Gulf Coast Aquifer, Texas

Lynn Lefebvre

Spring 2010
GO 571 - Hydrogeology
Dr. Marcia K. Schulmeister


Introduction

Geography and Climate

Geologic Setting

Groundwater

Hydraulic Properties

Contamination

References


Introduction

The Gulf Coast Aquifer is a coastal aquifer stretching all along the Gulf Coast, from Florida through Texas and into Mexico. It lies below the southern portions of Mississippi, Alabama, and Georgia, along with over half the states of Louisiana and Florida. The Gulf Coast Aquifer is over 100 miles wide in most sections. The aquifer is essential for providing drinking water for millions of people. Heavy irrigation of this resource is necessary to maintain large agricultural yields.

The focus for this research project is the Texas portion of the aquifer. This includes the major cities of Houston, Galveston, Corpus Christi, Brownsville, and Beaumont. More water is pumped out of the Gulf Coast Aquifer than any other aquifer in Texas, except for the Ogallala Aquifer (Mace, 2006).


Figure 1. Skyline of Houston, Texas (Houston, 2007).


Geography and Climate


Figure 2. Texas extent of Gulf Coast Aquifer (Chowdhury, 2006).
The region experiences a humid subtropical climate. Average high temperatures (F) range from the 60s in the winter to the 90s for the summer. Frost events are a rare occurrence. Being located on the Gulf Coast, this area is a highly active region for heavy rainfall, hurricanes, and tropical storms. The predominate southeast winds bring in warm moist air from the gulf. The amount of rainfall varies throughout the region. Gulf currents bring in over 55 inches precipitation a year in east Texas. However, the far south western portion sees only 20 inches annually (Mace, 2006). The area shows extremely gradual relief by rising only about one foot per mile coming in from the coast (Coplin, 2007). The relief increases slightly further inland. The combination of hurricanes and low relief makes the coastal region highly susceptible to flooding. The low elevation throughout the region also keeps the water table near the surface. The high water table allows little room for the unsaturated zone.

This aquifer is affected by the large amount of surface water nearby. Most notably the Gulf of Mexico and Galveston Bay. These bodies of water supply vast amounts of water to the aquifer. The low elevations allow for many thousands of acres of marshland along the coast. Other surface water features play a much lesser role in the aquifer. As shown on the map to the left, the region contains eleven major rivers as well as many smaller ones. Only a few large lakes and reservoirs lie in this area.

The Texas region of the Gulf Coast Aquifer features a population of over 8 million. This is over a third of the population of the state. Supporting this many people requires extensive pumping of available water resources (Mace, 2006). The Houston metropolitan area produces the highest well yields of 1,600 gallons per minute (Ashworth, 1995). Over 1.1 million acre-feet of groundwater pumped annually out of the aquifer (Chowdhury, 2006). This heavy pumping has led to water table declines of 200-300 feet in Harris County (Ashworth, 1995).

Land subsidence affects the city of Houston as a result of the extensive water pumping and also, to a lesser degree, petroleum pumping. The population explosion resulting from the oil boom began a steady subsidence of about one foot for every ten years (Coplin, 2007).


Geologic Setting

The aquifer is a large, leaky artesian aquifer system. The system is a complex of interbedded clays, silts, sands, and gravels. It is subdivided into four major components. The uppermost and easternmost component, the Chicot Aquifer, consists of alluvial deposits and low permeable clay layers with some interbedding sands. The next component is the Evangeline Aquifer. This layer includes two prominent sand formations, making it more permeable than the Chicot. The Burkeville confining system separates the Evangeline from the next component, the Jasper Aquifer. This formation is mostly impermeable due to the presence of the Oakville Sandstone. Below the Jasper is the Catahoula, a layer containing restricted sand layers. The maximum total sand thickness reaches 700 feet in the south and 1,300 feet in the northeast (Ashworth, 1995).


Figure 3. Major components of the Gulf Coast Aquifer (Ashworth, 1995).

Figure 4. Location of cross section dipicted on Figure 3 (Ashworth, 1995).


Groundwater

Most of the groundwater for the aquifer is used for irrigation and municipal purposes. The chart on the right illustrates the high usage. The demand is even higher a decade later with over 90% used for irrigation and city water supplies (Ashworth, 1995). The continuous growth of the Houston area keeps the need for municipal water on the rise. The predominate crops in the region are the water intensive crops of cotton and rice.

Typical groundwater composition samples contain more than 95% percent calcium, sodium, potassium, magnesium, bicarbonate, chloride, and sulfate. Total dissolved solids were generally less than 1000 mg/l. Three locations in the Chicot Aquifer contained values higher than 3000 mg/l. These locations had high salinity concontrations (Mace, 2006).


