User:Hannahclimbs/Edwards Aquifer

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.