Aquifer Storage and Recovery

Unlike the Edwards Aquifer with its high transmission rates, water in a sand aquifer tends stay in place or move very slowly. Water injected into unconfined sand forms a stationary dome, and if there are confining layers then water spreads out horizontally.

Either way, it is possible to store water in sand and come back years later and extract the exact same water. This is the concept behind Aquifer Storage and Recovery (ASR). When there is extra Edwards water around, it could be injected into locally occurring sands and extracted during times of shortage. Water from other sources could also be transported and stored in the sands for later use.

ASR technology addresses one of the region’s biggest problems: there is hardly any storage locations where water can be put in times of plenty for later use. One big advantage of storing water in sand instead of a reservoir is that no water evaporates. Also, it’s easy to ensure the people who pay for the project are the ones who benefit, which is not necessarily the case with other options such as additional Edwards recharge.

The rules are that before water can be injected, it has to meet drinking water quality standards so there is no chance that water already in the ground could be contaminated. ASR technology has been in use for many years on the east coast, in Florida, and in California. In Texas, there are now two operating ASR facilities - one was developed in Kerrville for the Upper Guadalupe River Authority in the early 1990’s, and the other was built by the San Antonio Water System in south Bexar county and came online in June 2004.

When source water is available, it is diverted to a treatment facility that can be disguised to blend in with the local character.  Water is treated to drinking water quality and injected into a suitable underground aquifer, often composed of sand.  The same wells are later used to extract the water and distribute it to users.  The water retrieved is the exact same water that was injected, and there is no evaporation.  The land overlying the wellfield can also continue to be used for other purposes such as agriculture or grazing.

San Antonio Project Development

In September 1996 the San Antonio Water System and Bexar Metropolitan Water District were awarded a $200,000 grant to study the possibility of storing water in the Carrizo-Wilcox and Glen Rose aquifers, the saline zone of the Edwards Aquifer, and the Austin Chalk and Anacacho limestone formations (2).  The study included looking at availability of water for storage, whether the source waters were compatible with water in the destination aquifers, and the quality and movement of water in each aquifer.  The Carrizo-Wilcox Aquifer was identified as having the characteristics necessary for Aquifer Storage and Recovery.  The Carrizo-Wilcox is composed mainly of sand interbedded with gravel, silt, clay, and lignite.  It extends from the Rio Grande in South Texas northeastward into Arkansas and Louisiana, passing through southern Bexar, Wilson, and Atascosa counties.  In some places the water has a high iron content, and hydrogen sulfide and methane also occur.  Carrizo water is easily treated by conventional methods, and lots of people currently use it without treatment.  

In September 1999 the San Antonio Water System purchased a 261 acre farm over the Carrizo-Wilcox near the Atascosa-Bexar county line, and added two more large adjacent tracts in February 2000 for a total of over 3,200 acres (6, 7). SAWS developed an aggressive project timetable to have the facility online and producing water in late 2002 or early 2003, and plans included leaving the farms in agricultural production.

In February 2001 the SAWS' Board approved $7.53 million in engineering design contracts to move the project forward (8).  Engineers began preparing plans for a facility capable of producing about 30,000 acre-feet of water per year, enough for about 60,000 families. Overall, the project was estimated to cost about $215 million. 

Twin Oaks ASR Treatment Plant Site	The Twin Oaks
In 2001, engineers began preparing plans to transform this site off Hardy Road in south Bexar county into a state-of-the-art water treatment plant for the Twin Oaks Aquifer Storage and Recovery Facility. Regulations require that water has to be treated to drinking water quality before it can be injected into the ground and stored for later use. The Twin Oaks are on the right.	During an initial site visit by architects, engineers, and SAWS water treatment experts, the late-morning heat drove the group to seek shade under this tree. A SAWS staffer dubbed the facility being envisioned the "Twin Oaks plant", and the name stuck.

In October 2001, fearing that SAWS' plans to pump water from the Carrizo-Wilcox aquifer would affect their own wells, south Bexar county residents asked to be included in the neighboring Evergreen Underground Water District (10).  A small portion of the Carrizo-Wilcox aquifer in Bexar county is not within any groundwater district and therefore not regulated. Inclusion in the District would mean that SAWS could be subject to pumping limitations.  On October 30 the Board of the Evergreen District accepted the resident's petition and scheduled a February 2 vote by approximately 8,000 landowners that use the Carrizo-Wilcox.  SAWS officials appealed to the Board to delay calling for an election, asking for time to explore a joint management agreement that could provide more protection than regulation.  The utility proposed a contract with the District that would provide for mitigation, paying to remedy any problems caused by declining aquifer levels.  SAWS would also fund a well monitoring program so that impacts could be measured.  

