones Fault Zone Aquifer

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.

Springs

In-Stream Sinkholes

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Featured Studies and Datasets

Aquifer-scale studies and the datasets they produce are a key component to understanding how karst aquifers behave, and the quality of water within them.

• Barton Springs Aquifer Groundwater Quality — Annual summertime sampling of about a dozen groundwater wells, during non-stormflow conditions, for a variety of constituents.
• Barton Springs Characterization — A 2-year water-quality study of the 4th largest spring in Texas, which produced an interpretive report.
• National Water-Quality Assessment, South-Central Texas — A program to describe the status and trends in water quality of a large, representative part of the Nation's surface-water and groundwater resources.

Additional Information

The following websites are additional sources of information about this aquifer:

• The Edwards Aquifer Website (by Gregg Eckhardt)
• Edwards Aquifer Authority
• Barton Springs Edwards Aquifer Conservation District
• San Marcos Salamander Photos - U.S. Fish and Wildlife Service National Digital Library
• Barton Springs Salamander Photos - U.S. Fish and Wildlife Service National Digital Library