Ground water is a valuable resource both in the United States and throughout the world. Where surface water, such as lakes and rivers, are scarce or inaccessible, ground water supplies many of the hydrologic needs of people everywhere. In the United States. It is the source of drinking water for about half the total population and nearly all of the rural population, and it provides over 50 billion gallons per day for agricultural needs. 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.
The water stored in the ground can be compared to money kept in a bank account. If you withdraw money at a faster rate than you deposit new money you will eventually start having account-supply problems. Pumping water out of the ground faster than it is replenished over the long-term causes similar problems. 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:
Pumping ground water at a faster rate than it can be recharged can have some negative effects of the environment and the people who make use of the water:
The most severe consequence of excessive ground-water pumping is that the water table, below which the ground is saturated with water, can be lowered. For water to be withdrawn from the ground, water must be pumped from a well that reaches below the water table. If ground-water levels decline too far, then the well owner might have to deepen the well, drill a new well, or, at least, attempt to lower the pump. Also, as water levels decline, the rate of water the well can yield may decline.
As the depth to water increases, the water must be lifted higher to reach the land surface. If pumps are used to lift the water (as opposed to artesian wells), more energy is required to drive the pump. Using the well can become prohibitively expensive.
There is more of an interaction between the water in lakes and rivers and ground water than most people think. Some, and often a great deal, of the water flowing in rivers comes from seepage of ground water into the streambed. Ground water contributes to streams in most physiographic and climatic settings. The proportion of stream water that comes from from ground-water inflow varies according to a region's geography, geology, and climate.
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. 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.
The basic cause of land subsidence is a loss of support below ground. In other words, sometimes when water is taken out of the soil, the soil collapses, compacts, and drops. This depends on a number of factors, such as the type of soil and rock below the surface. Land subsidence is most often caused by human activities, mainly from the removal of subsurface water.
One water-quality threat to fresh ground-water supplies is contamination from saltwater saltwater intrusion. All of the water in the ground is not fresh water; much of the very deep ground water and water below oceans is saline. In fact, an estimated 3.1 million cubic miles (12.9 cubic kilometers) of saline ground water exists compared to about 2.6 million cubic miles (10.5 million cubic kilometers) of fresh ground water (Gleick, P. H., 1996: Water resources. In Encyclopedia of Climate and Weather, ed. by S. H. Schneider, Oxford University Press, New York, vol. 2, pp.817-823). Under natural conditions the boundary between the freshwater and saltwater tends to be relatively stable, but pumping can cause saltwater to migrate inland and upward, resulting in saltwater contamination of the water supply.
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 several states. The extents of the resulting effects depend on several factors including pumpage and natural discharge rates, physical properties of the aquifer, and natural and human-induced recharge rates. Some examples are given below.
Atlantic Coastal Plain - In Nassau and Suffolk Counties, Long Island, New York, pumping water for domestic supply has lowered the water table, reduced or eliminated the base flow of streams, and has caused saline ground water to move inland.
Many other locations on the Atlantic coast are experiencing similar effects related to ground-water depletion. Surface-water flows have been reduced due to ground-water development 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).
The chart below shows monthly-mean water levels from 1964 to 2003 for a well in Cook County, sourthwest Georgia. The well is used for irrigation and public-supply purposes and offers a good visual representation of long-term ground-water declines due to excessive pumping. [d] - data for the chart is available.
West-central Florida - Ground-water development in the Tampa-St. Petersburg area has led to saltwater intrusion 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 experiencing effects related to ground-water depletion:
High Plains - The High Plains aquifer (which includes the Ogallala aquifer) underlies parts of eight States and has been intensively developed for irrigation. Since predevelopment, water levels have declined more than 100 feet in some areas and the saturated thickness has been reduced by more than half in others.
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.
Desert Southwest - Increased ground-water pumping to support population 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 subsequently as much as 12.5 feet of subsidence has been measured. Additionally, lowering of the water table has resulted in the loss of streamside vegetation.
These pictures show a reach of the Santa Cruz River south of Tucson, Arizona. In the 1942 picture vegetation is growing in the riparian (river bank) area the river, indicating that sufficient water in the soil existsed at a level that plant roots could access it. The same site in 1989 shows that the riparian trees have largely disappeared as a result of lowered ground-water levels.
Chicago-Milwaukee area - Chicago has been using ground water since at least 1864 and 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.
This map shows contours of water-level declines, in feet, in the Chicago-Milwaukee area from 1864 to 1980.
Sources and more information
• Ground-Water Depletion Across the Nation, USGS Fact Sheet 103-03
• Sustainability of Ground-Water Resources, USGS Circular 1186
U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu/gwdepletion.html
Page Contact Information: Howard Perlman
Page Last Modified: Friday, 07-Nov-2008 15:42:51 EST
