An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. This article is about the chemical substances. ... Gravel being unloaded from a barge Gravel is rock that is of a certain grain size range. ... For other uses, see Sand (disambiguation). ... Silt is soil or rock derived granular material of a specific grain size. ... The Gay Head cliffs in Marthas Vineyard are made almost entirely of clay. ... Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of geologic formations. ... Cable tool water well drilling rig in Kimball, West Virginia. ... Hydrogeology (hydro- meaning water, and -geology meaning the study of rocks) is the part of hydrology that deals with the distribution and movement of groundwater in the soil and rocks of the Earths crust (commonly in aquifers). ...

Classification

Typical aquifer cross-section

This diagram indicates typical flow directions in a cross-sectional view of a simple confined/unconfined aquifer system (two aquifers with one aquitard between them, surrounded by the bedrock aquiclude) which is in contact with a stream (typical in humid regions). The water table and unsaturated zone are also illustrated. Image File history File links A diagram showing the relation between confined and unconfined aqufiers; from USGS circular 1186. ... In geometry, a cross section is the intersection of a body in 3-dimensional space with a plane, or of a body in 2-dimensional space with a line, etc. ... An aquitard is a geological formation of layers comprised either of clay, with tiny connected pores, or on non-porous rock that resrict water flow from one aquifer to another. ... A running stream. ... Humidity is the quantity of moisture in the air. ... Cross section showing the water table varying with surface topography as well as a perched water table The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure. ... The vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the water table, and is thus not considered groundwater (vadose is Latin for shallow). It comprises the unsaturated portion of the soil, regolith or bedrock, as well as the saturated capillary fringe...

Saturated versus unsaturated

Groundwater can be found at nearly every point in the earth's shallow subsurface, to some degree; although aquifers do not necessarily contain fresh water. The earth's crust can be divided into two regions: the saturated zone or phreatic zone (e.g., aquifers, aquitards, etc.), where all available spaces are filled with water, and the unsaturated zone (also called the vadose zone), where there are still pockets of air that can be replaced by water. Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of geologic formations. ... For the village on the Isle of Wight, see Freshwater, Isle of Wight. ... Water content is a ratio used in hydrogeology and soil mechanics to indicate the amount of water a porous medium contains. ... The term phreatic is used in geology to refer to matters relating to underground water below the water table (the word originates from the Greek phrear, phreat- meaning well or spring). The phreatic zone is the layer(s) of soil or rock below the water table in which voids are... The vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the water table, and is thus not considered groundwater (vadose is Latin for shallow). It comprises the unsaturated portion of the soil, regolith or bedrock, as well as the saturated capillary fringe...

Saturated means the pressure head of the water is greater than atmospheric pressure (it has a gauge pressure > 0). The definition of the water table is surface where the pressure head is equal to atmospheric pressure (where gauge pressure = 0). Unsaturated conditions occur above the water table where the pressure head is negative (absolute pressure can never be negative, but gauge pressure can) and the water which incompletely fills the pores of the aquifer material is under suction. The water content in the unsaturated zone is held in place by surface adhesive forces and it rises above the water table (the zero gauge pressure isobar) by capillary action to saturate a small zone above the phreatic surface (the capillary fringe) at less than atmospheric pressure. This is termed tension saturation and is not the same as saturation on a water content basis. Water content in a capillary fringe decreases with increasing distance from the phreatic surface. The capillary head depends on soil pore size. In sandy soils with larger pores the head will be less than in clayey soils with very small pores. The normal capillary rise in a clayey soil is less than 1.80 m (six feet) but can range between 0.3 and 10 m (1 and 30 feet). [1] Diurnal (daily) rhythm of air pressure in northern Germany (black curve is air pressure) Atmospheric pressure is the pressure at any point in the Earths atmosphere. ... Cross section showing the water table varying with surface topography as well as a perched water table The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure. ... It has been suggested that Hydraulic head (hydrology) and Head (hydraulic) be merged into this article or section. ... Suction is the creation of a partial vacuum, or region of low pressure. ... Hydrogeology (hydro- meaning water, and -geology meaning the study of rocks) is the part of hydrology that deals with the distribution and movement of groundwater in the soil and rocks of the Earths crust (commonly in aquifers). ... Dew drops adhering to a spider web Adhesion is the molecular attraction exerted between bodies in contact. ... Hydrogeology (hydro- meaning water, and -geology meaning the study of rocks) is the part of hydrology that deals with the distribution and movement of groundwater in the soil and rocks of the Earths crust (commonly in aquifers). ... The word isobar derives from the two ancient Greek words, ισος (isos), meaning equal, and βαρος (baros), meaning weight. In meteorology, an isobar is a line of equal or constant pressure on a graph, plot, or map; an isopleth of pressure. ... This article or section does not cite its references or sources. ... The capillary fringe , or tension-saturated zone, is the subsurface layer in which water molecules seep up from a water table by capillary action to fill pores. ... For other uses, see Sand (disambiguation). ... The Gay Head cliffs in Marthas Vineyard are made almost entirely of clay. ...

