The Carrizo-Wilcox Aquifer in Texas

Scott Jones

Spring 2008

Introduction to Hydrogeology   *    Emporia State University

Hydrologic Setting

Hydrologic & Geologic Properties

Water Resources

Groundwater Quality & Contamination



(ACBL District 16, 2006).

Hydrologic Setting

The Carrizo-Wilcox Aquifer in Texas makes up part of the larger Texas Coastal Uplands Aquifer System, but the Carrizo-Wilcox is no small aquifer. It extends from the Rio Grande in South Texas and into parts of Arkansas and Louisiana, supplying water to 60 Texas counties. Many of the studies completed on the Carrizo-Wilcox Aquifer have focused on the Texas portion of the aquifer, and in Texas the aquifer can be broken down into three regions: Northern, Central, and Southern (Figure 1). The Northern region is bounded in the northeast by the Red River in Louisiana and Arkansas and by the surface water basin divide between the Trinity and Brazos Rivers in the southwest. The Central region is bounded by the course of the San Antonio River in the southwest and then by the aquifer outcrop in Van Zandt County, across part of the East Texas Basin and the Sabine Uplift, and into the deep part of the Carrizo–Wilcox aquifer in the northeast. The Southern region is bounded laterally on the northeast by the surface water basin divide between the Guadalupe and Colorado Rivers and to the southwest by the Rio Grande (Deeds et al., 2003; Dutton et al., 2003; Fryer et al., 2003).

Figure 1-Carrizo-Wilcox Aquifer in Texas (Dutton et al., 2003).

The average annual precipitation in the Carrizo Wilcox Aquifer region ranges from approximately 21 inches in the Rio Grande Valley to approximately 50 inches at the Louisiana border. Generally, precipitation amounts in the region of the aquifer increase from west to east. The average annual temperature is approximately 65ºF, but average monthly temperatures can range anywhere from 32ºF to 95ºF. Evaporation rates are usually greatest in the Southern region of the aquifer, and the rates tend to decrease in the Central and Northern regions (Deeds et al., 2003; Dutton et al., 2003; Fryer et al., 2003).

The aquifer is recharged with water from rainfall or from streams infiltrating the outcrop. The Northern region of the aquifer receives the majority of its water from the Brazos, Trinity, Neches, Sabine, and Red River basins. The drainage area of these river basins is nearly 115,000 square miles, and there are 48 major reservoirs. These rivers are perennial and gain flow from the underlying geology. The Central region not only relies on water from the Brazos and Trinity River basins, but also from the Colorado River basin. These rivers are deeper and broader than the major rivers to the south. The Southern region of the aquifer obtains the majority of its water from the Rio Grande, Nueces, San Antonio, Guadalupe, Colorado, and Lavaca River basins. These rivers tend to be perennial as they often lose water to the underlying material during dry times, but the drainage basins encompass about 117,000 square miles. There are also 21 major reservoirs in this region (Deeds et al., 2003; Dutton et al., 2003; Fryer et al., 2003).

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Hydrologic & Geologic Properties

The Carrizo-Wilcox Aquifer is made up of the Wilcox Group and the overlying Carrizo Formation of the Claiborne Group. The Carrizo Sand and Wilcox Group crop out along a narrow band that lies parallel to the Gulf Coast and dips beneath the land surface toward the coast (see Figure 2), except in the East Texas structural basin where a trough has formed (Deeds et al., 2003; Dutton et al., 2003; Fryer et al., 2003).

Figure 2-Outcrop and direction of Carrizo-Wilcox Aquifer in Texas (TWBD, 2008).

The aquifer is composed mainly of hydraulically connected sands followed by gravel, silt, clay, and lignite deposited during the Tertiary Period. The thickness of the sand can range anywhere from 0 ft to 3,000 ft (see Figure 3), but the total aquifer thickness of the aquifer can reach a total thickness of 10,000 ft. The Carrizo is a homogeneous sand unit overlying the thicker, more heterogeneous Wilcox Group (Deeds et al., 2003; Dutton et al., 2003; Fryer et al., 2003).

Figure 3-Depth of sand in the Carrizo-Wilcox Aquifer in Texas (Ryder, 1996).

In the Central region, the Wilcox Group can be broken down into the Hooper (oldest), Simsboro, and Calvert Bluff (youngest) Formations. See Figure 4 for rock sequences in the Central Carrizo-Wilcox Aquifer. The Carrizo and Simsboro Formations are important in determining groundwater availability because they contain more permeable and thicker sands. The Calvert Bluff and Hooper Formations are made of clay, silt, sand, and lignite mixtures; they generally have a low vertical permeability, which makes them act as leaky aquitards that confine fluid pressures in the Carrizo and Simsboro and restrict groundwater movement between the layers (Dutton et al., 2003).

