Decline in ground-water levels in the sand- stone aquifer, Chicago and Milwaukee areas, 1864-1980 (Alley and others, 1999).
of Mexico into several of the aquifers (Taylor and Alley, 2001).
In the Houston, Texas, area, exten- sive ground-water pumping to support economic and population growth has caused water-level declines of approx- imately 400 feet, resulting in extensive land-surface subsidence of up to 10 feet. Among other issues, subsidence is responsible for increased suscepti- bility to flooding and the permanent inundation of some areas.
Continued pumping since the 1920s by many industrial and municipal users from the underlying Sparta aquifer have caused significant water- level declines in Arkansas, Louisiana, Mississippi, and Tennessee. Such declines have caused concerns about the Sparta’s sustainability resulting in the aquifer being declared “critical” in Arkansas. The Memphis, Tennessee, and West Memphis, Arkansas, area is one of the largest metropolitan areas in the world that relies exclusively on ground water for municipal supply. These large withdrawals have caused regional water-level declines of up to 70 feet, and have resulted in interstate
concerns over continued and increased pumping in the Memphis area.
High Plains The High Plains aquifer (which includes the Ogallala aquifer) underlies parts of eight States and has been intensively developed for irriga- tion. Since predevelopment, water levels have declined more than 100 feet in some areas and the saturated thick- ness has been reduced by more than half in others. Water levels are recover- ing in some areas due to management by State and local agencies, improved irrigation efficiency, low crop prices, and agricultural programs (McGuire and others, 2003).
Pacific Northwest Ground-water development of the Columbia River Basalt aquifer of Washington and Oregon for irrigation, public-supply, and industrial uses has caused water- level declines of more than 100 feet in several areas; management efforts to reduce withdrawals have reversed some of the declines. The Snake River Plain aquifer in Idaho provides water for extensive irrigation as well as much of the flow of the Snake River through springs. Since 1950, water levels and spring discharge have decreased due to intensive use of ground water for agriculture (Burns, 1997).
Chicago-Milwaukee area Since the first documented water well was completed in the Chicago area in 1864, ground water has been the sole source of drinking water for about 8.2 million people in the Great Lakes watershed. This long-term pumping has lowered ground-water levels by as much as 900 feet in the sandstone aquifer underlying the Chicago area and eastern Wiscon- sin. Concern over how such pumping affected surface water in the Great Lakes region led to the reduction of ground-water withdrawals in much of the area. Water levels are recovering in some areas, however, declines continue in others (Grannemann and others, 2000).
Desert Southwest Increased ground- water pumping to support popula- tion growth in south-central Arizona (including the Tucson and Phoenix areas) has resulted in water-level declines of between 300 and 500 feet in much of the area. Land subsidence was first noticed in the 1940s and subse- quently as much as 12.5 feet of subsid- ence has been measured. Additionally, lowering of the water table has resulted in the loss of streamside vegetation as documented by historical photographs.
In 1999, Las Vegas, Nevada, was the fastest growing municipal area in the United States. In places, ground-water levels have declined 300 feet since the
Locations in the basins of southern California, Nevada, Utah, Arizona, and New Mexico where substantial ground-water level declines have been measured. In some areas, water levels have recovered in response to reduction in pumping and increased recharge efforts (Leake and others, 2000).