Each and very day in San Antonio, when we listen to our daily newscasts, in addition to the news and weather reports, we are told the status of the Edwards Aquifer expressed in feet. If you're like most people, this conjures a vision of a vast underground pool of water and someone measuring the depth with a really long tape measure!

      Here's what it's actually all about, reprinted from The Edwards Aquifer Website by Gregg Eckhardt. The Edwards Aquifer is a unique groundwater system and one of the most prolific artesian aquifers in the world. It is one of the greatest natural resources on Earth, serving the diverse agricultural, industrial, recreational, and domestic needs of almost two million users in south central Texas. Within this region and poised on the edge of the vast Chihuahuan desert lies San Antonio, America's 7th largest city. The city has a semi-arid climate, and water from the Edwards is the reason that 18th century Spanish missionaries were able to establish footholds like the Alamo here on the New World frontier. For over two centuries, San Antonio and many other cities in the surrounding region were able to grow and prosper without developing surface water or other water resources because of the Edwards Aquifer.

     And here's how it's measured: The J-17 index well is located in the small building at the base of the large water tower near the national cemetery at Fort Sam Houston in San Antonio. It is on a major Edwards flowpath and responds quickly to pumpage and recharge, so it has been used since 1956 to record changes in the level of the Aquifer in the San Antonio area. The level of the J-17 well has ranged from 612 feet during the 1950's drought to 703 feet after historic rains in 1991 and 1992. There is much confusion about what the reported Aquifer level means. When weathercasters say the Aquifer stands at 650 feet, it does NOT mean there is 650 feet of water left or that it is 650 feet to the top of the Edwards formation. The number is simply an indication of relative pressure being exerted on water at the location of the test well. The figure below illustrates this concept...the Edwards formation is between 300 and 700 feet thick, so it is about as thick as the Tower of the Americas is tall.



      Out to the west in the recharge zone, the Edwards outcrop at the land surface is higher than the top of the Tower. Water tends to flow downhill, and it so happens that "downhill" is directly under most of San Antonio. Water is heavy stuff, and as new water enters the formation in the recharge zone, it places tremendous pressure on water already deep inside, forcing water up through cracks and wells toward the land surface. So water rises in the test well because of pressure being exerted by water higher up in the Edwards formation out to the west. It does not rise all the way to the elevation of water to the west because of friction. When water does rises all the way to the top of a well in this manner, the well is called artesian and water flows out without pumping. A good index well such as J-17 is one in which pressure is never sufficient to cause the well to become artesian. To get water out of J-17, it would have to be pumped. The land surface at the top of the J-17 well is at 730.8 feet above sea level, and the downtown area around the Tower of the Americas is around 650 feet above sea level. A reported Aquifer level of 650 feet, for example, would indicate the top of water in the well is about even with the bottom of the Tower. The water in the well is still 80.8 feet below the land surface, so to extract water from the well it would have to be pumped that distance. It is still another 400 feet from 650 to the top of the Edwards limestone formation.