The capillary rise of water in a small diameter tube is this same physical process. The water table is the level to which water will rise in a large diameter pipe (e.g. a well) which goes down into the aquifer and is open to the atmosphere. Diameter is an AAA (authentication, authorization and accounting) protocol for applications such as network access or IP mobility. ...

See also: Water content and Soil moisture

Water content is a ratio used in hydrogeology and soil mechanics to indicate the amount of water a porous medium contains. ... This article or section does not cite its references or sources. ...

Aquifers versus aquitards

Aquifers are typically saturated regions of the subsurface which produce an economically feasible quantity of water to a well or spring (e.g., sand and gravel or fractured bedrock often make good aquifer materials). An aquitard is a zone within the earth that restricts the flow of groundwater from one aquifer to another. An aquitard can sometimes, if completely impermeable, be called an aquiclude or aquifuge. Aquitards comprise layers of either clay or non-porous rock with low hydraulic conductivity. Economically feasible is a relative term; for example, an aquifer that is quite adequate for local domestic use, as in a rural area, might be considered an inadequate aquitard for industrial, mining, or urban water supply. Cable tool water well drilling rig in Kimball, West Virginia. ... A natural spring on Mackinac Island in Michigan. ... For other uses, see Sand (disambiguation). ... Gravel being unloaded from a barge Gravel is rock that is of a certain grain size range. ... Bedrock is the native consolidated rock underlying the Earths surface. ... Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of geologic formations. ... The Gay Head cliffs in Marthas Vineyard are made almost entirely of clay. ... The rocky side of a mountain creek near Orosí, Costa Rica. ... written by AmerHydraulic conductivity, mathematically represented as , is a property of soil or rock, in the vadose zone or groundwater, that describes the ease with which water can move through pore spaces or fractures. ... This article is about mineral extraction. ... An urban area is a term used to define an area where there is an increased density of human-created structures in comparison to the areas surrounding it. ... A water supply system provides water to the locations that need it. ...

In non-mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated alluvium. They are typically composed of mostly horizontal layers of materials deposited by water processes (rivers and streams), which in cross-section (looking at a two-dimensional slice of the aquifer) appear to be layers of alternating coarse and fine materials. Coarse materials, because of the high energy needed to move them, tend to be found nearer the source (mountain fronts or rivers), while the fine-grained material will make it farther from the source (to the flatter parts of the basin or overbank areas - sometimes called the pressure area). Since there are less fine-grained deposits near the source, this is a place where aquifers are often unconfined (sometimes called the forebay area), or in hydraulic communication with the land surface. Alluvium (from the Latin, alluvius, from alluere, to wash against) is soil or sediments deposited by a river or other running water. ...