The Northern region is composed of sediments that are part of a gulf-ward thickening wedge of Cenozoic sediments deposited in the East Texas Basin and the Houston Embayment of the Gulf Coast Basin. See Figure 4 for rock sequences in the Northern Carrizo-Wilcox Aquifer. The southwest part of the Northern region is similar to the Central region. The northeast part though is missing the Simsboro Formation; therefore the Wilcox is divided into an upper and a lower. The Carrizo Sand, composed of homogenous fluvial sands, unconformably overlies the more heterogeneous Wilcox Group. The northeastern part of this portion of the aquifer is known as the Cypress Aquifer due to the lack of confining units between aquifers (Fryer et al., 2003).

The sediments of the Southern region are part of a gulf-ward thickening wedge of Cenozoic sediments deposited in the Rio Grande Embayment of the northwest Gulf Coast Basin. The main sediments consist of fluvial-deltaic sediments of the upper Paleocene and lower Eocene Wilcox Group and Carrizo Sand. See Figure 4 for rock sequences in the Southern Carrizo-Wilcox Aquifer. Each of the depositional sequences was deposited during the Paleogene, and each is bounded by marine shales and finer grained sediments. The Reklaw Formation, consisting of mud and sand, bounds the aquifer from above in the northeast, which creates a semi-confining unit between the Carrizo Sand and the shallow aquifer of the Queen City Formation. The Bigford Formation is the same type of formation as the Reklaw but it is located in the southwest (Deeds et al., 2003)

Figure 4-Stratigraphic section of Carrizo-Wilcox Aquifer (Fryer et al., 2003).

Hydraulic conductivity (K) is highly variable in this aquifer. In the Wilcox Group, areas where the sand deposits are present the K value can range from 20 to 60 ft/day, yet the areas with inter-channel sands and muds can have average K values between 3 and 7 ft/day. The more uniform Carrizo Formation has K values similar to the sandy Wilcox Group’s K values, but there is still a large range (Thorkildsen and Price, 1991). For the entire aquifer the average K value is around 6 ft/day, but the K value can range from 0.01 ft/day to 4,000 ft/day. The average transmissivity is approximately 300 ft2/day but can range from around 0.1 ft2/day to about 10,000 ft2/day. The hydraulic conductivity and transmissivity vary both vertically and areally throughout the Carrizo-Wilcox Aquifer (Mace et al., 1999).

Storativity in the aquifer ranges from about 10-6 to 10-1 with an average storativity of 3.0 * 10-4. Specific storage ranges from about 10-7 to 10-3 with an average of 4.5 * 10-6. Lower values occur at shallower depths where the aquifer is unconfined (Mace et al., 1999).

Groundwater in the aquifer exists under both water-table and artesian conditions. Water-table conditions usually occur in the outcrop areas, and artesian conditions occur where the aquifer is overlain by confining beds with lower hydraulic conductivity rates. Well yields are usually 500 gal/min, but some may reach 3,000 gal/min downdip where the aquifer is under artesian conditions (Thorkildsen and Price, 1991).

The groundwater in the Carrizo-Wilcox Aquifer is naturally fresh to slightly saline in some places. The chemical quality of the groundwater in the aquifer declines with depth. The deeper waters have higher temperatures and have been under greater pressures, which have led to greater mineralization. The worst chemical character of the groundwater is that it is high in iron content, which was dissolved from rocks and soils (Thorkildsen and Price, 1991). Flouride and chloride are also present in the groundwater as natural chemicals that dissolved from rocks and soil, but chloride can also be found in the salty water deep within the aquifer (Follett, 1974). Hydrogen sulfide and methane may be present in some areas (Thorkildsen and Price, 1991).

The area above the aquifer does include an unsaturated zone. In some areas this zone is larger than others due to the vegetative cover, type of sediments, evaporation rates, thickness of the underlying units, and other factors.

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Water Resources

The groundwater pumped from the Carrizo-Wilcox aquifer is used primarily for municipal public water supply, rural domestic use, and manufacturing in approximately 60 counties in Texas. Approximately 35 percent of the total groundwater removed from the aquifer is for municipal water supply; the largest public water supplies, Bryan-College Station, Lufkin-Nacogdoches, and Tyler, make up a large portion of that percentage as they receive all of their water from the Carrizo-Wilcox aquifer. These major municipalities include over 370,000 people, but the aquifer as whole provides water to ten to twelve million people (National Wildlife Federation et al, 2006). The Southern region of the aquifer is usually pumped heavily for irrigation purposes, but irrigation pumping occurs throughout the aquifer accounting for nearly 51 percent of the total groundwater removed (see Figure 5 for water use percentages). Lignite mining operations also remove a significant portion of groundwater from the central area of the aquifer. Much of the water from the streams, rivers, and lakes never reaches the water table because of high evaporation rates in this humid region (Deeds et al., 2003; Dutton et al., 2003; Fryer et al., 2003).

Figure 5-Use of groundwater withdrawn from the Carrizo-Wilcox in 1985, in percent (Ryder, 1996).

