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== Terms & Synonyms ==
<big>Types of aquifers</big>
== Official WHO Definition ==
== Other Definitions ==
== <!-- IGNORE THIS --> ==
{{GlossaryRelated}}
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== Interpretations and Explanations ==
== Description ==
[[Image:Aquifer.jpg|thumb|An unconfined aquifer]]
An aquifer is a water bearing layer in the underground. See also: [[Aquifer]].
There are all kinds of aquifers. The most commonly used types are the <b>confined</b> (or artesian) aquifer, the <b>unconfined</b> (or free, phreatic) aquifer and the <b>semi-confined</b> aquifer. There are also <b>complex aquifer systems</b> consisting of a number of different aquifer types.
<u>confined (artesian) aquifer</u><br>
A confined (artesian) aquifer is an aquifer bounded both at the bottom and at the top by an impermeable stratum (aquiclude) and fully filled with water which is usually under (artesian) pressure. When perforating a hole into the ground until reaching the confined aquifer, one will see the water level in the hole rise to above the top of the aquifer and perhaps even above the land surface. In the latter case the water will flow out from the hole over the land surface. In the upstream part the confined aquifer gradually changes into an unconfined aquifer (see below) which forms the recharge zone where rain water reaches the water-table.<br>
The <i>Al Hasa oasis</i> around Al Hofuf near Dahran in Saudi Arabia is underlain by an enormous artesian aquifer and there are a multitude of natural cracks through which the groundwater pours out. The spring water is hot as it comes from a great depth. The aquifer carries fresh water even into the sea and to the island of Bahrain. The aquifer is fed by the infiltration of rain water falling in the hilly central plateau of the peninsula. Although the annual rainfall is low (only 400 mm), the runoff from the hills can be considerable and it collects in the local depressions. Here, part of the water has time to infiltrate and percolate to the underground before it evaporates. The depressions have enough natural vegetation to serve as rangeland for large herds of camels.
<u>unconfined aquifer</u><br>
An unconfined (free, phreatic) aquifer is an aquifer underlain by an impermeable stratum, but the top of the the aquifer consists of soil layers that are permeable enough to provide easy passage of water, at least in vertical sense. Such an aquifer has a free water table or phreatic surface.<br>
When perforating a hole into the ground until it fills with water, and letting the water come to rest, the water level in the hole can be observed and it indicates the level of the water table in the aquifer outside the hole. Many alluvial fans and river plains have unconfined aquifers in the upper part of the sediment deposits.<br>
The <i>Indus basin</i> in Pakistan provides examples of extensive and deep unconfined aquifers. Where the water quality of the aquifer is acceptable, the aquifer is intensively pumped from wells, mostly for irrigation purposes. South of the barrage near Sukkur, the groundwater quality worsens due to salinity.
<u>semi-confined aquifer</u>
[[Image:Aquifers.svg.png|right|300px]]
A semi-confined aquifer is an aquifer underlain by an impermeable stratum and bounded at the top by soil layers of relatively low permeability (hydraulic conductivity), especially in horizontal sense. These layers form the semi-confining layer (the aquitard) in which a free water table is found.<br>
The flow of water in the top layer is mainly vertical while the horizontal flow is negligible. Semi-confined aquifers are often found in river delta's and coastal plains.<br>
When drilling a hole into the semi-confining layer  until reaching the aquifer proper and placing a tight fitting tube in the hole (this is a piezometer), one may observe that the water level in the tube (the piezometric level) is either above, at, or below the level of the water-table in the semi-confining layer. When the piezometric level is above the level of the water table, the aquifer has an overpressure and groundwater will flow upwards from the aquifer into the semi-confining layer. When, reversely, the piezometric level is below the level of the water-table, the aquifer has an under-pressure and there will be downward flow of water (deep percolation, natural drainage) into the aquifer.<br>
The <i>Nile delta</i> in Egypt is gives an example of a semi-confined aquifer with an upper, semi-confining, layer of dense clay soil with a relatively low hydraulic conductivity overlying thick course textured sediments with a relatively high hydraulic conductivity. In the upper part of the delta, towards its apex near Cairo, the aquifer has an under-pressure and the topsoil drains naturally to the aquifer. This is the recharge zone of the aquifer. More to the north the situation gets reversed: the groundwater seeps from the aquifer up into the topsoil and the water-table is shallow. If there is no drainage system, the seepage water will evaporate and leave salts behind.(See also:  "[[Salinity control|Soil salinity]]"  and  "[[Watertable control|Land drainage]]" ). The discharge zone of the aquifer further extends into the Mediterranean sea.
== Flow velocity ==
Although aquifers can transport large quantities of water, the velocity of the flow is slow. <br>
For example, assuming an aquifer with a very high hydraulic conductivity of 100 m/day and a relative large hydraulic gradient of 1% (i.e. 1 m hydraulic head loss over a distance of 100 m), the (apparent)flow velocity is only 1 m/day, which is very small compared to the water velocity in rivers and channels. With this velocity, an aquifer of 100 m depth transports 100 m<sup>3</sup>/day per m width or 100000 m<sup>3</sup>/day per km width. <br>
Most aquifers have a flow velocity that is much smaller than 1 m/day.
== References ==
<references/>
'''[http://www.who.int/thelexicon/entry.php?list&langue1=en&langue2=fr&langue3=es&langue4=ru&langue5=de&langue6=zh&orderby=term_en&submit=Search&searchfilter=all&methode=contient&term={{SUBPAGENAME}} WHO-Lexicon page] (translations and examples)'''
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== See also ==
Internal links:
* [[Groundwater]]
* [[Groundwater table]]
* [[Soil hydrology|Soil hydrology]]
* [[Watertable control]]
* [[Salinity control]]
== External Resources ==
An overview of land forms and aquifer types can be consulted in chapter 2: "Groundwater Investigations" of ILRI publication 16: "Drainage Principles and Applications" in: http://www.alterra.wur.nl/NL/publicaties+Alterra/ILRI-publicaties/Downloadable+publications/
An example of free software on a spatial-agro-hydro-salinity groundwater model based on a polygonal network <i>SahysMod</i> that gives the opportunity to differentiate unconfined and semi-confined aquifers can be downloaded from : [http://www.waterlog.info/sahysmod.htm] , with an explanatory paper : [http://www.waterlog.info/pdf/sahysmod.pdf]
Calculations of the drainage of aquifers by a well field in unconfined and semi-confined aquifers can be done with a computer program <i>WellDrain</i> downloadable from : [http://www.waterlog.info/weldrain.htm] , with an explanatory paper : [http://www.waterlog.info/pdf/wellspac.pdf]
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== Attachments ==
[[Category:Hydrology]]
[[Category:Groundwater Resources]]
[[Category:Glossary]]
<layout name="Glossary" />
Templates used on this page:
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