See also: Hydraulic conductivity and Storativity

written by AmerHydraulic conductivity, mathematically represented as , is a property of soil or rock, in the vadose zone or groundwater, that describes the ease with which water can move through pore spaces or fractures. ... Specific storage, storativity and specific yield (Ss, S and Sy) are aquifer properties; they are measures of the ability of an aquifer to release groundwater from storage, due to a unit decline in hydraulic head. ...

Confined versus unconfined

There are two end members in the spectrum of types of aquifers; confined and unconfined (with semi-confined being in between). Unconfined aquifers are sometimes also called water table or phreatic aquifers, because their upper boundary is the water table or phreatic surface. Typically (but not always) the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer (an aquitard or aquiclude) between it and the surface. Unconfined aquifers usually receive recharge water directly from the surface, from precipitation or from a body of surface water (e.g., a river, stream, or lake) which is in hydraulic connection with it. Confined aquifers have the water table above their upper boundary (an aquitard or aquiclude), and are typically found below unconfined aquifers. The term "perched" refers to ground water accumulates above a low-permeability unit. This term is generally used to refer to a small local area of ground water that occurs at an elevation higher than a regionally-extensive aquifer. The difference between perched and unconfined aquifers is their size (perched is smaller). Cross section showing the water table varying with surface topography as well as a perched water table The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure. ... River Gambia flowing through Niokolokoba National Park A river is a large natural waterway. ... A running stream. ... A man-made lake in Keukenhof, Netherlands A lake is a body of water or other liquid of considerable size surrounded by land. ...

If the distinction between confined and unconfined is not clear geologically (i.e., if it is not known if a clear confining layer exists, or if the geology is more complex, e.g., a fractured bedrock aquifer), the value of storativity returned from an aquifer test can be used to determine it (although aquifer tests in unconfined aquifers should be interpreted differently than confined ones). Confined aquifers have very low storativity values (much less than 0.01, and as little as 10^-5), which means that the aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities. Unconfined aquifers have storativities (typically then called specific yield) greater than 0.01 (1% of bulk volume); they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water (up to the drainable porosity of the aquifer material, or the minimum volumetric water content). An Aquifer test is conducted to evaluate an aquifer by stimulating the aquifer through constant pumping, and observing the aquifers response (drawdown) in observation wells. ... Specific storage, storativity and specific yield (Ss, S and Sy) are aquifer properties; they are measures of the ability of an aquifer to release groundwater from storage, due to a unit decline in hydraulic head. ... Specific storage, storativity and specific yield (Ss, S and Sy) are aquifer properties; they are measures of the ability of an aquifer to release groundwater from storage, due to a unit decline in hydraulic head. ... Hydrogeology (hydro- meaning water, and -geology meaning the study of rocks) is the part of hydrology that deals with the distribution and movement of groundwater in the soil and rocks of the Earths crust (commonly in aquifers). ... Water content is a ratio used in hydrogeology and soil mechanics to indicate the amount of water a porous medium contains. ...

See also: Porosity and Storativity

Used in geology, building science and hydrogeology, the porosity of a porous medium (such as rock or sediment) is the proportion of the non-solid volume to the total volume of material, and is defined by the ratio: where Vp is the non-solid volume (pores and liquid) and Vm... Specific storage, storativity and specific yield (Ss, S and Sy) are aquifer properties; they are measures of the ability of an aquifer to release groundwater from storage, due to a unit decline in hydraulic head. ...

