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Located in South Central Texas, the Edwards Aquifer incorporates an area of approximately 4,350 square miles that extends into parts of 11 counties. 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. The aquifer is hydrologically separated into the Austin and San Antonio regions by a groundwater divide near the town of Kyle in Hays County.

The total area of the aquifer forms roughly the shape of a slight upward curve and approximately measures 160 miles east to west at its furthermost boundaries and 80 miles north to south at its widest section. The aquifer is geographically divided into four distinct regions: the total drainage area, recharge zone, artesian zone, and saline zone. These zones run east to west, with the drainage area forming the northernmost portion of the aquifer and the saline zone forming the southernmost portion. The artesian zone intersects the saline zone to the south and west at the fresh water - saline water boundary (FW-SW).

Hydrogeology[edit]
The Edwards Aquifer is highly productive karst aquifer made up of Edwards Group limestones. The Edwards limestone is variable in hydrologic character, but is generally highly porous and permeable, which makes it able to hold and move a lot of water. The limestone is broken by faults and joints. Water flows through these fractures and continues to dissolve the limestone, creating larger and larger pore spaces over time. Some units also store water in eroded fossil burrows that formed through the burrowing action of worms and crustaceans at the seafloor. The effective porosity, or the amount of water that is capable of being recovered, of the Edwards aquifer is estimated to be about 5%. The aquifer ranges in thickness from about 300 to 700 feet (100 – 200 meters). Main Barton Spring in Austin, Texas, a prominent fracture (visible here) in limestone rock. Through this artesian karst spring water emerges to the surface from the karstic Edwards Aquifer. This spring is situated near the diving board in Barton Springs Pool. Photo provided by US Geological Survey.

Unlike sand and gravel aquifers that store water in very small pore spaces, karst aquifers store water in large pockets or caverns, forming underground "rivers" and "lakes". The rate at which groundwater will move through these conduits can vary tremendously. In the Edwards Aquifer some water may barely move, while in other areas water may travel miles (thousands of meters) in a single day. On average, the Edwards aquifer has been modeled with a transmissivity of about 100 ft2/day (9.29 m2/day).

In the south, the Edwards Aquifer dips beneath the lowland plains of the gulf coast. This area south of the recharge zone is referred to as the Artesian Zone, where the water is held under pressure by low permeability layers, and can flow to the surface without the assistance of pumps through openings like springs and artesian wells.

Edwards Aquifer Saline Zone
The saline zone of the Edwards Aquifer is described as having greater than 1000 mg/L of total dissolved solids (TDS). The zone is always associated with geochemically reducing conditions.The FW-SW interface (FWSWI) delineates a hydrogeologic boundary to the south and east of the Edwards Aquifer where ground water salinity changes from a concentration of <500 mg/L to >1000mg/L and water flow rates shift from fast to slow moving. The vertical and lateral distributions vary due to porosity and permeability, pumping, recharge rates, aquifer stratigraphy, and structural faulting.

Sources of Saline Water
Because the Edwards Group was deposited as a broad carbonate platform, the basic composition and mineralogy of the rocks on either side of the FWSWI are similar. The primary difference between the rocks in the FW zone and the SW zone is the degree of late diagenesis and extent of rock dissolution. The FW zone rocks are exposed to fresh meteoric waters, which enhances dissolution, but the SW zone rocks are not and have lower permeability. Ions from minerals that are not dissolving are not being carried out of the SW zone as rapidly as those in the FW zone, which results in increased dissolved ion concentrations, or basically TDS. The presence of evaporite minerals, such as gypsum and anhydrite, in the rock units also contributes to higher TDS. At one time, the FW zone also had evaporites; however, with higher transmissivity rates and exposure to meteoric water, these minerals dissolved rapidly.

In 1956, researchers first noted that faults controlled the presence of the FWSWI. However, recent studies instead reveal wedges of saline water trapped under FW and bound by faults and that the SW zone is compartmentalized due to faults, which impede water to flow both up-dip and down-dip.

Stratigraphically down-dip, petroleum reservoirs leak trace concentrations of hydrocarbons into the Edwards Aquifer, which have been measured from throughout the SW zone. Brine solution from the petroleum reservoirs can also contribute to salinity by migrating stratigraphically updip and mixing with FW at the FWSWI.

Water Chemistry
The SW zone of the Edwards Aquifer has five geochemical facies, or distinct water types defined by major ion compositions. The facies change along the strike of the aquifer. From west to east, the facies are:


 * Facies A, a Ca-SO4 water type with low Cl concentrations in the westernmost portion of the Balcones Fault Zone. Faulting is minimal in this portion of the aquifer, thus cross-formational flow is minimal. In south and southwestern Kinney County and Val Verde County, facies A results from the dissolution of gypsum and anhydrite. A few hypotheses have been posed for the low concentrations of chloride, including flushing from the aquifer rocks or low primary porosity.


 * Facies B, a Ca-Mg-SO4 water type, with high concentrations of Na and Cl. High concentrations of Mg result from the dissolution of dolomite or dedolomitization. Dedolomitization is a process by which dolomite is converted to calcite by the following reaction:   Excess calcium is supplied by gypsum and anhydrite, which dissolve due to meteoric water. High chloride concentrations result from mixing of FW with deep brine solutions and leakage of high chloride water from underlying stratigraphic units. There has been some indication that flow between the FW and SW zones fluctuates depending on pumping and recharge rates.
 * Facies C, a Na-Cl water type with high Ca-SO4 in Bexar County. Facies C is the result of dissolution of carbonates, evaporites, and possibly from dedolomitization. There is also potential cross-formational flow from underlying stratigraphic units. It is likely that sodium and chloride originate from down-dip Edwards Group oil field brines.
 * Facies D, a Na-Cl-SO4-HCO3 water type. This facies overlies the San Marcos Arch and is similar to facies C, except that it has higher sodium and chloride concentrations. This facies is also associated with the area of greatest fault displacement along the Balcones Fault Zone. The rocks associated with this facies still contain high proportions of dolomite. Sodium and chloride originate from brines.
 * Facies E, a Na-SO4-Cl water type and facies E' is a Na-Cl-SO4-HCO3 water type found in Williamson and Bell Counties. The FW changes composition across the artesian zone in Williamson County, from Ca-HCO3 to NaSO4-Cl. The progression of the facies composition suggests that the area is a mixing zone between Ca-HCO3 waters and high TDS waters from the underlying stratigraphic formation or perhaps down-dip brines.

Ecology
The Edwards Aquifer has a diverse community of stygobionts (obligate ground-water dwelling fauna). In the SW zone, the main food source is derived from chemolithoautotrophic sulfide- and methane-oxidizing microorganisms. Many stygobitic invertebrates, such as ostracods, isopods, and amphipods, graze on microbial mats comprised of these microorganisms.

The saline zone also has two species of blind catfish, including the predatory widemouth blindcat, Satan eurystomus, and the toothless blindcat, Troglolanis pattersoni.