The water supply has not been monitored or managed carefully over the years. Water-level declines developed in semi-arid regions that were heavily dependent on ground water for irrigation. Water levels have declined as much as 100 feet in some areas and more than 250 feet in a few counties. Municipal and industrial pumping has caused major declines in northeast, and areas underlying Tyler and Lufkin-Nacogdoches region have experienced declines as much as 500 ft since the 1940s. On a positive note, some areas have made the conversion to more surface-water use rather than groundwater use (Deeds et al., 2003; Dutton et al., 2003; Fryer et al., 2003). The Texas Natural Resource and Conservation Commission and the Texas Groundwater Protection Committee are the major groups that oversee the management and quality control of the groundwater from the Carrizo-Wilcox aquifer, and these groups are taking actions to safe-guard the water supply in the aquifer.

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Groundwater Quality & Contamination

The Carrizo-Wilcox Aquifer is naturally fresh to slightly saline in some places. Ground water samples taken throughout the aquifer indicate excessive dissolved solids, high iron, high flouride and high chloride concentrations. Many of the samples taken have indicated dissolved solid levels lower than 1,000 mg/L (see Figure 6), and the majority of the areas have dissolved solid levels lower than 500 mg/L (Follett, 1974).

Figure 6-Dissolved-solids concentrations in the Carrizo-Wilcox (Ryder, 1996).

In some areas pesticides and insecticides have been detected, but the concentrations do not exceed EPA (Environmental Protection Agency) standards. There are some areas where the alluvium is in hydraulic connection with the Carrizo Sand, which would mean any contaminant in the rivers would infiltrate sand and contaminate a portion of the aquifer. There no known drinking water wells are present in these areas where the aquifer sand and alluvium are connected. Overall, the Carrizo-Wilcox Aquifer is thought to be at a low to moderate level of risk to a spill in the vicinity of the aquifer.

When contaminants are discovered the Texas Development Water Board along with the Texas Commission on Environmental Quality, Department of State Health Services, Texas Department of Agriculture, Texas State Soil and Water Conservation Board, Texas Alliance of Groundwater Districts, Texas Agricultural Experiment Station, Bureau of Economic Geology, Railroad Commission of Texas, and Texas Department of Licensing and Regulation are the groups that may be involved in the clean-up. If the contamination is large enough the EPA will also aid in clean-up efforts (Texas Groundwater Protection Committee, 2006).

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The groundwater of the Carrizo-Wilcox Aquifer in Texas is one of the greatest assests of the East-Central region of Texas. It provides water for agriculture, industry, and human consumption and use. Over-pumping has caused major declines in some areas, and with an increasing population pollution becomes more of an issue. Now and in the future, the aquifer needs to be carefully monitored, managed, and protected so it can provide millions of people the water they need to survive.

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American Contract Bridge League (ACBL), District 16 . 2006. [retrieved on 1, May 2008].

Deeds, N., Kelley, V., Fryar, D., Jones, T., Whallon, A.J., and Dean, K.E. 2003. Groundwater availability model for the Southern Carrizo-Wilcox Aquifer. Final report, prepared for Texas Water Development Board. [retrieved on 1, May 2008].

Dutton, A.R., Harden, B., Nicot, J.P., O’Rourke, D., and Tinker, S.W. 2003. Groundwater availability model for the central part of the Carrizo-Wilcox Aquifer in Texas. Final report, prepared for Texas Water Development Board. [retrieved on 1, May 2008].

Fryar, D., Senger, R., Deeds, N., Pickens, J., Jones, T., Whallon, A.J., and Dean, K.E. 2003. Groundwater availability model for the Northern Carrizo-Wilcox Aquifer. Final report, prepared for Texas Water Development Board. [retrieved on 1, May 2008].

Follett, C.R. 1974. Ground-water resources of Brazos and Burleson Counties, Texas. Report prepared by United States Geological Survey for the Texas Water Devlopment Board, USGS Report 185, 105 p.

Mace, R.E, Smyth, R.C., Xu, L., and Liang, J. 1999. Transmissivity, hydraulic conductivity, and storativity of the Carrizo-Wilcox Aquifer in Texas. Bureau of Economic Geology, The University of Texas at Austin, final report and CD-ROM submitted to the Texas Water Development Board, 76 p. [retrieved on 1, May 2008].

Ryder, P.D. 1996. Texas Coastal Upland Aquifer System. GROUND WATER ATLAS of the UNITED STATES, Oklahoma, Texas, USGS Report #HA 730-E. [retrieved on 1, May 2008].

Texas Groundwater Protection Committee. 2006. Joint groundwater monitoring and contamination report—2005, 84 p. [retrieved on 1, May 2008].

Texas Water Development Board, (TWDB). last update unknown. Carrizo-Wilcox Aquifer. [retrieved on 1, May 2008].

National Wildlife Federation, Environmental Defense, and Lone Star Chapter of the Sierra Club. 2006. The 16 water regions of Texas. [retrieved on 1, May 2008].

Thorkildsen, D. and Price, R.D. 1991. Ground-water resources of the Carrizo-Wilcox Aquifer in the Central Texas Region. Texas Water Development Board, Report #332, Austin, TX, 46 p. [retrieved on 1, May 2008].

This webpage was designed for Introduction to Hydrogeology
Instructor: Dr. Marcia K. Schulmeister of Emporia State University
For questions or comments contact Scott Jones
Created on May 1, 2008