Groundwater lies below the Earth's surface almost everywhere. More than two million cubic miles of fresh water is stored in the Earth, and half of that is within a half mile of the surface. Fifty percent of the U.S. population depends on groundwater for their daily needs. Groundwater is also one of our most important sources for irrigation.

Stretching across portions of ten counties, the San Antonio segment of the Balcones Fault Zone (Edwards Aquifer) is 180 miles long with a width that varies between 5 and 40 miles. Its primary geologic component is Edwards Limestone, and it is one of the most permeable and productive aquifers in the United States. The Edwards Aquifer occurs in three distinct segments: the drainage area, the recharge zone and artesian zone.

Drainage Area

The area north and west of the aquifer is called the Edwards Plateau or more commonly, the Texas Hill Country. Portions of this area serve as the catchment or drainage area of the aquifer.

The drainage area is the largest component of the aquifer system, spanning approximately 5,400 square miles, and includes all or part of thirteen counties, Edwards, Kinney, Real, Uvalde, Kerr, Bandera, Medina, Gillespie,Kendall, Bexar, Blanco, Comal, and Hays counties. Rain falling in the drainage area soaks into the limestone of the plateau forming spring-fed streams. These streams flow over relatively less permeable older rock formations until they reach the recharge zone.

Recharge Zone

The recharge zone is located in an area geologically known as the Balcones Fault Zone. In the recharge zone porous and permeable Edwards Limestone is exposed at the surface and provides a path for water to reach the artesian zone.

Recharge is water that enters the aquifer through features such as fractures, sinkholes and caves. Streams from the Edwards Plateau flow across the recharge zone, percolating into the ground. Rain falling directly on the recharge zone also percolates into the ground and enters the Edwards Aquifer.

The recharge zone encompasses approximately 1,250 square miles and forms the northern boundary of the artesian zone in Kinney, Uvalde, Medina, Bexar, Comal and Hays counties. Although average precipitation is greater in the eastern counties, the largest amount of recharge to the Edwards Aquifer occurs in the stream basins of the western counties. The Nueces River basin, the Frio-Sabinal River basins and the Seco-Hondo Creek and Medina River basins (located in Kinney, Uvalde and Medina counties) supply about 70 percent of the total recharge to the aquifer. These western basins are characterized by larger catchment areas and larger recharge areas than those in the east.

Artesian Zone

The Edwards Aquifer has a great capacity for storing and moving water. The artesian zone is a complex network of interconnecting spaces varying from microscopic pores to open caverns. The artesian zone differs from the recharge zone because it is located between two relatively less permeable layers that confine water to the system.

The artesian zone is about 2,100 square miles and underlies all or a portion of the ten counties south and east of the Balcones Fault Zone. Those ten counties are Kinney, Uvalde, Medina, Bexar, Comal, Hays, Atascosa, Guadalupe, Frio and Zavala counties. Water cannot seep directly into the artesian zone from the ground surface because of impermeable layers, such as clays, between the surface and the aquifer.

In certain places where there is enough artesian pressure, some of the water is forced to the surface through faults, forming springs. Artesian pressure can also cause some wells to flow without a pump. Water leaving the aquifer is referred to as discharge. Water is discharged from the aquifer through springs and wells.