The Edwards Aquifer
The Edwards Aquifer is extends about 6,000 square miles in the area just North of San Antonio down in Texas. It is one of the largest artesian aquifers in the world. The water resides in a very porous limestone that is is usually 300 to 700 feet thick. The Edwards is divided into three parts: the contributing zone, the recharge zone and the artesian zone. The aquifer is mostly confined, the contributing zone, which is the largest part,is confined below by a rock called Glen Rose Limestone and above by mostly impermeable Del Rio Clay. The artesian zone is also confined in this way. The part that is unconfined is the smaller recharge area.
The Edwards aquifer is often referred to as a fault-zone aquifer because recharge in the recharge zone occurs because the limestone is heavily faulted and fractured as it follows the Balcones Fault Line. The recharge area is about 1,500 square miles and nearly all recharge occurs when the streams and rivers cross this area. Medina Lake, a reservoir built in the recharge area, also contributes quite a bit of water to the aquifer. A small percentage of recharge comes from rain falling directly in the recharge zone. The average recharge for the aquifer is usually about 500 million gallons of water per day.
The Edwards aquifer is highly permeable and the most transmissive aquifer in Texas or Oklahoma. Transmissivity of the aquifer is estimated at anywhere from 200,000 to 2,000,000 feet squared per day depending on pumping activity and other factors. Storativity is very small at .00001 and specific yield is relatively small at 3-4%. The average hydraulic conductivity ranges from 4-11 feet per day.
The water of the Edwards is considered to be very healthy. It has a very good mix of ions and minerals that make it very good for drinking. Large amounts of magnesium and calcium are already found in the water. Naturally occurring elements like iron and copper are found well below the water quality standards. Along with these, there is almost nothing found in the aquifer that did not come there naturally, pesticides or herbicides are rarely found. This is good because the water from the aquifer is used by around 2,000,000 people in South Central Texas. Most of the water is used for agricultural and public use; the small amount left is used for domestic and industrial use. The aquifer is even more important because of its location in a semi-arid climate.
Although demand is so high and the water is used by so many, the aquifer is presently in no danger of overuse. The aquifer usually stays 90 to 95% full. The only problem is the that the springs will stop flowing if it gets down to about that low. In 1993, the Edwards Aquifer Authority was put into place by the state to regulate the withdrawal of the Edwards and monitor the other aspects of it.