In mid-June, almost 65% of Texas was experiencing conditions ranging from abnormally dry weather to exceptional drought, according to US Drought Monitor. For Texans in general, and water resource planners and suppliers in particular, it brought back bitter memories of the drought of 2011.
In October 2011, the driest year on record, Texas State Climatologist John Nielsen-Gammon’s briefing for the state legislature described the grim scenario: Spring wildfires were followed by dying crops, if they emerged at all, while grazing lands withered and stock-water tanks went dry. “This drought has been the most intense one-year drought in Texas since at least 1895 when statewide weather records begin, and . . . it probably already ranks among the five worst droughts overall,” he writes.
In some areas of Texas, strong population growth is further straining water resources and driving consideration of innovative water supply sources. America 2050, the Regional Plan Association’s national infrastructure planning and policy program, estimates that by the middle of the century, 35 million people–70% of the population–will live in the metropolitan areas comprising the Texas Triangle, which is anchored by Houston, Dallas-Fort Worth, and San Antonio.
El Paso Leads in Brackish Groundwater Development
For some time, water resource planners and suppliers in Texas have been digging deeper for a reliable, drought-resistant water supply, often using surface water and fresh groundwater in conjunction. Recently, they have begun developing brackish water aquifers to create new freshwater sources. A joint project of El Paso Water Utilities (EPWU) and the US Army/ Ft. Bliss led the way. The presence of brackish groundwater in the aquifers had been long established, but as treatment technologies improved and water demand increased, it made sense to explore the resource further. In 1997, EPWU, water utilities and other agencies in Texas and Mexico commissioned the US Geological Survey to analyze the Hueco Bolson aquifer for freshwater, brackish water, and flow patterns. These studies helped planners determine the best location for wells to produce the brackish groundwater and for placement of the inland desalination facility. Ten years later, EPWU opened Kay Bailey Hutchison Desalination Plant–one of the world’s largest inland desalination plants–which uses reverse osmosis (RO) to produce 15.5 million gallons a day (MGD) of permeate, and blends it with 12 MGD of fresh groundwater for a total production of 27.5 MGD. The plant increases EPWU’s fresh water production by approximately 25% based on current demand, and augments existing supplies to ensure that El Paso and Ft. Bliss have sufficient water for the future.
Currently, San Antonio Water System (SAWS) is developing Phase I of a brackish groundwater desalination facility. Phase I, that is scheduled to be completed in 2016, will produce approximately 12 MGD from the brackish Wilcox Aquifer. Phases II and III, anticipated to be completed in 2021 and 2026, respectively, will produce a combined total of over 30 MGD.
Available, Good Quality Water Resource
In 2004, LBG-Guyton Associates published a preliminary study for the Texas Water Development Board (TWDB), which identified and characterized brackish groundwater supplies based on available data. This report increased recognition of the potential for large-scale development of brackish aquifers as a new source of freshwater for Texas.
Although brackish groundwater has not been well defined throughout the industry, a practical working definition is groundwater that contains 1,000–10,000 mg/L of total dissolved solids (TDS). Based on an analysis of water-quality data from over 40,000 existing wells in Texas (excluding deep saline wells), much of the state’s brackish water falls within the lower end of this range, which is generally the most cost-effective to treat. While brackish groundwater is scattered throughout the state, there are concentrations in the deeper portions of many dipping aquifers east of the I-35 corridor, in west Texas, and in south Texas, near the Mexico border.
Based on the results of this study and other research on brackish groundwater and treatment, the TWDB’s 2012 State Water Plan recommended brackish groundwater desalination projects across the state, in particular, along the Texas/Mexico border roughly between Brownsville and Rio Grande City, and in the San Antonio area. Many more projects are being contemplated in areas where brackish groundwater is available and municipal needs cannot be met by other water sources.
SAWS injection well
Cost-Effective to Treat and Transmit
Although the TWDB has also recommended new reservoirs in some areas, most of the state’s surface water is already appropriated, and permitting of new surface water supplies is time-consuming and costly. Groundwater already makes up 45–55% or more of the water supply in about half of the counties in the state–and perhaps closer to 100% in West Texas, especially under drought conditions. Brackish groundwater can be used to meet long-term demand or can serve as a drought supply for systems that use both groundwater and surface water. It can also serve as a small portion of a larger diversified water supply portfolio or as a transitional water supply source, for example, in rapidly growing suburban areas where long-term surface water supplies are being developed.
Desalination of brackish groundwater is also a cost-effective alternative to seawater desalination for drought-plagued inland locations. Pretreatment and treatment costs are generally lower than those for seawater because seawater contains higher concentrations of TDS (generally about 35,000 mg/L). And because it is located close to water demand, long-distance transmission costs can be reduced.
