About the Edwards Aquifer
The San Antonio Segment of the Balcones Fault Zone Edwards Aquifer (Aquifer) in South-Central Texas is one of the most productive aquifers in the United States. The Edwards Aquifer is a karst aquifer and is characterized by the presence of sinkholes, sinking streams, caves, large springs and highly productive water wells. Karst aquifers are considered triple permeability aquifers. Water is contained in the rock matrix, in fractures and faults and in caves and conduits. Conduits or solution channels within the Aquifer range from the size of a finger to tens of feet in diameter. The interconnected fractures and conduits in the Edwards Aquifer accounts for its extremely high yielding wells and springs. As is characteristic of many karst aquifers, the Aquifer exhibits extremely high (cavernous) porosity and permeability, allowing for the transmission of large volumes of water and enabling groundwater levels within the Aquifer to respond quickly to rainfall events (known as recharge). The large interconnected openings in the rock also exhibit a diverse fauna of more than 40 species including eyeless salamanders, shrimp and two species of catfish.
Geographically, the Aquifer extends through parts of Kinney, Uvalde, Zavala, Medina, Frio, Atascosa, Bexar, Comal, Guadalupe and Hays counties and covers an area approximately 180 miles long and five to 40 miles wide. The total surface area overlying the Aquifer is approximately 3,600 square miles. The Aquifer is the primary water source for much of this area, including the City of San Antonio and its surrounding communities. Historically, the cities of Uvalde, San Antonio, New Braunfels and San Marcos were founded around large springs that discharged from the Aquifer. As the region grew, wells were drilled into the Aquifer to supplement the water supplied by those springs. The Aquifer also serves as the principal source of water for the region’s agricultural and industrial activities and provides necessary springflow for endangered species habitat, as well as recreational purposes and downstream uses in the Guadalupe, Nueces and San Antonio river basins.
Edwards Aquifer Hydrogeology
Water circulates through the Edwards Aquifer as part of the hydrologic cycle from recharge areas to discharge locations (springs and wells). Approximately 1,250 square miles of Edwards Limestone is exposed at the ground surface and composes the Recharge Zone where water enters the Aquifer. Surface water from springs and streams originating on the Contributing Zone reaches the Recharge Zone where much of the flow sinks into the Edwards Limestone. Some water also enters the Edwards Aquifer through interformational flow (from rock formations adjacent to the Edwards Limestone) and from direct precipitation on the Recharge Zone.
Water from the Recharge Zone flows down gradient to the Artesian Zone where the Aquifer is contained between less permeable beds of the Del Rio Clay (above) and the Upper Glen Rose Limestone (below). Portions of the Artesian Zone are as much as 3,400 feet below the surface where it still contains fresh water. The southern boundary of the Artesian Zone marks the Aquifer’s transition from freshwater to saline water (water with a total dissolved solids concentration greater than 1,000 mg/L). Groundwater moves through the Artesian Zone and ultimately discharges from a number of locations, such as Leona Springs in Uvalde County, San Pedro and San Antonio springs in Bexar County, Hueco and Comal springs in Comal County and San Marcos Springs in Hays County. In addition, domestic, livestock, municipal, agricultural and industrial wells throughout the region withdraw water from the Aquifer. The residence time of water in the Aquifer ranges from a few hours or days to much longer, depending on depth of circulation, location and other aquifer parameters.
Edwards Aquifer Karstification
The Edwards Aquifer is a karst aquifer. Karst aquifers form over soluble rock types such as limestone by reacting with mildly acidic waters. For example, rain water contains carbonic acid (created when carbon dioxide (CO2) from the air or decaying vegetation on the ground comes in contact with water droplets).
Because of this reaction, the Edwards Aquifer exhibits extremely high (cavernous) porosity and permeability. In contrast, aquifers that occur in sand and gravel or in other rock types such as sandstone have a much lower permeability. Because the Edwards Aquifer has areas of such high permeability, it allows for the transmission of large volumes of water, enabling groundwater levels to respond quickly to rainfall (recharge) events and to times of drought.
It is important to note, however, that the amount of water in the aquifer does not equal the amount of water that can be recovered from it. There are many factors that determine how much water is available for recovery. First, there are physical factors, such as depth and pressure that limit how much groundwater can be recovered through withdrawal. Second, there are considerations such as economics, well depth and location and the potential impact of other wells. Third, there are the minimum springflow requirements established pursuant to the Endangered Species Act (ESA) combined with administrative limitations established by the EAA’s permitting system mandated within its specific enabling legislation.