Local Geology Professor debates SAWS’ Desalination Project

Above graphic shows representation of how water will be extracted from Wilcox Aquifer.

Above graphic shows representation of how water will be extracted from Wilcox Aquifer.

At the February 28 Evergreen Underground Water Conservation District meeting, Geologist Arthur Troell, professor Emeritus of Geology at San Antonio College, stated his position on SAWS’ plans to expand production into the brackish water of the Wilcox formations. Their project, called the Expanded Brackish Water Desalination Project Phase 1, involves producing brackish (slightly salty) water from the Wilcox aquifer, desalinate it at a facility in San Antonio, and provide that water to San Antonio ratepayers. The Wilcox aquifer underlies the Carrizo aquifer throughout most of Atascosa County.

On Tuesday, March 4, around 9 a.m. the SAWS Board of Directors approved the budget for the Expanded Brackish Water Desalination Project Phase 1, which will cost more than $1,400 per acre foot, and will pump 13,000 acre-feet (4,236,068,570 gallons) annually. SAWS also approved Black and Veatch to be the primary contractor on the project.

Black and Veatch produced the Expanded Brackish Groundwater Desalination Project Concept Study (B&V Project No. 177185, Texas Firm Registration No. 258t) and in that study, they state that they used the Southern Queen City-Sparta Groundwater Availability Model to determined that water levels in the Wilcox formation would fall as much as 200 feet in the city of Pleasanton by 2060 as interpreted from Figure 10: SAWS Brackish Modeling South Bexar 33 MGD + Wilson County 50.5 MGD Year 2060 Drawdown in their concept study.

Troell says that drawing water out of the Wilcox formation will likely cause the water levels in the Carrizo to drop, saying “if there is leakage between the Carrizo and Wilcox, which we believe there will be, it will be slow at first and increase in time as water is withdrawn, from the Lower Wilcox beneath, through time.”

SAWS Intergovemental Relations Officer William Peche said that pulling water from the Wilcox should not present too many problems “The best available science indicates that production from the Wilcox Aquifer will have minimal impacts to levels in the Carrizo formation over a fifty year period.” Peche further explained “The confining clay layer between the Wilcox and the overlying Carrizo thins to the west (into Atascosa County) and is very thin in the western portion of Atascosa. In eastern most Atascosa and in Wilson County, the confining clay layer, or aquitard, is up to 400 feet thick, and will provide adequate separation between the Wilcox and Carrizo formations to minimize the possibility of any communication between the two aquifers.”

Troell says that SAWS cannot know beyond a shadow of a doubt that there will be no communication between the two aquifers, saying that there are faults that can breach the aquitard. Also, Troell explained that the aquifers have hills and valleys just like the land surface above.

Troell explained that many millennia ago, the Wilcox formation was formed underwater from the deposition of sediments. Around the time that the deposition was finished, the land under the Wilcox was uplifted to above the surface of the water, exposing it to wind and rain. That wind and rain caused hills and valleys to be cut into Wilcox and the Carrizo aquifers over time, and those channels where the aquitard is thin or non-existent can be places where water infiltrates from the Carrizo into the Wilocx. Troell said that some SAWS lease agreements from 2006 imply that they know that Carrizo water may leak into the Wilcox; they stipulate that any water produced in Wilcox wells will be considered brackish regardless of its TDS (total dissolved solids). If the fresher Carrizo water infiltrated in the saltier, Wilcox water, then TDS would drop. Brackish water, which is under different regulations, is defined as having a TDS between 1000 to 10,000 milligrams per Liter.

Peche said that SAWS will do its due diligence, saying “Although SAWS believes production from the Wilcox will have minimal impacts to levels in the Carrizo over a fifty year period, SAWS will proactively monitor the Carrizo levels in and around the current Brackish Groundwater Desalination plant in Bexar County and eventually in Wilson County if SAWS chooses to pursue expanded desalination efforts. SAWS would have monitoring wells as described in Evergreen UWCD Rule 5.7: Large Scale Groundwater Pumping Projects.”

Peche also explained that falling water levels might not be SAWS’ fault “Of course, declines in the Carrizo would have to take into consideration production from existing Carrizo wells in the vicinity of the Desal project in order to determine if and what impacts SAWS has. If there are impacts attributed to the SAWS project; SAWS would work with the Evergreen to find solutions.”

Troell says another side of this issue is that SAWS will pump out water from the Wilcox that will not be replenished, which means that SAWS will draw all the recoverable water out of the Wilcox; water that Atascosa county could tap in the future for desalination. “SAWS solution is not sustainable over the course of time. SAWS will pump us dry.” Whereas SAWS has the ability to implement desalinating seawater, says Troell, Atascosa County would not be able to afford such a project.

The South Central Texas Regional Water Planning Group Region L (SCTRWPG Region L) has concerns about the project. The authors of the 2011 South Central Texas Regional Water Plan Water Management Strategy Summary Sheet, said of the SAWS project “The wells would be developed in the deep, confined section of the Wilcox, which is substantially removed from the outcrop area. There is concern about the possibility of interaction between the Wilcox and Carrizo layers of the Carrizo- Wilcox Aquifer. If the interaction is significant, pumping the Wilcox will cause some downward leakage of Carrizo water into the Wilcox, which will cause some lowering of water levels in the Carrizo”

Troell asserts that most residents, farmers, cities and industries in Atascosa draw from the Carrizo aquifer, and that falling water levels would translate into higher costs for local farmers.

Troell suggested that, instead of drilling wells in the Wilcox, that SAWS should consider desalination project at the coast. “The desalination plant should be built at the coast and then the treated water pumped to San Antonio. The conden- sate could then be returned to the ocean.”

Peche says that desalinating water from the gulf is a possibility but is cost prohibitive. “Ocean water is twenty times more salty than brackish water, and would have to be piped up hill for 150 miles. All of this adds unnecessary costs when there is an ocean of brackish water beneath our feet from which the entire region can benefit. “

However, Troell says that Atascosa County residents had resisted SAWS attempts to drill wells in Atascosa County before. “If the public (Atascosa County citizens) speak out loudly through public forums, the media, legislators, etc., that we want and need to preserve our underground water resources for this county’s future growth and needs in this rapidly growing county, it can have an effect.”

Peche alluded that there is support for the SAWS project in Atascosa county, saying “In fact, the current brackish groundwater desalination project in southern Bexar County is a result of careful work with a science committee comprised of Wilson and Atascosa Counties stakeholders that asked us to “prove up” the science and operation of the project within Bexar County.”

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