Brackish water RO treatment plants also have the advantage of scalability. A water supply entity can start small and double, triple, or quadruple its capacity to meet demand if the treatment facilities are designed with growth in mind.
On account of these factors, water suppliers are considering implementing desalination technology in existing groundwater pumping and treatment infrastructure in locations where brackish water is close to demand.
The major challenge associated with brackish groundwater desalination is the disposal cost for the concentrate, which can range from 5,000–15,000 mg/L TDS. Injection wells are typically the most cost-effective alternative for average to high volume of concentrate; for example, both the EPWU and SAWS facilities use this alternative. Small volumes of concentrate have been processed and blended through a surface water treatment plant or discharged to irrigation drainage ditches and other surface water features that already contain high TDS water.
Four Key Development Components
A brackish groundwater development project’s feasibility and, ultimately, its success depend on a thorough, simultaneous investigation of four key components: groundwater availability and quality, treatment and transmission, concentrate disposal, and permitting.
The investigation generally begins with a “desktop” evaluation of available data, including previous studies, geologic data, geophysical or electric log data, water chemistry and production data from existing wells, and regulations. For example, existing geophysical logs from the oil and gas industry enable hydrogeologists to identify the aquifers, rock and sediment characteristics, and make a basic assessment of salinity. Hydrogeologists use these data to identify potential locations for well fields and estimate well depth.
Field investigations–including test drilling, water-quality sampling and assessment, aquifer pumping tests–are essential to confirm the preliminary desktop assessment.
Assessing the Concentrate for EPWU
During development of the Kay Bailey Hutchison Desalination Plant, EPWU tapped LBG-Guyton for assistance in establishing brackish water monitoring and production wells, and for characterization, testing, and permitting of the proposed underground concentrate injection facility.
In 2002, EPWU drilled and monitored nine test wells to characterize a section of the aquifer and determine where fresh or brackish groundwater was present. Existing wells were also analyzed for their potential to supply the desalination facility. An RO pilot plant was constructed to test the process. In February 2005, US Army consultants completed environmental studies and published the Final Environmental Impact Study.
As noted above, concentrate disposal can be a challenging component of such a project. LBG-Guyton completed a hydrogeologic assessment to identify potential injection zones and coordinated the Texas Commission on Environmental Quality Underground Injection Control (TCEQ-UIC) permitting requirements necessary to identify and characterize the site location and the underground injection reservoir. This involved characterizing a 2.5-mile radius area-of-review for the sites, drilling and testing of wells completed to depths near 3,000 feet, surface and subsurface geophysical surveys, and water-quality sampling. The data were used to identify an injection zone that receives and contains up to 3 MGD of RO concentrate without affecting the shallower local fresh water supplies. The concentrate is pumped in a 22-mile pipeline to the deep injection wells.
SAWS Examined All of Its Options
The brackish desalination program for San Antonio is part of the SAWS 2012 Water Management Plan, which is designed to meet the city’s water needs for the next 50 years while reducing dependence on the Edwards Aquifer.
In 2006, LBG-Guyton completed an evaluation of the brackish groundwater resources of the Wilcox Formation in three counties to determine the feasibility of a long-term brackish groundwater supply for desalination by the City of San Antonio. The study included: collection of available geophysical logs, structural mapping of top and base of the Wilcox, delineation of Upper and Lower Wilcox units, mapping of net fresh and brackish sand thickness, determination of salinity, construction and testing of three deep production wells and two deep monitoring wells, and numerical groundwater modeling of the Wilcox and Carrizo aquifers.
Three test wells and two monitor wells were constructed in the Wilcox aquifer to depths ranging from approximately 1,700–2,600 feet, and tested at average rates ranging from 850–1,050 gallons per minute (gpm). Salinity of individual sand beds was initially estimated by comparing resistivity values from geophysical logs for individual sands to actual water chemistry of the same sands. Test wells produced groundwater with a range of 1,200–1,500 mg/L TDS. A numerical model was run to assess long-term impacts and to design the best well configuration for long-term production from well fields.
The RO plant will be located at the existing SAWS Twin Oaks Aquifer Storage & Recovery Site, which is close to the brackish water source and near the proposed areas for concentrate disposal. Eight of 12 Wilcox production wells, ranging from 1,200–1,800 feet deep, have been drilled on SAWS property.
Concentrate disposal will be accomplished through the use of injection wells located on SAWS property in nearby Wilson County. SAWS has completed the drilling and testing of a concentrate disposal well into the Edwards Formation. Results from this test injection well provide information regarding water quality, depth, injection pressures and characteristics of the disposal formation. This information will enable permitting, design, and construction of the remaining injection wells.
As these projects demonstrate, brackish groundwater can offer an alternative new supply for drought-plagued locations. Because feasibility and cost depend on many site-specific factors, it is best to consider financial, regulatory, hydrogeologic, and engineering issues simultaneously.
