Geology and ground-water resources of Galveston County, Texas.




Petitt, B.M., Jr.
Winslow, A.G.

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Galveston County, on the upper Texas Gulf Coast, is underlain by alternating beds of sand and clay. These strata crop out in belts roughly paralleling the coastline and dip gently southeastward at an angle greater than the slope of the land, thereby providing conditions favorable for artesian water. 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 age. Most of the potable water is obtained on the mainland of Galveston County. The water from wells on Galveston Island generally is highly mineralized. Prior to 1948 the water for all public supplies and nearly all industrial supplies in the county was derived from wells. The largest quantities of ground water used in the county are now pumped in the areas around Alta Loma and Texas City. The average daily pumpage in these areas increased from about 6 million gallons in 1938 to 17.8 million gallons in 1970 and reached a peak of about 34 million in 1945. From 1945 until 1948 the rate of pumpage was nearly constant. Surface water was diverted from the Brazos River to supply some of the industries in the Texas City area in 1948 and, as a result, the use of ground water was reduced by about 30 percent. Since 1948 the average daily pumpage in these areas has fluctuated between 22 and 24 million gallons. It is estimated that, in 1951, the total average daily withdrawal of ground water in Galveston County was approximately 28 million gallons. Water levels in wells declined as the pumpage increased however, since 1948, the water levels in many wells in the county have risen as a result of the decrease in pumpage, and others have tended to become stabilized. Subsidence of the land surface has occurred in a large part of the county, particularly in the Texas City area, and is probably the result of withdrawal of ground water. Salt-water encroachment has been a problem to water users 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 supplies were developed. Encroachment either from below or from down-dip has occurred with the lowering of the artesian pressure in the Alta Loma and Texas City areas. Results of pumping tests indicate that the average coefficient of transmissibility of the Alta Loma sand is 102,000 in the Alta Loma area and 153,000 in the Texas City area. The coefficients of transmissibility of the sands in the upper part of the Beaumont clay in the Texas City area average 27,300. Surface water from the Brazos River has been used for the irrigation of rice since 1942. In 1948 surface water was made available to the industries in Texas City and is being utilized increasingly instead of ground water. The water from the Brazos River is variable in quality, but probably it can be utilized on a somewhat larger scale than at present. Much additional ground water is available from both the Alta Loma sand and the upper part of the Beaumont clay, especially in the northern and western parts of the county. However, if large developments are planned, the proposed areas of development should be explored by test drilling. The problems of well spacing and pumping rates should be thoroughly studied, in order to permit planning the development so as to combat salt-water encroachment. Current observations should be continued, with special emphasis on the progress of salt-water encroachment.


219 p.


ground water, water resources, geology, water use, surface water, water levels