Economic Costs of Desalination in South Texas: A Case Study

Abstract

Estimated at more than 2.7 billion acre‐feet (LBG-Guyton, 2003), brackish groundwater (water with total dissolved solids concentration of 1,000 to 10,000 milligrams per liter) constitutes an important desalination water supply option in Texas. However, one of the more challenging issues—and a potential roadblock to more widespread implementation of desalination—is the lack of detailed information on parameters important to desalination for the brackish sections of Texas Water Development Board (TWDB) designated aquifers. In 2009, TWDB established the Brackish Resources Aquifer Characterization System program to map and characterize brackish groundwater in the state and facilitate the planning of desalination projects. As part of the program, the Pecos Valley Aquifer in Regional Water Planning Area F was selected for a pilot study. In addition to mapping and characterizing brackish water in the aquifer, the goals of the project were to develop techniques of data analysis and build a database management system that could be used in future brackish aquifer mapping projects and other geologic studies. The Pecos Valley Aquifer underlies an area of about 8,650 square miles in the Trans-Pecos area of West Texas and New Mexico. It is the primary source of water in the area. The underlying Edwards-Trinity (Plateau), Dockum, Rustler, and Capitan Reef Complex aquifers provide smaller volumes. For the study, we collected, analyzed, and interpreted thousands of water well and geophysical well logs to map the geologic units and establish stratigraphic relationships. We also gathered water chemistry, water level, and aquifer test data from a wide variety of sources to characterize groundwater in the Pecos Valley Aquifer. The Pecos Valley Aquifer consists of more than 1,700 feet of Tertiary and Quaternary alluvial sediments that are present in two hydrologically separate, approximately north-south trending solution basins known as the Pecos and Monument Draw troughs. Because there are several sub-basins within the two solution troughs that have not been penetrated by water wells, complete water chemistry for the entire aquifer could not be evaluated. Nevertheless, the chemical quality of water in the aquifer appears to be highly variable, changing with location and depth. In the Pecos Valley Aquifer, the concentration of total dissolved solids ranges from less than 200 to more than 10,000 milligrams per liter; silica from 1 to 83 milligrams per liter; iron from 0.01 to 4.5 milligrams per liter; sulfate from 2 to 4,208 milligrams per liter; and chloride from 3 to 7,280 milligrams per liter. In places, water quality has deteriorated as a result of past irrigation practices and oil and gas activities. We estimate that the Pecos Valley Aquifer contains about 15 million acre-feet of fresh water (0 to 1,000 milligrams per liter of total dissolved solids), 85 million acre-feet of brackish groundwater (1,000 to 10,000 milligrams per liter of total dissolved solids), and 1 million acre-feet of very saline water (>10,000 milligrams per liter of total dissolved solids). Brackish water is present almost everywhere in the aquifer but appears to be more prevalent in the central and western parts—areas where the saturated thickness of the aquifer is the greatest. The 2010 approved Region F water plan projects water shortages of about 28,887 acre-feet in 2010 increasing to 35,342 acre-feet in 2060. Desalination of brackish groundwater present in the Pecos Valley Aquifer may be one option to meet at least some of the projected shortages. While the project report presents important new information about the Pecos Valley Aquifer on a regional scale, the real value of the project is the new database and GIS datasets that were built and raw well records assembled for the project. These data sources—which were hitherto not available to the public—contain a wealth of groundwater data (raw and processed). Water planners can customize and use the data to develop more site-specific information to meet their needs. The pilot study has helped lay the foundation for future Brackish Resources Aquifer Characterization System projects by developing a database management system in which a variety of data can be stored and processed. Information contained in the report is not intended to serve as a substitute for site-specific studies that are required to evaluate local aquifer characteristics and groundwater conditions for a desalination plant.