Background (Encyclopedia of Global Resources)
Groundwater is one of the most valuable natural resources: It serves as the source of a significant percentage of the water used for all purposes. However, even though it is so widely used and so vital to the health and economy of all nations, the occurrence of groundwater is not only poorly understood but also subject to many misconceptions. For example, one common misconception is that groundwater flows in large underground rivers that resemble surface streams. Folklore has it that these streams can be detected by certain special individuals who practice “water dowsing.” Such misconceptions have hindered the development and conservation of groundwater and have negatively affected the protection of water quality.
(The entire section is 112 words.)
Infiltration (Encyclopedia of Global Resources)
Groundwater is a major component of the hydrologic cycle, which is the constant movement of water above, on, and below the Earth’s surface. That fraction of precipitation that can infiltrate the Earth’s surface can become part of the subsurface component of the hydrologic cycle. Infiltration rates vary enormously, depending upon the intensity and duration of precipitation, land use, and the physical characteristics and moisture content of the soil. For example, the infiltration rates can range from a high of 25 millimeters per hour in mature forests on sandy soils to only a few millimeters per hour in clayey and silty soils to zero in paved areas.
(The entire section is 107 words.)
The Unsaturated Zone (Encyclopedia of Global Resources)
Subsurface water occurs in two distinct zones in the ground. The uppermost zone contains both water and air and is called the unsaturated zone. It is divided into three parts: a soil zone or soil-water belt, an intermediate zone, and the upper part of the capillary fringe. The soil-water belt extends from the top of the land surface to a maximum depth of about 1 to 2 meters. The porosity (the amount of openings in earth material) and the permeability (velocity of fluid flow within the Earth material) are higher in the soil-water belt than in the underlying intermediate zone. The capillary fringe is located in the lowest part of the unsaturated zone and results from the attraction between water and rocks. The thickness or depth of the unsaturated zone varies from zero in swamps to a few meters in humid regions to more than 300 meters in deserts.
(The entire section is 146 words.)
The Saturated Zone (Encyclopedia of Global Resources)
The zone below the unsaturated zone has all interconnected openings filled with water and is called the saturated zone. The top of the saturated zone is marked by the water table, which is the level at which the hydraulic pressure is equal to atmospheric pressure. Water in the saturated zone is the only subsurface water that supplies wells, springs, and base flow to streams and is the only water which is properly called groundwater.
(The entire section is 74 words.)
Groundwater Movement (Encyclopedia of Global Resources)
In sharp contrast to surface water, groundwater moves very slowly. For example, surface water can move tens of kilometers per day, whereas groundwater flow ranges approximately from 1.5 meters per day to as low as 1.5 meters per year. This slow movement means that any contaminant that gets into groundwater will be there for a long time.
As part of the hydrologic cycle, groundwater also furnishes the stream with base flow or dry-weather flow. This is why streams in humid areas have water flowing in the channel days after precipitation has occurred. In fact, a large portion of streamflow is derived from base flow which is groundwater.
(The entire section is 105 words.)
Groundwater Recharge (Encyclopedia of Global Resources)
The source of groundwater is precipitation in the recharge area that has percolated through the unsaturated zone and reached the water table. Once there, groundwater flows down the hydraulic gradient to discharge areas along floodplains and streams. Average annual recharge rates in the United States range from zero in desert areas to as much as 600 millimeters per year in rural areas in Long Island, New York, and similar places along the Atlantic coastal plain that are underlain by permeable sands. These high recharge rates account for as much as 50 percent of average annual precipitation.
The rate of groundwater movement from recharge areas to discharge areas depends upon the permeability and porosity of the Earth material. Shallow groundwater flow to discharge areas can be measured in days as compared with deep groundwater flow which can take decades, centuries, or even millennia to reach a discharge area.
(The entire section is 145 words.)
Groundwater Quality and Groundwater Pollution (Encyclopedia of Global Resources)
Water is often referred to as the universal solvent because of its ability to dissolve at least small amounts of almost all substances that it contacts. Since groundwater moves very slowly, it has plenty of time to dissolve earth materials. Thus, groundwater usually contains large amounts of dissolved solids.
Groundwater pollution refers to any degradation of water quality that results from anthropogenic activities. In urban and suburban areas, these activities include disposal of industrial and municipal wastes in unlined landfills, leaking sewers, and application of lawn fertilizers, herbicides, and pesticides. Groundwater can be polluted in rural areas by septic tanks, animal feedlots, and application of crop fertilizers, herbicides, and pesticides. Other sources of groundwater pollution include leaking gasoline and home-heating oil tanks, salt coming from unprotected stockpiles, and saltwater encroachment in coastal areas that have been overpumped. There have been numerous instances of groundwater pollution: Municipal wells on Long Island were forced to close because of a pre-World War II application of fertilizers to potato fields; public supply wells were closed in Massachusetts because of excessive road salt applications; and well fields were contaminated by saltwater encroachment in Dade County, Florida, and Southern California (Manhattan Beach).
