Geology and ground-water resources of Galveston County, Texas

Abstract

Galveston County, on the Texas gulf coast, is underlain by alternating beds of sand and clay. These sand and clay strata crop out in belts that roughly parallel the coastline and dip gently southeastward at an angle gre? +,er than the slope of the land, thereby creating artesian aquifers. The formations that yield potable water to wells are the Lissie formation, the "Alta Loma" sand and other sands of the Beaumont clay, and beach and dune sands of Recent aie. Most of the potable water is obtained on the mainland of Galveston County. The water from most wells on Galveston Island is highly mineralized. Before 1948, water for all public use and nearly all industrial use was derived from wells. Most ground water now used in the county is pumped from areas around Alta Loma and Texas City. The average daily pumpage in these areas increased from 6 million gallons in 1938 to 17.8 million gallons in 1940 and reached a peak of about 34 million gallons in 1945. Between 1945 and 1948 the rate of pumpage was nearly constant, but in 1948 surface water was diverted from the Brazos River to supply some of the Texas City industries and, as a result, the use of ground water was reduced about 30 percent. Water levels declined in county wells as the pumpage increased during the years prior to 1948. Since water from the Brazos River has been utilized the levels have risen in many wells and tended to become constant in others. Subsidence of the land in a large part of the county, particularly in the Texas City area, is attributed to the excessive withdrawal of ground water. Salt-water encroachment has been a problem in the county for many years. Salt water was present in the lower part of the "Alta Loma" sand in the Alta Loma and Texas City areas and throughout that sand on Galveston Island when the first wells were drilled. Encroachment from either below or downdip took place with the lowering of artesian pressure in the aquifer in the vicinity of Alta Loma and Texas City. Pumping tests reveal that the average coefficient of transmissibility of the "Alta Loma" sand is 102,000 at Alta Loma and 153,000 at Texas City. The coefficients- of transmissibility of sands in the upper part of the Beaumont clay around Texas City average 27,300. Surface water from the Brazos River, used for the irrigation of rice since 1942, was made available in 1948 to industries in Texas City as a substitute for ground water. The water from the Brazos River is variable in quality, but probably can be utilized on a somewhat larger scale than at present. Much additional ground water could be obtained from both the "Alta Loma" sand and the upper part of the Beaumont clay, especially in the northern and western parts of the county. Before large developments of supplies are planned, however, these areas should be explored by test drilling. The problems of well spacing and pumping rates should be thoroughly studied in order to determine the maximum development permitted by the ground-water supply. Current observations should be continued with special emphasis on the progress of salt-water encroachment.