Instruct: Given a question, retrieve relevant documents that answer the question.
Query: How large is Edward's Aquifer in Texas?
Located in South Central Texas, the Edwards Aquifer incorporates an area of approximately 4,350 square miles that extends into parts of 11 counties.[13] The aquifer's boundaries begin at the groundwater divide in Kinney County, East of Brackettville, and extend Eastward through the San Antonio area and then Northeast where the aquifer boundary ends at the Leon River in Bell County.[3] The aquifer is hydrologically separated into the Austin and San Antonio regions by a groundwater divide near the town of Kyle in Hays County.[13]
Scientists with the United States Geologic Survey have developed numerical groundwater flow models for the San Antonio and Barton Springs aquifer segments to determine the amount of water in the aquifer, the direction it is flowing, and its velocity. These are used to estimate the sustainable levels of groundwater withdrawal throughout the aquifer.[10] Given ample data is needed for numerical simulations, yet often lacking,[11] regional modeling of large aquifers is difficult, but modeling segments within an aquifer is common and provides useful information for water users throughout the aquifer.
More than 1.7 million people rely on water from the Edwards Aquifer for municipal, industrial and daily use.[46] One of the major cities on the aquifer is San Antonio, America's 7th largest city, with a population of over 1 million. San Antonio is fully dependent on the Edwards Aquifer for their municipal, industrial and daily use.[44] Another major city on the aquifer is Austin. More than 50,000 people in the city of Austin (6% of Austin's population) rely on the Barton Springs segment of the Edwards Aquifer.[46]
The Comal Springs are fed by the Edwards Aquifer, a large karst aquifer that runs through most of central Texas. The aquifer consists of porous, water-bearing limestone features which channel rainfall and surface runoff from the aquifer's recharge zone down to various discharge zones, including the springs.
Aquifers are basins that yield a usable amount of groundwater. An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer in central Texas. This carbonate aquifer has historically been providing high quality water for nearly 2 million people, and even today, is completely full because of tremendous recharge from a number of area streams, rivers and lakes. The primary risk to this resource is human development over the recharge areas.
Edwards Aquifer Authority regulates withdrawal permits, transfers, and groundwater conservation plans under authority granted by the Texas legislature. Groundwater law in the state of Texas is governed by the Rule of Capture, which gives landowners the right to pump groundwater beneath their land, with the exception of drilling a lateral well extending under a neighbor’s property, wasting water, or pumping with the intention of causing harm to a neighbor’s well.[67] In order to construct a well to withdraw water from the Edwards Aquifer, however, a user requires a permit that is granted by the Edwards Aquifer Authority. Permits for existing users are determined by maximum historical use, taking into consideration the overall availability of water in the aquifer.[68]
Due to Sierra Club v. Babbitt the Edwards Aquifer Act 1993 was passed which created the Edwards Aquifer Authority to oversee pumping regulations.[77] In the case Barshop v. Medina Under. Wat. Cons. Dist. 1996, Medina County Under Water Conservation District challenged the Edwards Aquifer Authority over the constitutionality of the Edwards Aquifer Act.[78] The challenge was over whether property owners have the constitutional right to pump water from their land, or whether concerns for water conservation and endangered species take precedence. Barshop v. Medina Under. Wat. Cons. Dist. went to the Texas Supreme Court where the Edwards Aquifer Act was upheld.[73][78]
The Edwards Aquifer underlies 38 counties in South and Western Texas.[47] West Texas is regionally defined by jobs in the oil and gas industries, but is also home to mining support, agriculture, and transportation support, among other sectors.[48] South Texas is regionally defined by recent economic growth in shipping industries, irrigation based farming, and manufacturing.[49] According to the Texas Comptroller and Texas Water Development Board, the Southern region's economic growth and irrigation practices have put pressure on water demands that exceed supply, and this is expected to increase with economic and demographic trends between 2010 and 2060.[49]
In the 1950s, however, Texas experienced the worst drought on record.[63] Legislature for the Edwards aquifer began in 1959 with the creation of the Edwards Underground Water District, which created and supplied maps and worked with licensing departments for development interests. Starting in the 1970s, the Texas Water Quality Board (TWQB) first recognized the aquifer and issued regulations regarding surface recharge zones. Following these first steps, regulations began to include the need for geologic assessments prior to development, design standards for underground storage tanks and pipes, and fees for development.[64]
North and west of the faults are the Stockton, Edwards, and Comanche plateaux; these define a crustal block that was upthrown during the Neogene. This large region of central Texas, which extends from Brewster County east to Bexar, and northeast to the Red River features extensive Cretaceous shale and limestone outcrops. The limestone in particular is important, both economically for its use in cement manufacture and as a building material, as well as practically; a porous limestone formation in the Texas Hill Country is the reservoir of the Edwards Aquifer, a vital water source to millions.
In Sierra Club v. Babbitt the plaintiffs claimed that the defendants were not fulfilling their duties under the Endangered Species Act, to protect endangered species and their ecosystems. The endangered species included the Fountain Darter, San Marcos Salamander, San Marcos Gambusia, Texas Blind Salamander, and Texas Wild-rice. The ecosystems for these species depends on water from Comal Springs and San Marcos Springs which have the potential to run dry if too much water is withdrawn from Edwards Aquifer.[71]
Historically, the Edwards Aquifer has served as the sole source of water for the city of San Antonio.[53] This eight-county metropolitan area is the second fastest-growing area in the state of Texas and depends on the aquifer for both recreational use and clean drinking water.[54][55] San Antonio Water System (SAWS) is the largest public water utility system that serves the eight counties of the San Antonio metropolitan area. A total of 92 water wells with a daily pumpage rate of 203.7 million gallons supply water to SAWS’ customers.[56]
Five groups of stakeholders have played significant roles in shaping the use and conservation of the aquifer, including the Edwards Aquifer Authority (EAA), New Braunfels, San Marcos, San Antonio, and Texas State University.[59][60] Additionally, federal entities including US Geological Survey, US Fish and Wildlife Service, and US Environmental Protection Agency have been involved in water steward activities and recovery management plans of the Edwards aquifer system.[53][54][55]
Comanche Springs was an aquifer of six artesian springs located in the Trans-Pecos region of West Texas. The groundwater source originated from a Comanchean limestone fault combined within the bountiful Edwards Aquifer and the Glass Mountains.
