Digging Deep for Texas Water March 2017
When we talk about meeting the future water needs of our state, one of the most fundamental questions that has to be answered is, "Where will the water come from?" We depend on science to help us answer that question. Because no one is able to create water to fulfill demands, the water that’s currently on Earth is what we are limited to. As a result, we have to be creative and knowledgeable in how we identify and use the water we already have.
Brackish groundwater is becoming an important water source that can help reduce the demand on fresh water supplies. Brackish groundwater contains salts with total dissolved solid concentrations ranging from 1,000 to 10,000 milligrams per liter, and it is found in 26 of the 30 major and minor aquifers in Texas. (Aquifers can have both brackish and fresh water.) Mapping of Texas’ saline water resources dates back to 1956, but the amount of brackish groundwater wasn’t defined until a 2003 study estimated that Texas has more than 2.7 billion acre-feet in storage.
A deep understanding of the water source is required before utilities can drill into the earth, pump out the brackish water, and desalinate it to make it drinkable. There’s a lot of science behind understanding brackish groundwater and determining whether it can be used as a reliable water source.
And science is one of our specialties. In 2009, the 81st Texas Legislature approved funding to implement the Brackish Resources Aquifers Characterization System (BRACS) program to help us more thoroughly characterize brackish aquifers (in particular, characterizations such as hydrogeologic framework, the water-bearing lithologic units, the properties related to groundwater flow, and water salinity and quality). The goal of the program is to map and characterize the brackish portions of the aquifers in Texas in sufficient detail to provide useful information and data to regional water planning groups and other entities interested in using brackish groundwater as a water supply.
In 2010, with the aid of legislative funding, the TWDB funded three initial research projects to support the BRACS program. The first project identified geophysical well logs (graphic charts that provide quantitative information about the subsurface geology of the changes in the physical properties of the rocks encountered by wells drilled for oil, gas, or water) that could be used to map the geologic structure of aquifers and estimate the salinity of groundwater across Texas. The logs were then scanned into digital images and entered into a database. The BRACS Database now has more than 52,000 logs available.
The second project compiled a bibliography of more than 7,500 reports, articles, and graduate research papers with an emphasis on Texas geologic formations containing brackish groundwater into a relational database. This database serves as a source of reference for existing geologic information for a project area. The third project assessed computer software programs capable of modeling different densities of groundwater found in brackish aquifers. From the assessment, a project report and a modeling code selection tool were developed to help users select the appropriate software.
In 2011, the TWDB completed a pilot study on the Pecos Valley Aquifer in West Texas to establish the methods of data analysis for future brackish groundwater studies. For each BRACS study, the TWDB—or our contractors—collects as much geological, geophysical, and water well data as available in the public domain and uses the information to map and characterize both the vertical and horizontal extent of the aquifers in great detail. Groundwater is classified into five salinity classes: fresh, slightly saline, moderately saline, very saline, and brine. Multiple salinity classes may be found in an aquifer, and the volume of groundwater in each class is estimated based on the three-dimensional mapping of the salinity zones. Project data and reports are available on the TWDB website, and digital geophysical well logs used for the studies may be downloaded from the TWDB Water Data Interactive website. We are currently working on two new studies, one of the Lipan Aquifer and another of the Wilcox, Carrizo, Queen City, Sparta, and Yegua aquifers in Central Texas.
Taking the mapping and characterization of aquifers one step further, the 84th Texas Legislature passed House Bill 30 in 2015. House Bill 30 provided $2 million in funding and directed the TWDB to identify and designate brackish groundwater production zones in four aquifers: an area of the Carrizo-Wilcox Aquifer located between the Colorado River and Rio Grande, the Gulf Coast Aquifer and sediments bordering that aquifer, the Blaine Aquifer, and the Rustler Aquifer. These production zone designations were due to the legislature by December 1, 2016.
House Bill 30 also directed the TWDB to study the remainder of the state's aquifers. The TWDB is required to report our progress on the studies every two years in the biennial desalination report, and all mapping must be completed by December 1, 2022. Contracted studies on three additional aquifers (the Trinity, the Blossom, and the Nacatoch) will be completed in August 2017.
House Bill 30 requires the TWDB to determine the amount of brackish groundwater a production zone can produce over 30- and 50-year periods without causing a significant impact to water availability or water quality in surrounding aquifers. The TWDB is also required to make recommendations on reasonable monitoring to observe the effects of brackish groundwater production within the zone.
To complete these requirements, we used the same methods as in our BRACS studies to analyze the lithology (the general physical characteristics of rocks) and stratigraphy (the arrangement of rock layers) of the aquifers. We mapped the salinity of the aquifers using existing water quality samples and interpreted geophysical well logs. We then evaluated exclusion areas, identified potential production areas, and developed models and simulations. Stakeholder engagement also played a key role in gathering information.
As a result of this research and local input, the Board designated one production zone in the Carrizo-Wilcox Aquifer with a potential pumping volume of 43,000 acre-feet per year, four zones in the Gulf Coast Aquifer with a total potential volume of 45,000 acre-feet per year, and three zones in the Rustler Aquifer with a total potential volume of 15,680 acre-feet per year. No zones were designated in the Blaine Aquifer, as it is prominently used for agricultural and municipal purposes. Each of these zones contains groundwater that is slightly to moderately saline and is an area where the water could successfully be pumped without significantly impacting other portions of the aquifer—like the fresh water portions.
The value of this scientific work extends well beyond the production zones. The analysis will be useful for anyone considering brackish desalination in other parts of the aquifers and for general knowledge moving forward. Understanding Texas aquifers and the water they contain benefits our state in numerous ways, demonstrating that Texas is leading the charge on this innovative water supply and continuing to plan ahead for the future.
For more information on BRACS, visit the TWDB website, or "The Future of Desalination in Texas: 2016 Biennial Report on Seawater and Brackish Groundwater Desalination."