Achieving water supply diversity
through efficiency and innovation.
 he Twin Oaks Aquifer Storage and Recovery facility (ASR) is a key component of SAWS’ 50-Year Water
Supply Plan. In June of 2004 SAWS opened ASR—the first major project funded by SAWS customers through the Water Supply Fee.
ASR is a large-scale underground water storage facility in south Bexar County. It uses relatively new technology to store excess Edwards Aquifer drinking water during rainy times for use during our dry south Texas summers.
What are the Benefits to ASR?
- Environmentally-friendly method of storing
Edwards Aquifer drinking water in the Carrizo-
Wilcox Aquifer.
- Water stored during the year can be used
during dry, hot months.
- Maximizes use of pumpage allocations from the
Edwards throughout the year.
- Underground storage means less evaporation.
- Less vulnerable to contamination than surface storage.
- Most land directly above the ASR can continue its prior use.
The ASR technology is a proven method of storing water underground. SAWS enlisted the help of some of the world’s leading ASR experts on this project. The concept is simple. Water is pumped from the Edwards Aquifer throughout the year and stored in the Carrizo Aquifer in south Bexar County. Later, during the hot, dry months, the drinking water is pumped back into the existing distribution system to help meet summer water demands.
Why an ASR in San Antonio?
- Allocations from Edwards Aquifer are being
limited by legislation.
- During wet periods there was no method to store
allocations that were not used.
- There is no carryover or credit for pumping rights
not used in a given year. This means " use it or lose it."
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We strive to be environmentally sensitive leaving our job sites — like this one near the ASR project — preserved in their natural splendor.
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Saving for a Sunny Day
An additional benefit is the ability to pump limited amounts of Carrizo Aquifer water. Up to 30,000 acre-feet* of excess Edwards Aquifer water can be diverted to storage in the Carrizo Aquifer (injection mode). The water is
withdrawn during dry periods (recovery mode), to help maintain springflows
in New Braunfels and San Marcos which ensure protection of endangered species.
Phase I of the project, completed in 2004 includes:
- 30 million gallons per day treatment facility
- 16 wells
- A high service pump station
- And 30 miles of pipeline to convey
water to the SAWS Artesia and Seale pumping stations.
Production Mode
Up to two acre-feet of groundwater (about 6,400 acre-feet) may be produced from the native groundwater of the Carrizo-Wilcox Aquifer for every surface acre of land owned. This is in accordance
with an agreement with the Evergreen Underground Water Conservation
District.
Water Treatment
Also located at the facility site is SAWS’ first drinking water treatment plant. The Twin Oaks treatment facility is designed to make native Carrizo Aquifer
water compatible with Edwards Aquifer supplies during production mode.
Carrizo Aquifer water is typically higher in iron and manganese and has lower hardness
and pH than Edwards Aquifer water.
More Facts About Twin Oaks ASR
- The ASR water treatment plant will serve as a key relay and treatment point for additional water sources currently under development.
- Phase I was constructed and completed ahead of schedule and under budget in 21 months at a cost of $185 million.
- Phase II includes construction of 12 additional wells, interconnecting
pipeline, a second clearwell storage and other improvements.
Scheduled completion: Summer 2007.
- While not required by state law, a mitigation program has been implemented to assist area well owners that may be impacted by draw down.
- Upon completion of Phase II, Twin Oaks will be the second
largest ASR facility in the nation.
* An acre-foot of water is equivalent to 325,851 gallons
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