Hydrology
This 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.
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.
The Edwards Aquifer receives water primarily from streams and rivers originating from the catchment areas on the Edwards Plateau (Figure 2). 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.
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," BalconesEscarpment "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.