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A hydrograph showing ground-water-level declines in the Buckman well field, which supplies water for Santa Fe, New Mexico. No measurements were made between August 1988 and June 1997, during which time water levels declined nearly 300 feet, emphasiz- ing the importance of continual monitoring. Long-term data that document the evolv- ing response of aquifers to ground-water development are particularly important for calibrating ground-water-flow models used to forecast future conditions.

Reduced surface-water flows In most areas, the surface- and ground-water systems are intimately linked. Ground- water pumping can alter how water moves between an aquifer and a stream, lake, or wetland by either intercepting ground-water flow that discharges into the surface-water body under natural conditions, or by increasing the rate of water movement from the surface- water body into an aquifer. In either case, the net result is a reduction of flow to surface water, though the full effect may take many years to develop.

A related effect of ground-water pumping is the lowering of ground- water levels below the depth that streamside or wetland vegetation needs to survive. The overall effect is a loss of riparian vegetation and wildlife habitat.

Subsidence Land subsidence is “a gradual settling or sudden sinking of the Earth’s surface owing to subsurface movement of earth materials.” Though several different earth processes can cause subsidence, more than 80 percent of the subsidence in the United States is related to the withdrawal of ground water (Galloway and others, 1999).

Deterioration of water quality Coastal aquifers tend to have wedge- shaped zones of saltwater underlying

the potable freshwater. Under natural conditions the boundary between the freshwater and saltwater tends to be rel- atively stable, but pumping can cause saltwater to migrate inland, resulting in saltwater contamination of the water supply. Inland aquifers can experience similar problems where withdrawal of good-quality water from the upper parts of inland aquifers can allow underly- ing saline water to move upward and degrade water quality. Additionally, where ground water is pumped from an aquifer, surface water of poor or differing quality may be drawn into the aquifer. This can degrade the water quality of the aquifer directly or mobi- lize naturally occurring contaminants in the aquifer.

Where does ground-water depletion occur in the United States?

Ground-water depletion has been a concern in the Southwest and High Plains for many years, but increased demands on our ground-water resources have overstressed aquifers in many areas of the Nation, not just in arid regions. In addition, ground-water depletion occurs at scales ranging from a single well to aquifer systems underlying

A 1942 photograph (top) of a reach of the Santa Cruz River south of Tucson, Arizona, shows stands of mesquite and cottonwood trees along the river. A photograph (bottom) of the same site in 1989 shows that the riparian trees have largely disappeared, as a result of lowered ground-water levels.

Photos: Robert H. Webb, USGS

several states. The extents of the result- ing effects depend on several factors including pumpage and natural dis- charge rates, physical properties of the aquifer, and natural and human-induced recharge rates. Some examples from east to west across the Nation are given below.

Atlantic Coastal Plain In Nassau and Suffolk Counties, Long Island, New York, water pumped for domestic supply is used and sent to a wastewa- ter-treatment plant and then discharged into the surrounding saltwater bodies. As a result of these actions, the water table has been lowered, the base flow of streams has been reduced or elimi- nated, the length of perennial streams has been decreased, and saline ground water has moved inland.

Many other locations on the Atlantic coast are experiencing similar effects related to ground-water deple- tion. Surface-water flows have been reduced due to ground-water devel- opment in the Ipswich River basin, Massachusetts. Saltwater intrusion is occurring in coastal counties in New Jersey; Hilton Head Island, South Carolina; Brunswick and Savannah, Georgia; and Jacksonville and Miami, Florida (Barlow, 2003).

West-central Florida Ground-water development in the Tampa-St. Peters- burg area has led to saltwater intru- sion and subsidence in the form of sinkhole development and concern about surface-water depletion from lakes in the area. In order to reduce its dependence on ground water, Tampa has constructed a desalination plant to treat seawater for municipal supply.

Gulf Coastal Plain Several areas in the Gulf Coastal Plain are experienc- ing effects related to ground-water depletion:

Ground-water pumping by Baton Rouge, Louisiana, increased more than tenfold between the 1930s and 1970, resulting in ground-water-level declines of approximately 200 feet. Baton Rouge is underlain by a series of aquifers, and pumping has shifted among them with time. The large water-level declines have resulted in saltwater encroaching from the Gulf

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