Achieving water supply diversity
through efficiency and innovation.
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By every measure, San Antonio's Aquifer Storage and Recovery facility has exceeded expectations and become a critical water management tool for the city and surrounding region.
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 he Twin Oaks Aquifer Storage and Recovery facility (ASR) is a key component of SAWS’ 50-Year Water Resource Plan.The first major project funded by SAWS customers through the Water Supply Fee, SAWS opened the Twin Oaks ASR in June 2004.
ASR technology is relatively new. SAWS stores excess Edwards Aquifer drinking water during rainy times in a large-scale underground water storage facility in south Bexar County for use during our dry south Texas summers.
What are the Benefits of ASR Technology?
- 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 underground reservoir can continue its prior use.
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. If water is not required to be recovered during the current year, it can remain in storage until required in a future year.
As of January 2011, more than 90,000 acre-feet of water was stored underground.
Why use an ASR technology in San Antonio?
- Allocations from Edwards Aquifer are limited by legislation and Edwards Aquifer Authority rules.
- During wet periods, there was no method to store allocations that were not used.
- There is no carry over or credit for pumping rights not used in a given year. This means "use it or lose it."
- Stored water recovery can lessen the effect of drought on the Edwards Aquifer.
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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. |
Saving for a Sunny Day
About 100,000 acre-feet* or more 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 spring flows in New Braunfels and San Marcos, ensuring the protection of endangered species. This also lessens the effect of drought on the Edwards Aquifer.
Production Mode
Up to 2 acre-feet of groundwater per surface acre controlled (about 6,400 acre-feet) may be produced from the native groundwater of the Carrizo-Wilcox Aquifer. 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 serves as a key relay and treatment point for additional water sources currently under development.
- Twin Oaks is the third largest ASR facility in the nation.
- While not required by state law, a mitigation program has been implemented to assist area well owners that may be impacted by draw down during the recovery phase.
- 29 ASR wells constructed
- 60 million gallons per day (mgd) well field capacity
- 30 mgd water treatment capacity
- 30 miles of 60-inch steel pipe and 10 miles of 42-inch steel pipe integrating water at Artesia, Seale, and Randolph pump stations.
- 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 underground reservoir can continue its prior use.
- Total cost for the ASR Project, Phase I and Phase II combined, is $255 million.
ASR and the Current Drought
Beginning in late March 2011, SAWS was producing 20 million gallons of water per day from Twin Oaks before Stage One restrictions were even in place. In late June 2011 as the Edwards Aquifer level dipped below 640 feet, San Antonio Water System increased production of diverse supplies from the Twin Oaks Aquifer Storage and Recovery facility. From that time and to present, SAWS has been producing 40 million gallons per day — enough to supply about 20 percent of San Antonio's current daily demand. Coupled with reduced demand from outdoor watering restrictions, diverse supplies have resulted in a delay of Stage Three restrictions.
* An acre-foot of water is equivalent to 325,851 gallons
Rev. 11/2011 |