Misconception

A common misconception is that groundwater exists in underground rivers (e.g. caves where water flows freely underground). This is only sometimes true in eroded limestone areas known as karst topography which make up only a small percentage of Earth's area. More usual is that the pore spaces of rocks in the subsurface are simply saturated with water — like a kitchen sponge — which can be pumped out and used for agricultural, industrial or municipal uses. A concept is an abstract, universal idea, notion or entity that serves to designate a category or class of entities, events or relations. ... Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of geologic formations. ... Lechuguilla Cave, New Mexico This article is about natural caves; for artificial caves used as dwellings, such as those in north China, see yaodong. ... Severe soil erosion in a wheat field near Washington State University, USA. Erosion is the displacement of solids (soil, mud, rock and other particles) by the agents of wind, water or ice, by downward or down-slope movement in response to gravity or by living organisms (in the case of... Limey shale overlaid by limestone. ... Karst topography is a three-dimensional landscape shaped by the dissolution of a soluble layer or layers of bedrock, usually carbonate rock such as limestone or dolomite. ... Used in geology, building science and hydrogeology, the porosity of a porous medium (such as rock or sediment) is the proportion of the non-solid volume to the total volume of material, and is defined by the ratio: where Vp is the non-solid volume (pores and liquid) and Vm... An electrically driven pump (electropump) for waterworks near the Hengsteysee, Germany. ...

The beach provides a model to help visualize an actual aquifer. If a hole is dug into the sand, very wet or saturated sand will be located at a shallow depth. This hole is a crude well, the wet sand represents an aquifer, and the level to which the water rises in this hole represents the water table. For other uses, see Beach (disambiguation). ... For other uses, see Sand (disambiguation). ... Cable tool water well drilling rig in Kimball, West Virginia. ... Cross section showing the water table varying with surface topography as well as a perched water table The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure. ...

Human dependence on groundwater

Most land areas on Earth have some form of aquifer underlying them, sometimes at significant depths. Fresh water aquifers, especially those with limited recharge by meteoric water, can be over-exploited and, depending on the local hydrogeology, may draw in non-potable water or saltwater (saltwater intrusion) from hydraulically connected aquifers or surface water bodies. This can be a serious problem especially in coastal areas and other areas where aquifer pumping is excessive. Earth (IPA: , often referred to as the Earth, Terra, the World or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. ... Hydrogeology (hydro- meaning water, and -geology meaning the study of rocks) is the part of hydrology that deals with the distribution and movement of groundwater in the soil and rocks of the Earths crust (commonly in aquifers). ... This article is about the term salt as referred to in chemistry. ... Saltwater intrusion is a natural process that occurs in virtually all coastal aquifers. ...

Aquifers are critically important in human habitation and agriculture. Deep aquifers in arid areas have long been water sources for irrigation (see Ogallala below). Many villages and even large cities draw their water supply from wells in aquifers. Trinomial name Homo sapiens sapiens Linnaeus, 1758 Humans, or human beings, are bipedal apes belonging to the mammalian species Homo sapiens (Latin for wise man or knowing man) under the family Hominidae (known as the great apes). ... High-altitude aerial view of irrigation in the Heart of the Sahara ( ) Irrigation is the replacement or supplementation of rainfall with water from another source in order to grow crops or plants. ... A village is a human residential settlement commonly found in rural areas. ...

Some aquifers are "riparian aquifers". These are related to rivers, fluvial deposits, or unconsolidated deposits along river corridors, and are usually rapidly replenished by infiltration of surface water. Some municipal well fields are specifically designed to take advantage of induced infiltration of surface (usually river) water, leaving them potentially vulnerable to water quality problems in the surface water body (chemical spills, petroleum spills, and bacteriological problems). River Gambia flowing through Niokolokoba National Park A river is a large natural waterway. ... It has been suggested that Externality be merged into this article or section. ... A chemical substance is any material substance used in or obtained by a process in chemistry: A chemical compound is a substance consisting of two or more chemical elements that are chemically combined in fixed proportions. ... Pumpjack pumping an oil well near Sarnia, Ontario Ignacy Łukasiewicz - inventor of the refining of kerosene from crude oil. ... Phyla Actinobacteria Aquificae Chlamydiae Bacteroidetes/Chlorobi Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Lentisphaerae Nitrospirae Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Verrucomicrobia Bacteria (singular: bacterium) are unicellular microorganisms. ...

