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SwRI scientists use well levels, structural geology, and water chemistry to map western Edwards Aquifer for Edwards Aquifer Authority

San Antonio -- October 26, 2006 -- Hydrologists and geologists from Southwest Research Institute® (SwRI®), in a study funded by the Edwards Aquifer Authority (the Authority), were able to confirm two areas where geological features have a significant impact on the west-to-east movement of Edwards Aquifer water in counties west of San Antonio. The findings identify and map the flowpath of water, which will help the Authority to create more accurate models for continued study of this expansive, natural groundwater system.

By performing a nearly simultaneous measurement of groundwater elevations in water wells across Uvalde and Kinney counties and comparing the chemistry of water samples from different wells, the SwRI scientists developed insights about the relationship between the Uvalde pool and the San Antonio pool of the Edwards Aquifer. This included the discovery of a sizeable, apparently separate pool in Kinney County.

The chemical analyses, which measured subtle differences in properties such as salinity, carbonate concentration and total dissolved solids, helped the SwRI hydrologists trace the direction and limits of groundwater movement by mapping locations where water samples contained similar chemical signatures. 

Water level elevations were taken at about the same time from 156 wells in the two counties. Comparing these elevations helped scientists determine where the Edwards limestone is most permeable and where underground intrusions restrict the movement of groundwater. 

A study of the Knippa Gap, a subsurface feature about 10 miles east of Uvalde that separates the Uvalde and San Antonio pools, indicated that up to 270,000 acre-feet of groundwater per year crosses through a relatively narrow area from the Uvalde to the San Antonio pool. Meanwhile, a structural narrowing or restriction of the permeable layers along most of the "gap" area explains why water elevations in the Uvalde pool vary only by about 10 to15 feet, compared to 65 to 80 feet in wells east of the Knippa Gap in the San Antonio pool, during different groundwater elevation stages in the Edwards Aquifer. 

A comparison of groundwater elevations and chemical component analyses also indicated that relatively little Edwards groundwater moves from Kinney County into Uvalde County because of a low-permeability zone that effectively creates a separate Kinney County pool. That pool is fed by an estimated 70,000 acre-feet of recharge per year. Much of that recharge comes from the West Nueces River, which was previously thought to recharge the Uvalde pool.

"The Edwards Aquifer is the primary source of water for San Antonio and many other South Central Texas communities. It supplies residential water for 1.7 million people, plus water for agriculture, industry and recreation," said Dr. Ronald Green, a staff scientist in SwRI's Geosciences and Engineering Division.

"It is estimated that 46 percent of the total average groundwater recharge of the San Antonio segment of the Edwards Aquifer can be attributed to Uvalde County," he said. "Understanding the water resources of Uvalde County, including recharge and discharge, is central to the development of a representative conceptual model of groundwater flow."

The study had a two-fold purpose: 1) develop an updated conceptual model of the groundwater systems in Uvalde County that incorporates existing and recently collected data on hydrology, geochemistry, and structural geology; and 2) clearly define the hydrogeologic relationship between the Uvalde pool and the San Antonio pool of the Edwards Aquifer. By extending the study to Kinney County, between Uvalde and Del Rio, the scientists discovered that water earlier believed to recharge the Uvalde pool instead probably recharges a separate Kinney County pool.

The study was conducted as part of an ongoing Authority research program dedicated to creating a better understanding of the hydrogeology, water quality and use of the Edwards Aquifer. Known as the Edwards Aquifer Optimization Program (EAOP), the comprehensive research initiative consists of a series of interrelated studies known as the Optimization Technical Studies (OTS). The studies were conceived to better understand the needs of protected species that depend on springflow from the aquifer, to increase the understanding of Edwards Aquifer flowpaths, and to evaluate opportunities for increasing recharge to the aquifer. 

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For more information contact Joe Fohn at (210) 522-4630, Communications Department, Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510 or Roland Ruiz at (210) 477-5143 or

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