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Edwards Aquifer History

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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.

Edwards Plateau Map
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.

Edwards Aquifer map2

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.

Ariel View of Spring Lake

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.

Past, Present and Future

Early Europeans first settled along perennial streams sustained by natural spring flows from the Edwards Aquifer (Maclay and Land, 1988). Substantial well discharge from the aquifer began in the late 1800's and and steadily increased from 101,900 ac-ft in 1934 to a record high of 542,400 ac-ft in 1989. The groundwater is used extensively for public water supply and agriculture, and accounted for, respectively, 56.6% and 30.1% of the total well discharge during 1981-91.

Water Use Pie chart
The continual increase in well discharge has had an effect on the natural spring discharge, which comprises of 42.2% (1981-90) of the total withdrawal from the Edwards Aquifer. San Pedro and San Antonio Springs in San Antonio have become intermittent. Historically, the Comal Springs stopped flowing for two months in the summer of 1956.

In 1984 and 1990, some of the higher Comal Springs ceased to flow and water levels in the index well dropped to within twelve feet of the 1956 record.

Annual Pump Age Graph

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).

Waterlevel Graph

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 Commision 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.

Annual Recharge to Edwards Aquifer

Hydrology

The chart below shows the stratigraphic units of a typical Edwards Aquifer cross section. The aquifer is an association of Early Cretaceous age limestones overlying the Glen Rose Limestone and underlying the Del Rio Clay. The base of the aquifer is confined by the upper part of the Glen Rose Formation and in the artesian section, the top of the aquifer is confined by the Del Rio Formation. The lateral boundaries of the Edwards Aquifer consist of groundwater divides on the east and west near Kyle in Hays County and near Brackettville in Kinney County, respectively. The aquifer is bounded on the south by the "bad water line", (line marking water with more than 1000 mg/l of total dissolved solids), and on the north by the northern most edge of the Balcones Fault Zone.

Edwards Aquifer Cross Section

The Edwards Group and associated limestones were deposited as shallow marine platform carbonates consisting of reef and deep marine lithofacies in the western portion of the aquifer region. Most of the Edwards Group and associated limestones within the San Marcos Platform and the Devils River Reef Trend were subjected to subaerial exposure after deposition which enhanced the development of secondary porosity (Maclay and Land, 1988). Enhanced porosity has also occurred along the high angle faults and fractures of the Balcones Fault Zone.

Campers Tubing

The Edwards Aquifer receives water primarily from streams and rivers originating from the catchment areas on the Edwards Plateau. Except for the Guadalupe River, all streams and rivers that cross the outcrop of the Edwards Aquifer lose major portions of their flow to the aquifer through joints, faults, and sink holes. There are three river basins that cross the aquifer area: the Nueces, the San Antonio, and the Guadalupe River. Extending from the west, the Nueces River Basin covers over a half of the aquifer area. Several major tributaries in the basin traverse the aquifer recharge zone including the Nueces; the West Nueces; the Frio; the Dry Frio; the Sabinal Rivers; the Seco; and the Hondo Creeks. The portion of the San Antonio River Basin that is located in the recharge zone extends from the Medina River to the Cibolo Creek and includes headwaters of the Helotes, Leon, and Salado Creeks. Only a small portion of the Guadalupe River Basin intersects the eastern aquifer area. However, two of the basin tributaries, the Comal and San Marcos Rivers, are primarily fed by the aquifer at the Comal and San Marcos Springs.

Spring Lake Ariel Map

The tributaries crossing the recharge area include the Guadalupe River, the Blanco River, and headwaters of the Comal and San Marcos Rivers. Small creeks in the headwaters are the Dry Comal, Alligator, York, Purgatory, and Sink Creeks.

Based on the USGS estimates, the aquifer has an average annual recharge of 651,700 ac-ft with about 58.5% contributed by the Nueces River Basin. Generally, the water flows south-southeastward from the recharge zone under steep hydraulic gradients and low permeabilities within the unconfined portion of the aquifer. As the water flows into the confined portion of the aquifer, the flow direction changes toward the east and northeast within the low gradient, highly permeable grabens in the artesian zone. The water then discharges from several springs -- mostly the Comal and San Marcos Springs -- which account for 355,500 ac-ft annually. The two major springs constribute about 25% of flow in the Guadalupe River downstream. The contribution was about 66% during the drought year of 1956.

An excellent reference entitled the, Balcones Escarpment contains papers by EARDC staff Longley and Ogden that provide interesting information regarding the aquifer. This reference also has several other illustrated papers about the aquifer.