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Further Reading (Encyclopedia of Global Resources)
Appelo, C. A. J., and D. Postma. Geochemistry, Groundwater, and Pollution. 2d ed. New York: Balkema, 2005.
Fetter, C. W. Applied Hydrogeology. 4th ed. Upper Saddle River, N.J.: Prentice Hall, 2001.
_______. Contaminant Hydrogeology. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 1999.
Palmer, Christopher M. Principles of Contaminant Hydrogeology. 2d ed. Boca Raton, Fla.: CRC Lewis, 1996.
Price, Michael. Introducing Groundwater. 2d ed. New York: Chapman & Hall, 1996.
Todd, David Keith, and Larry W. Mays. Groundwater Hydrology. 3d ed. Hoboken, N.J.: Wiley, 2005.
Younger, Paul L. Groundwater in the Environment: An Introduction. Malden, Mass.: Blackwell, 2007.
Zektser, Igor S., and Lorne G. Everett, eds. Ground Water Resources of the World and Their Use. Paris: UNESCO, 2004. Reprint. Westerville, Ohio: National Ground Water Association Press, 2006.
Natural Resources Canada. Groundwater. http://atlas.nrcan.gc.ca/site/english/maps/freshwater/distribution/groundwater/1
U.S. Environmental Protection Agency. Aquatic Biodiversity: Groundwater. http://www.epa.gov/bioiweb1/aquatic/ground-r.html
U.S. Geological Survey. USGS Groundwater Information Pages. http://water.usgs.gov/ogw
(The entire section is 164 words.)
Definition (Encyclopedia of Global Warming)
There are approximately 1.385 billion cubic kilometers of water on, above, and in the Earth. However, the vast majority of the Earth’s water (around 97 percent) is found in its oceans. Groundwater, though constituting less than 2 percent of the world’s water, accounts for more than 98 percent of its available freshwater resources. In the United States, more than half of the population depends on groundwater, while in semiarid to arid regions the percentage of people using groundwater is almost 100 percent.
Groundwater is water found in pore spaces (pores) in soil and rocks. It differs from soil water in that groundwater is found below a water table—the demarcation separating saturated pores from unsaturated pores. Soil or rock that is saturated with water and is capable of supporting a well is called an aquifer. An aquifer can comprise one or multiple rock or sediment layers, ranging in size from a few meters to several hundred kilometers long, several meters thick, and meters to kilometers in width. An aquifer could be unconsolidated material, such as sand, or consolidated material, such as sandstone and other rocks. Limestone also makes good aquifers.
Aquifers may be divided into two main types. Open aquifers enjoy direct access to surface water. Closed aquifers sit underneath confining layers of semi- or nonpermeable material, which prevent such access. Water recharges aquifers by infiltration or seepage through...
(The entire section is 249 words.)
Significance for Climate Change (Encyclopedia of Global Warming)
The effect of global warming on groundwater varies from place to place, with differing effects on water quality and quantity. In semiarid to arid regions, the higher rate of evaporation increases the concentration of dissolved salts in the groundwater, as is the case in Lake Chad, in Africa, where the surface water in some closed areas is becoming more saline. This saline water may recharge the African groundwater system, making the water saline.
With global warming, snow and ice caps will melt, resulting in rapid sea-level rise. This rise may force saline waters into aquifers, reducing the water’s suitability for human and ecological use. Already, such seawater intrusion has occurred along some coastal aquifers, including one in El Paso, Texas.
Global temperature rise will lead to an increase in precipitation in the Northern Hemisphere. As snow and glaciers melt, the groundwater table will rise in most places. Sinkhole numbers may increase in limestone terrains with higher rates of solution weathering resulting from acid rain. These sinkholes could become conduits through which contaminants from the land surface could reach the groundwater.
Groundwater resident time (the length of time water stays in an aquifer) varies from a few days to several thousand years. Generally, the shorter the resident time, the greater the infiltration rate or the shallower the aquifer. In areas such as Libya,...
(The entire section is 437 words.)
Further Reading (Encyclopedia of Global Warming)
Anderson, M. P. “Introducing Groundwater Physics.” Physics Today 60, no. 5 (May, 2007): 42. Good, succinct summary of the principles of groundwater.