Encompassing an area of 8 counties across south-central Texas including the City of San Antonio and its surrounding communities,[76] the Edwards Aquifer is the main water supply source for the region, providing water for agricultural and industrial purposes and necessary water flow for endangered species habitat, as well as recreational purposes. As a result of a lawsuit filed by the Sierra Club under the Federal Endangered Species Act in 1991, the Edwards Aquifer Authority (EAA) was created to oversee the aquifer in its conservation efforts.[56]
In the Edwards aquifer, groundwater flow models have been developed for the San Antonio and Barton Springs aquifer segments in the San Antonio region of Texas. Two model simulations were conducted: steady state and transient. A steady-state groundwater flow model requires magnitude and direction of flow remain constant, whereas a transient model simulation allows for a change in water storage over time. Steady-state results suggest water leaving the aquifer occurs through springs (73.3 percent), water well pumping (25.7 percent), and to the Colorado River (0.6 percent). Inflow of water to the aquifer mostly occurs through natural recharge (93.5 percent) and water delivered through the aquifer’s regional boundaries (6.5 percent). The transient simulation model also suggests discharge primarily occurs through springs, followed by water well pumping; however, changes in water storage is heavily dependent upon the amount of monthly precipitation and water well pumping volumes.[12]
In 1992, the TWQB declared the Edwards aquifer an underground river due to the precense of endangered species, but this was overturned later the same year. In 1993, Texas Senate Bill 1477 established the Edwards Aquifer Authority to manage the aquifer and to limit pumping to protect the spring flow levels.[65]
The Texas Legislature directed the EAA to regulate pumping from the aquifer, implement critical period management restrictions, and pursue measures to ensure minimum continuous spring flows of the Comal and San Marcos Springs are maintained to protect endangered and threatened species to the extent required by federal law.[76] In 2013, the U.S. Fish and Wildlife Service approved Edwards Aquifer Authority’s Habitat Conservation Plan (HCP), which is a regional 15-year plan designed to protect the water flow and species in the Edwards Aquifer region.
Today, SOS works with groups in the larger Edwards Region with a collective goal of conserving the Greater Edwards Aquifer Ecosystem, which is now chartered in the "Edwards Aquifer Protection Plan." The Edwards Aquifer is the source of water to over 4 million people, including residents of San Antonio. It also provides necessary heat relief and recreation to central Texas residents, and it is the home of many rare plant and animal species. 
Barton Springs is the main discharge point for the Barton Springs segment of the Edwards Aquifer of Texas, a well known karst aquifer. Geologically, the aquifer is composed of limestone from the Cretaceous period, about 100 million years old. Fractures, fissures, conduits, and caves have developed in this limestone. Both physical forces, such as faulting, and chemical forces, such as dissolution of limestone by infiltrating water, have enlarged these voids. This results in a karst aquifer made up of limestone with large void spaces. Water then enters the aquifer and fills the voids.
The EAA was created as a result of Edwards Aquifer Authority Act enacted by Texas State Legislature in 1993. The main purpose of EAA is to oversee the permitting system for water withdrawals from the aquifer system. A subdivision of state government, EAA is more of a liaison between federal agencies (e.g. USFWS, USEPA, USGS), state agencies (e.g. Texas Water Development Board, Texas Commission on Environmental Quality, etc.) and non-governmental organizations (e.g. Texas Water Conservation Association, Texas Association of Groundwater Districts).[53]
In 1997, Chapter 36 of the Texas Water Code was amended by Senate Bill 1 of the 75th Texas Legislature to require all underground water conservation districts in Texas to develop a groundwater management plan and submit it for approval by the Texas Water Development Board every five years on the anniversary of initial approval (September 17, 1998 for the Edwards Aquifer Authority). The initial requirements of the groundwater management plans were that they address the efficient use of groundwater, methods of controlling and preventing waste of groundwater, conjunctive surface water issues, natural resource issues that affect the use and availability, of groundwater, and methods of controlling and preventing subsidence.
In recharge zones of the aquifer permits are required to store regulated substances that could damage water quality. Additionally, The Texas Commission of Environmental Quality requires special permits for construction in the recharge zones of the aquifer.[70]
Edwards Aquifer is home to a large number of invertebrate species, 40 of which have been described.[33] The most diverse groups are the prosobranch gastropods and amphipod crustaceans. The Edwards Aquifer has the highest recorded diversity of stygobites in the world.[35] In the United States, only the fauna of the Edwards Aquifer of Texas has a significant component of marine-derived species. Of the major karst regions in the United States, it is the only one with a significant marine component.[36] Of the 64 stygobionts known from the Edwards Aquifer, 17 are marine relics.[37]
Spanish missionaries who arrived in Texas in the 1700s looked to the Edwards Aquifer as their primary source of water. Springs fed by the aquifer played a key role in deciding the location of the Alamo mission and other settlements in the Texas Hill Country.[61] As Europeans continued to settle the region, and as Texas was acquired by the United States, the Edwards Aquifer continued to supply water for farming, ranching, and rural domestic use.[62]