The withdrawal of larqe amounts of ground water in the Houston-Galveston region, Texas, has resulted in water-level declines of as much as 250 feet (76 meters) in wells completed in the Chicot aquifer and as much as 300 feet (91 meters) in wells completed in the Evangeline aquifer during 1943-77. Since late 1976, changes in pumping distribution resulting from efforts to control subsidence and the introduction of surface water from Lake Livingston have altered the pattern of water-level changes. In the Johnson Space Center and Baytown-La Porte areas (Chicot aquifer), and in the Pasadena area (Evangeline aquifer), water levels rose about 20 feet (6.1 meters) during 1973-77. However, in the western Houston area (Evangeline aquifer), water levels have continued to decline at an increasing rate through 1977.
The declines in water levels have caused pronounced regional subsidence of the land surface. The center of regional subsidence is the Pasadena area, where more than 9 feet (2.7 meters) and possibly as much as 10 feet (3.0 meters) of subsidence occurred between 1906 and 1978. Almost 9 feet (2.7 meters) of subsidence occurred between 1943 and 1978. Localized centers of subsidence exist throughout the region, especially in the Baytown-La Porte and Texas City areas.
Evaluation of tide records from five gages in Galveston Bay and the tidal reaches of Buffalo Bayou indicates that changes in elevations of significantly less than 0.5 foot (150 millimeters) and possibly as little as 0.1 foot (30 millimeters) can be detected.
The unit measure of compressibility, specific-unit compaction, ranged from 1.0 x 10-5 to 4.0 x 10-5 feet-1 (3.28 x 10-5 to 1.31 x 10-4 m-1) of compaction per foot of clay thickness per foot of average water-level change for 1906-78. The greatest compressibility was at the Clear Lake site and the least compressibility was at the Lake Houston site. The data indicate that the compressibility is related to the age of the sediments and the depth of burial of the sediments.
