Reconnaissance investigation of the ground-water resources of the Gulf Coast Region, Texas.
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The Gulf Coast region, as the term is used in this report, includes all or parts of 51 counties adjacent to the Gulf of Mexico between the Rio Grande on the Mexico border and the Sabine River on the Louisiana border, about 35,000 square miles. Its population is about 2,900,000. The climate ranges from semiarid in the southwestern part to humid in the northern part. Irrigation is practiced throughout the region, rice being the principal irrigated crop in the northeastern half. Cotton, amize, and other row crops are irrigated during dry periods in all but the northeastern fourth, and citrus fruit and vegetables are irrigated in the Lower Rio Grande Valley. Most of the Gulf Coast region is a smooth, featureless depositional plain rising from sea level to an altitude of 200 feet, although the interior boundary is as high as 900 feet. The sediments cropping out range in age from upper Locene to Recent. In vertical section, the geologic formations underlying the region form a series of gently dipping truncated wedges which thicken toward the wedge. Recent deposits form the wedge nearest the coast, and successively older crops consist of beds, lenses, and stringers of gravel and coarse to fine sand interbedded with silt and clay beds and lenses; downdip the lithology changes gradually to dominantly silt and clay. The principal aquifer includes the Goliad Sand, Willis Sand, and Lissie Formation. Of less importance is the aquifer that includes the Catahoula Sandstone, Oakville Sandstone, and the Lagarto Clay. The Beaumont Clay generally is an aquifer between the Nueces and Sabine Rivers, and the alluvium of Recent age is an aquifer in the Rio Grande Valley and the Brazos River Valley. Ground water in the region is classified as fresh--less than 1,000 ppm of dissolved solids--or slightly saline--1,000 to 3,000 ppm of dissolved solids. The base of the fresh to slightly saline water zone and the thickness of water-bearing sand in the zone are shown on maps prepared from electric logs of oil and water wells. The base of the fresh to slighly saline water zone is as much as 3,600 feet below sea level, and sands in the zone are as thick as 1,400 feet. The upper limits of dissolved solids used as a basis for the preparation of the map of the base of the fresh to slightly saline water varied according to the availability and use of the water in the region. Northeast of the Guadalupe River, most of the ground water classes as fresh to slightly saline contained less than 1,200 ppm of dissolved solids. In places southwest of the Guadalupe River, ground water that contains as much as 3,000 ppm of slightly saline water were based on that quantity. In general, the fresh to slightly saline water zone is thickest, contains the most water-bearing sand, contains the best quality water, and is capable of yielding the largest quantities of water to wells in the part of the region northeast of the Guadalupe River. In 1959, about 920,000 acre-feet of ground water, or about 820 million gallons per day, was pumped in the region. About 55 percent was for irrigation, about 23 % was for industry, and about 20 percent was for public supply. About 3 percent was from domestic and livestock wells, from miscellaneous small wells, and from flowing wells. A map showing the estimated transmissibility of all the sands in the fresh to saline water zone was prepared from the results of about 300 pumping tests and the map showing the thickness of the sand. To compare the potential quantity of ground water available in one area with that in another, the map was used with several assumptions to calculate the length of time necessary to lower the water level to a maximum depth of 400 feet along the line of discharge. The determined factors have a great bearing on the availability of ground water. Among these are the amount of recharge to the aquifers, the amount of natural lowered artesian pressure, and the amount of water that will be released by compaction of the clays as the artesian pressures are lowered. The computations maintain adequate recharge to equal or surpass the transmission capacity of the aquifers (quantity of water transmitted through a given width of an aquifer at a given hydraulic gradient). The aquifers in the part of the region that have the lowest potential recharge have small transmission capacities.