Fetter, C. W. Applied Hydrogeology. 4th ed. Upper Saddle River, N.J.: Prentice Hall, 2001. An easy-to-read textbook on hydrogeology. Presents several case studies and includes student versions of modflow, flownet, and aqtesolv software.
Hudak, Paul F. Principles of Hydrogeology. 3d ed. Boca Raton, Fla.: CRC Press, 2005. A simple book on hydrogeology that assumes readers have some geology and algebra skills.
Todds, David Keith, and Larry W. Mays. Groundwater Hydrology. Hoboken, N.J.: Wiley, 2005. Covers the basics of groundwater hydrology and includes modeling methods.
Younger, P. L. Groundwater in the Environment: An Introduction. Malden, Mass.: Blackwell, 2007. This is a good book for those interested in groundwater that does not require a geology background.
(The entire section is 130 words.)
Groundwater (Encyclopedia of Public Health)
An estimated 100 million Americans rely on groundwater for their source of drinking water. Approximately one-third of all public supplies and 95 percent of all rural domestic supplies use groundwater sources. In Asia, groundwater provides half of the drinking water, and in Europe the percentage is even much higher, as much as 98 percent in Denmark and 96 percent in Austria.
An aquifer is an underground formation of permeable rock or loose material that can produce useful quantities of water when tapped by a well. Groundwater is held within the tiny pores of the surrounding aquifer material. Aquifers vary in size from a few hectares to thousands of square kilometers of the earth's surface. The rate of groundwater flow is very slow compared to the flow of water on the surfacesually in the range of several inches per year to several feet per year. More than 96 percent of all available fresh water supplies occur in the form of groundwater, which is usually cleaner and more pure than most surface water sources.
Groundwater only partially fills unconfined aquifers. The upper surface of the groundwater, known as the water table, is thus free to rise and fall. The height of the water table will be the same as the water level in a well drilled in an unconfined aquifer. Unconfined aquifers can be vulnerable to contamination, especially if they are close to the surface. In these unconfined aquifers, gravity drives the movement of groundwater. Groundwater can leave the aquifer through the process of discharge, either when it reaches the land surface at a spring or other surface water body, or through the pumping of a well. Discharge can lead to contaminants in groundwater flowing into surface water bodies.
A confined aquifer (also known as an artesian aquifer) occurs between confining beds, which are layers of impermeable materials, such as clay, that impede the movement of water in and out of the aquifer. The groundwater in these artesian aquifers is under high pressure due to the confining beds. A recharge zone occurs where the confined aquifer is exposed to the surface. The confined aquifer is actually unconfined at the recharge zone. Confining beds serve two purposes. The first is to obstruct the movement of water into and out of the aquifer. The second is to bar the entry of contaminants from the overlying unconfined aquifers.
Aquifers are replenished with water from the surface through a process called "recharge." This occurs as a part of the hydrologic cycle when water from rainfall percolates into underlying aquifers. The rate of recharge can be influenced by different factors, such as soil, plant cover, water content of surface materials, and rainfall intensity. Groundwater recharge may also occur from surface water bodies in arid areas. Overwithdrawal of groundwater occurs when the discharge of groundwater in an aquifer exceeds the recharge rate over a period of time.
Groundwater can be polluted by landfills, septic tanks, leaky underground gas tanks, and from overuse of fertilizers and pesticides. This pollution poses a great risk to public health since the majority of the fresh water supply occurs as groundwater. Many of the groundwater pollutants are colorless, odorless, and tasteless. Degradation of groundwater supplies also occurs as a result of poor waste-disposal practices or poor land management.
MARK G. ROBSON
(SEE ALSO: Ambient Water Quality; Drinking Water; Groundwater Contamination; Water Quality)
Groundwater Foundation. Groundwater and Aquifers, 2000. Available at http://www.groundwater.com/groundwater_aquifer.html.
Groundwater Basics, 2000. Available at .
Koren, H., and Bisesi, M. (1996). Handbook of Environmental Health and Safety, 3rd edition, Vol. 2. Boca Raton, FL: Lewis Publishers.
Nadakavukaren, A. (2000). Our Global Environment. Prospect Heights, IL: Waveland Press.
Groundwater (World of Earth Science)
Groundwater occupies the void space in geological strata. It is one element in the continuous process of moisture circulation on Earth, termed the hydrologic cycle.