Aquifers that provide sustainable fresh groundwater to urban areas and for agricultural irrigation are typically close to the ground surface (within a couple of hundred meters) and have some recharge by fresh water. This recharge is typically from rivers or meteoric water (precipitation) that percolate into the aquifer through overlying unsaturated materials. High-altitude aerial view of irrigation in the Heart of the Sahara ( ) Irrigation is the replacement or supplementation of rainfall with water from another source in order to grow crops or plants. ...

Subsidence

In unconsolidated aquifers, groundwater is produced from pore spaces between particles of gravel, sand, and silt. If the aquifer is confined by low-permeability layers, the reduced water pressure in the sand and gravel causes slow drainage of water from the adjoining confining layers. If these confining layers are composed of compressible silt or clay, the loss of water to the aquifer reduces the water pressure in the confining layer, causing it to compress from the weight of overlying geologic materials. In severe cases, this compression can be observed on the ground surface as subsidence. Unfortunately, much of the subsidence from groundwater extraction is permanent (elastic rebound is small). Thus the subsidence is not only permanent, but the compressed aquifer has a permanently-reduced capacity to hold water.

Examples

An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer [2] in central Texas. This carbonate aquifer has historically been providing high-quality water for nearly 2 million people and, even today, is completely full because of tremendous recharge from a number of area streams, rivers and lakes. The primary risk to this resource is human development over the recharge areas. For other uses, see Texas (disambiguation). ... A man-made lake in Keukenhof, Netherlands A lake is a body of water or other liquid of considerable size surrounded by land. ...

One of the largest aquifers in the world is the Guarani Aquifer, with 1.2 million km² of area, from central Brazil to northern Argentina. The Guaraní Aquifer is a vast underground reservoir of fresh water which lies beneath the surface of Argentina, Paraguay, and Brazil. ...

Aquifer depletion is a problem in some areas, and is especially critical in northern Africa; see the Great Manmade River project of Libya for an example. However, new methods of groundwater management such as artificial recharge and injection of surface waters during seasonal wet periods has extended the life of many freshwater aquifers, especially in the United States. A world map showing the continent of Africa. ... The Great Manmade River or Great Man-made River (GMR) is a network of pipes that supplies water to the Sahara Desert in Libya from a fossil aquifer in the Sahara. ...

• The Great Artesian Basin is one of the largest groundwater aquifers in the world. It plays a large part in water supplies for remote parts of South Australia.

The Great Artesian Basin provides the only reliable source of water through much of inland Australia and is the largest artesian basin in the world. ...

North America

• Canada - Oak Ridges Moraine, north of the city of Toronto.

• United States - The Ogallala Aquifer of the central United States is one of the world's great aquifers, but in places it is being rapidly depleted for growing municipal use, and continuing agricultural use. This huge aquifer, which underlies portions of eight states, contain primarily fossil water from the time of the last glaciation. Annual recharge, in the more arid portions of the aquifer, is estimated to total only about ten percent of annual withdrawals.

• United States - The Mahomet Aquifer supplies water to some 800,000 people in central Illinois and contains approximately four trillion US gallons (15 km³) of water. The Mahomet Aquifer Consortium [3] was formed in 1998 to study the aquifer with hopes of ensuring the water supply and reducing potential user conflicts.

The Oak Ridges Moraine is an ecologically sensitive geological landform in south central Ontario, Canada. ... The Ogallala aquifer underlies portions of eight states. ... Overdrafting is the process of extracting groundwater beyond the safe yield or equilibrium yield of the aquifer. ... Fossil water is groundwater having remained in an aquifer for thousands or more years. ... Variations in CO2, temperature and dust from the Vostok ice core over the last 400 000 years For the animated movie, see Ice Age (movie). ... The Mahomet Aquifer is the most important aquifer in east-central Illinois. ... A water supply system provides water to the locations that need it. ...

See also

• Artesian Well
• List of aquifers

Geological strata giving rise to an Artesian well An artesian aquifer is an aquifer whose water is overpressurized. ... The following is a partial list of aquifers around the world. ...

External links

• Map of United States Aquifers
• Florida's Springs and Aquifer