U.S Geological Survey Fact Sheet 103-03
This fact sheet is available as a pdf.
Ground-water use has many societal benefits. It is the source of drinking water for about half the nation and nearly all of the rural population, and
U.S Geological Survey Fact Sheet 103-03
This fact sheet is available as a pdf.
Ground-water use has many societal benefits. It is the source of drinking water for about half the nation and nearly all of the rural population, and it provides over 50 billion gallons per day in support of the Nation’s agricultural economy. Ground-water depletion, a term often defined as long-term water-level declines caused by sustained ground-water pumping, is a key issue associated with ground-water use. Many areas of the United States are experiencing ground-water depletion.
An aquifer can be compared to a bank account, and ground water occurring in an aquifer is analogous to the money in the account. Hydrologists refer to this type of accounting as a water budget. Ground water can be recharged (deposited) by infiltration from precipitation, surface water, or applied irrigation water; it can be kept in storage (saved); and it can be discharged naturally to streams, springs, or seeps, or transpired by plants (withdrawn). In a ground-water system prior to development, the system is in longterm equilibrium—discharge is equal to recharge, and the volume of water in storage remains relatively constant. Ground-water levels fluctuate in time over a relatively small, natural range. Once pumping begins, however, this equilibrium is changed and ground-water levels decline. Just as a bank account must be balanced, withdrawals from an aquifer by pumping must be balanced by some combination of increased recharge, decreased discharge, and removal from storage (or depletion). An inventory of ground-water levels in wells reflects the volume of water stored (or occurring) in the aquifer, and is analogous to a financial statement.
The volume of ground water in storage is decreasing in many areas of the United States in response to pumping. Ground-water depletion is primarily caused by sustained ground-water pumping. Some of the negative effects of ground-water depletion include increased pumping costs, deterioration of water quality, reduction of water in streams and lakes, or land subsidence. Such effects, while variable, happen to some degree with any ground-water use. As with other natural resources, society must weigh the benefits against the consequences of such use. In order to provide the scientific information needed for informed decisions, these effects must be observed over time to determine their impact.
If intensive pumping from an aquifer continues, then adverse effects may occur.
Water-well problems Declining ground-water levels have three main effects on water wells. First, as the depth to water increases, the water must be lifted higher to reach the land surface. As the lift distance increases, so does the energy required to drive the pump. Thus, power costs increase as ground-water levels decline. Depending on the use of the water and the energy costs, it may no longer be economically feasible to use water for a given purpose. Second, ground-water levels may decline below the bottom of existing pumps, necessitating the expense of lowering the pump, deepening the well, or drilling a deeper replacement well. Third, the yield of the well may decline below usable rates.
|Ground-water budgets before and after development of the Gulf Coastal Plain aquifer system (all flows in cubic feet per second). The large withdrawals from the aquifers have been balanced by increases in recharge to the aquifer system and decreases in storage and discharge from the aquifer system (modified from Williamson and Grubb, 2001).|
|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, emphasizing the importance of continual monitoring. Long-term data that document the evolving 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 wedgeshaped zones of saltwater underlying the potable freshwater. Under natural conditions the boundary between the freshwater and saltwater tends to be relatively 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 underlying saline water to move upward and degrade water quality. Additionally, where grou