Meanwhile, in November of 2001, the utility received a permit from the Texas Natural Resource and Conservation Commission to construct and operate the system (11).  In January 2002 SAWS CEO Eugene Habiger indicated that, based on revised projections of future needs and other sources in SAWS' inventory, the most that SAWS would pump from the Carrizo would be 14,000 acre-feet per year, not 30,000 as initially proposed (14).  On January 16 the election scheduled for February 2 was halted by a federal judge after a community group filed a lawsuit alleging the election would violate the federal Voting Rights Act because there would be only one voting site (15).  The election was cancelled and re-scheduled for May 4 (16).  

Observers noted that even if voters decided to include the area in the Evergreen Underground Water District, it would not necessarily mean the end of the project. SAWS has an interlocal agreement with the District and the San Antonio River Authority to work together on ASR.  The District's board indicated it was not philosophically opposed to ASR, as long as it is proven to be feasible and safe (12).

Area residents who opposed the project were worried that pumping of large volumes by SAWS would cause their own wells to decline, and they were also worried about possible chemical reactions from the introduction of Edwards water.  SAWS officials conceded that drawdown of wells in the vicinity of the project will occur (9), but they promised that mitigation would include lowering of pumps, deepening of wells or drilling new ones, or the provision of access to potable-water service from a local purveyor.  Tests conducted on the feasibility of mixing Carrizo and Edwards water did not reveal any problems or adverse chemical reactions.  On February 19, 2002 the SAWS Board adopted a policy stating that SAWS would take water from the Carrizo only if Stage III drought restrictions are imposed and would take no more than 14,000 acre-feet per year.  In April 2002 SAWS revised its policy to say it would draw no more than 28,000 acre-feet over two years (17).

On May 4, 2002 south Bexar county voters rejected annexation into the Evergreen Underground Water District by a vote of 348 to 322.  Supporters of annexation had argued that inclusion in the District would safeguard their water supply, while opponents countered that inclusion would mean additional regulation and taxes without any benefit.  County Commissioner Robert Tejeda said "I think the people that were for the annexation will still be protected." (18).

On July 16, 2002 the SAWS Board of Trustees approved $110 million in construction and engineering contracts to build the treatment facility and 29 miles of pipelines and pumps to deliver and integrate the water into the SAWS distribution system (19). A groundbreaking ceremony was held on July 31, construction began on August 1, and the facility opened in June of 2004.

By the end of 2005, almost 18,000 acre-feet had been stored for later use. This water came in very handy during the drought of 2006, when more than 6,400 acre-feet of stored water was produced, deferring Edwards pumping and protecting springflows.

In August of 2007 a feud between Bexar Met and SAWS erupted over Bexar Met's plans to pump water from a wellfield across the street from Twin Oaks. Bexar Met had originally planned to pump 5 mgd from the wellfield but announced plans to almost triple that amount, and SAWS contended that modeling analysis indicated such a withdrawal rate would have an impact on Twin Oak's storage bubble. SAWS Trustee Douglas Leonhard said "I think this is really quite serious. Their pumping has the capability and really quite the certainty of moving that bubble of Edwards water over into the Staggs Ranch area where it can be pumped out by Bexar Met." (20).

Besides the SAWS project in Bexar county, there is another, larger regional ASR project being proposed by the South Central Texas Regional Water Planning Group. This one would also involve withdrawals from the Carrizo Aquifer in Wilson and Gonzales counties and could provide up to 50,000 acre feet per year. Once the sands are dewatered, they could be used for storage of other waters for recovery during dry times.

	Twin Oaks ASR Treatment Plant
Phase I of the project, completed in 2004, includes a 30 mgd treatment facility, 16 wells, a high service pump station, and 30 miles of large diameter transmission main to convey water to SAWS’ Artesia and Seale ground storage tanks.

The Treatment Process

When Edwards water is recovered, it does not require treatment before distribution. The treatment plant portion of the Twin Oaks facility is designed to make native Carrizo Aquifer water compatible with Edwards Aquifer supplies. Carrizo water is typically high in iron and manganese and has lower hardness and pH than Edwards water.

	Cascade Aerator
First, carbon dioxide and lime are added to the raw Carrizo water to increase pH, hardness, and carbonate alkalinity. A step-feed aeration process (right) removes any remaining carbon dioxide, provides oxidation of the iron and hydrogen sulfide, and increases the dissolved oxygen concentration.

	Solids Contactor
Polymer is added to the aerated water to assist coagulation of suspended solids into large, settleable particles. Potassium permanganate is then added to the water to oxidize manganese into an insoluble form that can be removed by sedimentation and filtration. Solids contact clarifiers (right) are designed to remove the settleable particles in the water.

	Dual-media Filters
Dual media filters (right) remove any remaining solid particles. Three recycle pumps and a filter backwash waste clarifier accommodate the backwashing of the filters.

	3 million gallon storage tank
A three mgd clearwell (right) stores the treated water prior to distribution.