Hydrology of the Texas Gulf Coast aquifer systems

A complex, multilayered ground-water flow system exists in the Coastal Plain sediments of Texas. The Tertiary and Quaternary clastic deposits have an areal extent of 114,000 square miles onshore and in the Gulf of Mexico. Two distinct aquifer systems are recognized within the sediments, which range in thickness from a few feet to more than 12,000 feet The older system--the Texas coastal uplands aquifer system-consists of four aquifers and two confining units in the Claiborne and Wilcox Groups. It is underlain by the practically impermeable Midway confining unit or by the top of the geopressured zone. It is overlain by the nearly impermeable Vicksburg-Jackson confining unit, which separates it from the younger coastal lowlands aquifer system. The coastal lowlands aquifer system consists of five permeable zones and two confining units that range in age from Oligocene to Holocene. The hydrogeologic units of both systems are exposed in bands that parallel the coastline. The units dip and thicken toward the Gulf. Quality of water in the aquifer systems is highly variable, with dissolved solids ranging from less than 500 to 150,000 milligrams per liter.

Substantial withdrawal from the aquifer systems began in the early 1900's and increased nearly continuously into the 1970's. The increase in withdrawal was relatively rapid from about 1940 to 1970. Adverse hydrologic effects, such as saltwater encroachment in coastal areas, land-surface subsidence in the Houston-Galveston area, and long-term dewatering in the Whiter Garden area, were among some of the factors that caused pumping increases to slow or to cease in the 1970's and 1980's.

Ground-water withdrawals in the study area in 1980 were about 1.7 billion gallons per day. Nearly all of the withdrawal was from four units: Permeable zones A, B, and C of Miocene age and younger, and the lower Claiborae-upper Wilcox aquifer. Ground-water levels have declined hundreds of feet in the intensively pumped areas of Houston-Galveston, Kingsville, Winter Garden, and Lufkin-Nacogdoches. Water-level declines have caused inelastic compaction of clays which, in turn, has resulted in land-surface subsidence of more than one foot in an area of about 2,000 square miles. Maximum subsidence of nearly 10 feet occurs in the Pasadena area east of Houston.

A three-dimensional, variable-density digital model was developed to simulate predevelopment and transient flow in the aquifer systems. The modeled area is larger than the study area, and includes adjacent parts of Louisiana and Mexico. The transient model calibration period was from 1910 (predevelopment) to 1982. Model-generated head distributions, water-level hydrographs, and land-surface subsidence were matched to measured data in selected, intensively pumped areas.

For the study area, mean horizontal hydraulic conductivity in the calibrated model ranges from 10 feet per day for the middle Wilcox aquifer to 25 feet per day for permeable zone A. Mean transmissivity ranges from about 4,600 feet squared per day for the middle Claiborne aquifer to about 10,400 feet squared per day for permeable zone D. Mean vertical hydraulic conductivity ranges from 1.1x10^-5 feet per day for the Vicksburg-Jackson confining unit, to 3.8x10^-3 feet per day for permeable zone A. Mean values of calibrated storage coefficient range from 52x10^-4 for the middle Claiborne aquifer to 1.7x10^-3 for the middle Wilcox aquifer and permeable zone C. Calibrated inelastic specific storage values for clay beds in permeable zones A, B, and C in the Houston-Galveston area are 8.5x10^-5, 8.0x10^-5, and 8.0x10^-6 feet^-1, respectively. These values are 85, 80, and 8 times greater than the estimated elastic specific storage value for the clays in permeable zones A, B, and C, respectively.

Recharge rates were mapped for predevelopment conditions as determined from a steady-state model calibration. A maximum rate of 3 inches per year was simulated in small areas, and the average rate for the study area was 034 inch per year. Total simulated recharge was 85 million cubic feet per day in the outcrop area. Recharge was equal to discharge in outcrop areas (79 million cubic feet per day) plus net lateral flow out of the study area (6 million cubic feet per day).

Rates of inflow and outflow to the ground-water system have nearly tripled from predevelopment to 1982 (85 to 276 million cubic feet per day) based on model simulation. Withdrawal of 231 million cubic feet per day was supplied principally by an increase in outcrop recharge and, to a lesser extent, from a decrease in natural discharge and release of water from storage in aquifers and compacting clay beds. The average simulated 1982 recharge rate for the study area was 0.52 inch per year, with a maximum simulated rate of 6 inches per year in Jackson and Wharton Counties.

Because withdrawal has caused problems such as saltwater intrusion, land-surface subsidence, and aquifer dewatering, the Texas Department of Water Resources has projected that ground-water use will decline substantially in most of the study area by the year 2030. Some areas remain favorable for development of additional ground-water supplies. Pumping from older units that are farther inland and in areas where potential recharge is greater will minimize adverse hydrologic effects.