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Aquifer

Aquifer

An aquifer is a body of sand or porous rock capable of storing and producing significant quantities of water . An aquifer may be a layer of loose gravel or sand, a layer of porous sandstone , a limestone layer, or even an igneous or metamorphic body of rock. An aquifer may be only a few feet to hundreds of feet thick. Aquifers occur near the surface or buried thousands of feet below the surface. It may have an aerial extent of thousands of square miles or a few acres. The key requirements are that the layer or body has sufficient porosity to store the water, sufficient permeability to transmit the water, and be at least partly below the water table . The water table is the elevation of the top of the completely saturated (phreatic) zone. Above the water table is the vadose or unsaturated zone where the pore spaces are only partially saturated and contain a combination of air and water.

Porosity and permeability are important measures of producibility in aquifers. Porosity is the ratio of the volume of voids in a rock or soil to the total volume. Porosity determines the storage capacity of aquifers. In sand or sedimentary rocks , porosity is the space between grains and the volume of open space (per volume) in fractures. In dense rocks such as granite , porosity is contained largely within the crack and/or fracture system. Permeability is the capacity of a rock for transmitting a fluid, and is a measure of the relative ease with which a fluid can be produced from an aquifer.

A rock that yields large volumes of water at high rates must have many interconnected pore spaces or cracks. A dense, low porosity rock such as granite can be an adequate aquifer only if it contains an extensive enough system of connected fractures and cracks to be permeable. In the shallow subsurface, this is common because nearly all (indurate) rocks are fractured, often heavily. For that reason, caution should be exercised before assuming a low porosity rock will be an aquitard (impermeable body) and not an aquifer.

Fluid pressure, measured in pounds per square inch (psi), in an aquifer depends on whether it is unconfined or confined. An unconfined aquifer is one that is hydraulically open or connected to the surface. Examples would include sand bodies on or near the surface and more deeply buried layers of rock or sand connected to the surface by fractures and/or faults. The fluid pressure in unconfined aquifers is equivalent to what one would measure at a point in a standing body of water and would increase linearly (at a constant rate) with depth. The elevation of the top surface of an unconfined aquifer is free to fluctuate with rainfall.

A confined aquifer is one that is surrounded on all sides by an aquitard, a formation that does not transmit fluid. The pressure in a confined aquifer can be different from that of an unconfined aquifer at the same elevation. A body of sand surrounded on all sides by a soft, impermeable clay or shale serves as a typical example.

See also Hydrogeology; Saturated zone; Water table

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aquifer

aquifer A body of permeable rock (e.g. unconsolidated gravel or a sand stratum) that is capable of storing significant quantities of water, that is underlain by impermeable material, and through which groundwater moves. An unconfined aquifer is one in which the water-table defines the upper water limit. A confined aquifer is sealed above and below by impermeable material. A perched aquifer is an unconfined groundwater body supported by a small impermeable or slowly permeable unit. See artesian water.

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"aquifer." A Dictionary of Ecology. . Encyclopedia.com. 22 Aug. 2017 <http://www.encyclopedia.com>.

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aquifer

aquifer A body of permeable rock, for example, unconsolidated gravel or sand stratum, that is capable of storing significant quantities of water, is underlain by impermeable material, and through which groundwater moves. An unconfined aquifer is one in which the water table defines the upper water limit. A confined aquifer is sealed above and below by impermeable material. A perched aquifer is an unconfined groundwater body supported by a small impermeable or slowly permeable unit. See PERMEABILITY.

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"aquifer." A Dictionary of Earth Sciences. . Encyclopedia.com. 22 Aug. 2017 <http://www.encyclopedia.com>.

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"aquifer." A Dictionary of Earth Sciences. . Retrieved August 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/aquifer

aquifer

aquifer A body of rock, unconsolidated gravel, or sand stratum that is capable of storing significant quantities of water, that is underlain by impermeable material, and through which ground water moves. An unconfined aquifer is one in which the water table defines the upper water limit. A confined aquifer is sealed above and below by impermeable material. A perched aquifer is an unconfined ground-water body supported by a small impermeable or slowly permeable unit. See ARTESIAN WATER.

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"aquifer." A Dictionary of Plant Sciences. . Encyclopedia.com. 22 Aug. 2017 <http://www.encyclopedia.com>.

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aquifer

aquifer Rock, often sandstone or limestone, which is capable of both storing and transmitting water owing to its porosity and permeability. Much of the world's human population depends on aquifers for its water supply. They may be directly exploited by sinking wells.

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aquifer

aq·ui·fer / ˈäkwəfər; ˈak-/ • n. a body of permeable rock that can contain or transmit groundwater.

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"aquifer." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 22 Aug. 2017 <http://www.encyclopedia.com>.

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aquifer

aquifer (ăk´wĬfər): see artesian well.

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"aquifer." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 22 Aug. 2017 <http://www.encyclopedia.com>.

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"aquifer." The Columbia Encyclopedia, 6th ed.. . Retrieved August 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/aquifer

aquifer

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"aquifer." Oxford Dictionary of Rhymes. . Encyclopedia.com. 22 Aug. 2017 <http://www.encyclopedia.com>.

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"aquifer." Oxford Dictionary of Rhymes. . Retrieved August 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/aquifer