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The withdrawal of large 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-') 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.

Fault creep has stopped or slowed in the eastern part of the Houston, Texas, land subsidence area where reductions in pumping of ground water have allowed water levels to recover partially. Creep has continued across faults in the western part where heavy pumping causes water levels to continue to decline. The observations support the long‐suspected relation between historical faulting and withdrawal of ground water and indicate that partial water‐level recoveries can reduce fault movement and structural damage.

Total withdrawals of ground water in the Houston district increased 9 percent from about 488 million gallons per day (21.4 cubic meters per second) in 1970 to about 532 million gallons per day (23.3 cubic meters per second) in 1974. The average annual rate of increase from 1960 to 1969 was about 6.3 percent. During 1970-74, increases in pumpage occurred in the Houston, Katy, and NASA areas; decreases occurred in the Pasadena and Alta Loma areas; and the pumpage in the Baytown-La Porte and Texas City areas remained almost constant. Water levels continued to decline throughout the district during 1970-74, but the rate of decline generally was not as great as in previous years. The greatest declines in the past several years were in the Houston area, but the center of decline is still in the Pasadena and Baytown-La Porte areas. The decrease in the rate of decline suggests that the aquifers in the Houston district could support the amount of pumping during 1970-74 with little, if any, further decline. Although saltwater encroachment has probably occurred in the district, particularly in Galveston County, no large increases in chloride were measured at the monitoring points. 4 DEVELOPMENT OF GROUND WATER IN THE HOUSTON DISTRICT, TEXAS, 1970-74 DEVELOPMENT OF GROUND WATER