Edwards Aquifer

Edwards Aquifer
Texas
Downstream from the headwaters of the San Marcos River
Country	United States
State	Texas
Region	Texas Hill Country
District	Edwards Plateau
Recharge zone	3,237 km2 (1,250 sq mi)
Geology	Limestone karst
Management	Edwards Aquifer Authority
Website: Edwards Aquifer Website

The Edwards Aquifer is one of the most prolific artesian aquifers in the world. Located on the eastern edge of the Edwards Plateau in the U.S. state of Texas, it is the source of drinking water for two million people, and is the primary water supply for agriculture and industry in the aquifer’s region. In addition, the Edwards Aquifer feeds the Comal and San Marcos springs, provides springflow for recreational and downstream uses in the Nueces, San Antonio, Guadalupe, and San Marcos river basins, and is home to several unique and endangered species.

Although between 25 and 55 million acre-feet of water may be present in the Edwards aquifer, only a small portion of this water is practically or legally available for use. Storage is the difference between recharge (inputs) and discharge (outputs) from the Edwards Aquifer.

Annual storage can be negative during dry years with high water use and positive during wet years with relatively low water use. A long-term negative imbalance between recharge and discharge in an aquifer may lead to the depletion of the available water in the aquifer.

Annual storage between 1955 and 2012 estimated from data provided by a continuing program between the U.S. Geologic Survey and the Edwards Aquifer Authority ranged from -633,000 to +1,653,000 acre-feet per year. The average storage during this period was 37,000 acre-feet per year.

Water enters the Edwards Aquifer in two ways: it either falls as precipitation and percolates directly into the aquifer, or it enters as streamflow flowing through the Recharge Zone. The Recharge Zone occurs along the Balcones Fault Zone where the Edwards Plateau drops steeply and meets the Gulf Coastal Plain. Here, highly fractured limestones are exposed at the Earth's surface, which allow rain and streamflow to infiltrate directly into the aquifer.

The Contributing Zone, which occurs on 5,400 mi2 (14,000 km2) of the Edwards Plateau (Texas Hill Country), collects precipitation and streamflow that drain to the Recharge Zone. Major streams draining the Contributing Zone include Cibolo Creek, Helotes Creek, Barton Creek, and Onion Creek. Water is unable to percolate into the aquifer in the Contributing Zone because much of the underlying geology is impermeable.