Almost all groundwater originates as surface water. Some portion of rain hitting the earth runs off into streams and lakes, and another portion soaks into the soil, where it is available for use by plants and subject to evaporation back into the atmosphere. The third portion soaks below the root zone and continues moving downward until it enters the groundwater. Precipitation is the major source of groundwater. Other sources include the movement of water from lakes or streams and contributions from such activities as excess irrigation and seepage from canals. Water has also been purposely applied to increase the available supply of groundwater. Water-bearing formations called aquifers act as reservoirs for storage and conduits for transmission back to the surface.
The occurrence of groundwater is usually discussed by distinguishing between a zone of saturation and a zone of aeration. In the zone of saturation, the pores are entirely filled with water, while the zone of aeration has pores that are at least partially filled by air. Suspended water does occur in this zone. This water is called vadose, and the zone of aeration is also known as the vadose zone. In the zone of aeration, water moves downward due to gravity, but in the zone of saturation it moves in a direction determined by the relative heights of water at different locations.
Water that occurs in the zone of saturation is termed groundwater. This zone can be thought of as a natural storage area or reservoir whose capacity is the total volume of the pores of openings in rocks.
An important exception to the distinction between these zones is the presence of ancient seawater in some sedimentary formations. The pore spaces of materials that have accumulated on an ocean floor, which has then been raised through later geological processes, can sometimes contain salt water. This is called connate water.
Formations or strata within the saturated zone from which water can be obtained are called aquifers. Aquifers must yield water through wells or springs at a rate that can serve as a practical source of water supply. To be considered an aquifer the geological formation must contain pores or open spaces filled with water, and the openings must be large enough to permit water to move through them at a measurable rate. Both the size of pores and the total pore volume depends on the type of material. Individual pores in fine-grained materials such as clay, for example, can be extremely small, but the total volume is large. Conversely, in coarse material such as sand, individual pores may be quite large but total volume is less. The rate of movement for fine-grained materials, such as clay, will be slow due to the small pore size, and it may not yield sufficient water to wells to be considered an aquifer. However, the sand is considered an aquifer, even though they yield a smaller volume of water, because they will yield water to a well.
The water table is not stationary, but moves up or down depending on surface conditions such as excess precipitation, drought, or heavy use. Formations where the top of the saturated zone or water table define the upper limit of the aquifer are called unconfined aquifers. The hydraulic pressure at any level with an aquifer is equal to the depth from the water table, and there is a type known as a water-table aquifer, where a well drilled produces a static water level which stands at the same level as the water table.
A local zone of saturation occurring in an aerated zone separated from the main water table is called a perched water table. These most often occur when there is an impervious strata or significant particle-size change in the zone of aeration, which causes the water to accumulate. A confined aquifer is found between impermeable layers. Because of the confining upper layer, the water in the aquifer exists within the pores at pressures greater than the atmosphere. This is termed an artesian condition and gives rise to an artesian well.
Groundwater can be pumped from any aquifer that can be reached by modern well-drilling apparatus. Once a well is constructed, hydraulic pumps pull the water up to the surface through pipes. As water from the aquifer is pulled up to the surface, water moves through the aquifer towards the well. Because water is usually pumped out of an aquifer more quickly than new water can flow to replace what has been withdrawn, the level of the aquifer surrounding the well drops, and a cone of depression is formed in the immediate area around the well.
Groundwater can be polluted by the spilling or dumping of contaminants. As surface water percolates downward, contaminants can be carried into the aquifer. The most prevalent sources of contamination are waste disposal, the storage, transportation and handling of commercial materials, mining operations, and nonpoint sources such as agricultural activities. Two other forms of groundwater pollution are the result of pumping too much water too quickly, so that the rate of water withdrawal from the aquifer exceeds the rate of aquifer recharge. In coastal areas, salty water may migrate towards the well, replacing the fresh water that has been withdrawn. This is called salt-water intrusion. Eventually, the well will begin pulling this salt water to the surface; once this happens, the well will have to be abandoned. A similar phenomenon, called connate ascension, occurs when a freshwater aquifer overlies a layer of sedimentary rocks containing connate water. In some cases, over pumping will cause the connate water to migrate out of the sedimentary rocks and into the freshwater aquifer. This results in a brackish, briney contamination similar to the effects of a salt-water intrusion. Unlike salt water intrusion, however, connate ascension is not particularly associated with coastal areas.
Groundwater has always been an important resource, and it will become more so in the future as the need for good quality water increases due to urbanization and agricultural production. It has recently been estimated that 50% of the drinking water in the United States comes from groundwater; 75% of the nation's cities obtain all or part of their supplies from groundwater, and rural areas are 95% dependent upon it. For these reasons every precaution should be taken to protect groundwater purity. Once contaminated, groundwater is difficult, expensive, and sometimes impossible to clean up.
See also Freshwater; Hydrogeology