What is the Edwards Aquifer?
The Edwards Aquifer (a naturally occurring subsurface reservoir capable of storing and yielding significant quantities of water) is one of the most permeable and productive sources of potable groundwater in the world. This karst aquifer is named for the Edwards Group—a collection of relatively soluble limestone and dolomitic strata deposited in warm, shallow seas that dominated the central Texas landscape about 60-100 million years ago during the Cretaceous Period.
The Edwards Aquifer is recharged primarily through leaky streambeds atop the porous outcrop of the Edwards Group along the northern margin of Balcones Fault Zone (a complex system of geologic faults and fractures). Here, surface-water runoff from the Catchment area—together with incident precipitation—enters the Recharge zone of the aquifer through faults, fractures, sinkholes, and caves. The Edwards Aquifer is recharged also by lateral, subsurface inflow across the northern limits of the fault zone where the upgradient Trinity Group is juxtaposed against the down-faulted Edwards Group.
Recharge through streambed leakage and precipitation on the aquifer’s outcrop is estimated to have ranged from nearly 44,000 acre-feet (during 1956) to almost 2,500,000 acre-feet (1992) and to have averaged about 730,000 acre-feet per year since 1934. The amount of recharge occurring as lateral, subsurface inflow from the Trinity Aquifer is estimated to range from less than 25,000 to perhaps 64,000 acre-feet per year. Two-thirds of all Edwards Aquifer recharge occurs west of San Antonio. The remaining one-third enters through updip, unconfined parts of the aquifer in Bexar, Comal, and Hays counties. Reference: Blome, C.D., J.R. Faith, and G.B. Ozuna, 2007, Geohydrologic framework of the Edwards and Trinity aquifers, south-central Texas: U.S. Geological Survey Fact Sheet 2006-3145, 6 p, http://pubs.usgs.gov/fs/2006/3145/
The entire Edwards (Balcones Fault Zone) Aquifer extends along the narrow belt of Balcones Fault Zone from the north of Georgetown through Austin, San Marcos, New Braunfels, San Antonio, Hondo, Sabinal and Uvalde to Brackettville. This limestone aquifer is separated into three portions by groundwater divides at Kyle in Hays county and at the Colorado River. The central portion, from Kyle to Colorado River is referred to as the Barton Springs segment.
A major problem facing the Edwards Aquifer is the threat of overdrafting the average annual recharge and maintaining natural springflows. Accordingly, the Edwards Aquifer Authority (EAA) was authorized by the state legislature to develop, implement, and enforce a Critical Management Plan (CMP). The Cities of San Antonio, New Braunfels, and San Marcos have had to enforce Water Conservation Plan ordinances and water use disputes among aquifer users have become more frequent. In the most current case, the Sierra Club sued the Secretary of the Interior and the U.S. Fish and Wildlife Service (FWS) for failure to perform duties under the Endangered Species Act (ESA) and for injunctive relief. The U. S. District Court for the Western District of Texas reached a decision on January 30, 1993, and ordered FWS to determine required springflows, and the Texas Water Commission (TWC, now the Texas Natural Resource Conservation Commission or TNRCC) to prepare a plan assuring springflows will not drop below jeopardy levels. The court threatened additional orders if the State Legislature did not set up a regulatory system to limit withdrawals from the aquifer. In response, the 73rd Texas Legislature passed the Senate Bill No. 1477 to create the Edwards Aquifer Authority (EAA) and abolish the Edwards Underground Water District (EUWD), effective September 1, 1993. Governed by an appointed Board of Directors, the authority's primary function is to regulate the aquifer pumpage by limiting the long-term annual withdrawal at 400,000 ac-ft. The limiting amount is believed to be adequate to maintain the springflows at the Comal and the San Marcos, although the court opinion stated this level to be 200,000 ac-ft. Aquifer modeling on a monthly basis is a part of the effort to improve the understanding of the quantitative relationship among recharge, pumpage, springflow, and water levels. Such models will allow efficient and prudent management options to be explored without actually implementing the plan that would cause take or jeopardy of federally-listed species.
San Marcos Springs, the headwaters of the San Marcos River. Average flow 160 cubic feet/second or approximately 100 million gallons per day. 
The Edwards Aquifer is approximately 160 miles long measuring from Brackettville to Kyle and varies in width from 5 to 40 miles. It extends to cover the major part of five counties namely, Uvalde, Medina, Bexar, Comal and Hays. It traverses several streams in three major river basins including the Nueces, San Antonio and Guadalupe. The aquifer is a very unique carbonate aquifer located in south-central Texas. Karst characteristics of the Edwards Aquifer make it one of the most productive aquifers in the United States and yet groundwater flow within the aquifer is very complex and difficult to predict. The Edwards Aquifer is designated by the EPA as a "sole source" drinking water supply for the 1.7 million people of San Antonio and the Austin-San Antonio corridor. The aquifer is also vital to the agricultural and light industrial economy of the region. Springflows from the Comal and San Marcos Springs provide water for the tourist and recreation industry, Critical Habitat of several endangered species, appropriated water use downstream on the Gulf Coastal Plain, and the San Antonio Bay ecosystem.
EARDC, and Aquifer