Figure 5. Groundwater use of the Gulf Coast Aquifer in 1985 (Ryder, 1996).


Hydraulic Properties

The geometric average for hydraulic conductivity ranged from 11-98 feet per day for each county in a nine county study of the central coast. The overall average hydraulic conductivity was 19 feet per day. Harris County, the most populous county and home to Houston had a hydraulic conductivity of 27 feet per day. The majority of the pumping tests in this study produced specific capacity values in the range of 6-10 feet per day with a few being higher or lower (Young, 2006).

The table below shows values from the Brazos Alluvium in the upper most part of the aquifer (Mace, 2006).

Hydraulic PropertyValues Range Average Value
Transmissivity50,000-300,000 gallons per day per foot
Hydraulic conductivity <1 - 2,400 feet per day about 290 feet per day
Specific yield 4-35% about 24%
Well yields 250-500 gallons per minute
Total dissolved solids <500 - >3,000 milligrams per liter
Storativity values between the Chicot Aquifer and the Evangeline Aquifer displayed little variation. The storativity range for Chicot was 10^-4 - 0.2 compared to 4 x 10^-5 -0.2 for the Evangeline (Mace, 2006).


Contamination

With the aquifer located next to the coast, the biggest concern for contamination is salt water intrusion. Overpumping in many areas along the coast has drawn more salt water inland. This concern along with the high rates of land subsidence has forced a reduction in pumping along the coast. As a result, more pumping occurs further inland.

The massive petrochemical industry in this part of Texas creates a large potential for contamination. Contamination can result from oil and gas pipeline leaks, irresponsible environmental practices, and accidental spills. This is avoided largely by numerous environmental regulators and hefty fines issued for violations.

Arsenic contamination is present in the Nueces River basin. Most wells tested contained less than 50 g/l. Contamination levels were measured as high as 600 g/l (Mace, 2006). However, both figures are well above the World Health Organization standard of 10 g/l for drinking water (Scientific, 2009). The higher concentrations came from the Jasper Aquifer further inland (Mace, 2006).


Figure 6. BP oil refinery in Texas City, the third largest refinery in the United States. BP p.l.c. (BP, 2010).

References

Ashworth, J.B. and J. Hopkins, 1995. Aquifers of Texas. Texas Water Development Board Report 345.
http://www.twdb.state.tx.us/publications/reports/GroundWaterReports/GWReports/R345 Aquifers of Texas/R345Complete.pdf. (accessed 26 Apr. 2010).

BP, 2010. Aerial shot of the Texas City refinery. http://www.bp.com/popuppreviewthreecol.do?categoryId=121&contentId=7027647. (accessed 27 Apr. 2010).

Chowdhury, A.H., and M.J. Turco, 2006. Geology of the Gulf Coast Aquifer, Texas. http://www.twdb.state.tx.us/publications/reports/GroundWaterReports/GWReports/R365/ch02-Geology.pdf. (accessed 26 Apr. 2010).

Coplin, L.S. and D. Galloway, 2007. Land Subsidence in the United States. Houston-Galveston, Texas: Managing coastal subsidence. U.S. Geological Survey Circular 1182. http://pubs.usgs.gov/circ/circ1182/pdf/07Houston.pdf. (accessed 28 Apr. 2010).

Houston Cityscape, 2007. Wikipedia. http://en.wikipedia.org/wiki/File:Houston_Cityscape.jpg. (accessed 27 Apr. 2010).

Mace, R.E., S.C. Davidson, E.S. Angle, and W.F. Mullican III, 2006. Aquifers of the Gulf Coast. Texas Water Development Board Report 365. http://www.twdb.state.tx.us/publications/reports/GroundWaterReports/GWReports/R365/R365_Composite.pdf. (accessed 26 Apr. 2010).

Ryder, P.E., 1996. Coastal Lowlands Aquifer System. U.S. Geological Survey Hydrologic Atlas 730-E. http://pubs.usgs.gov/ha/ha730/ch_e/E-text6.html. (accessed 26 Apr. 2010).

Scientific Facts on Arsenic, 2009. GreenFacts Scientific Board. http://www.greenfacts.org/en/arsenic/index.htm. (accessed 27 Apr. 2010).

Young, S.C., P.R. Knox, T. Budge, V. Kelley, N. Deeds, W.E. Galloway, and E.T. Baker, 2006. Stratigraphy, Lithology, and Hydraulic Properties of the Chicot and Evangeline Aquifers in the LWSP Study Area, Central Texas Coast. Aquifers of the Gulf Coast. Texas Water Development Board Report 365. http://www.twdb.state.tx.us/publications/reports/groundwaterreports/gwreports/r365/ch06-young_k&geology v2.pdf. (accessed 27 Apr. 2010).