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== Klasifikasi ==
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 are composed of layers of either [[clay]] or non-porous rock with low [[hydraulic conductivity]].
 
=== Jenuh dan tak jenuh ===
{{see also|Kadar air}}
Air tanah dapat ditemui di hampir setiap titik di permukaan Bumi, meskipun tidak semua akuifer di Bumi mengandung [[air tawar]]. Kerak Bumi dapat dibagi menjadi dua bagian, yakni zona [[Kadar air|jenuh]] atau [[Zona freatik|freatik]] dan zona tak jenuh atau [[Zona vados|vados]]. Zona freatik adalah area yang seluruh ruang kosongnya terisi oleh air, seperti akuifer, akuitard, dan sebagainya.<ref>{{Cite book|last=Pepper|first=Ian L.|last2=Gentry|first2=Terry J.|date=2015-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780123946263000041|title=Environmental Microbiology (Third Edition)|location=San Diego|publisher=Academic Press|isbn=978-0-12-394626-3|editor-last=Pepper|editor-first=Ian L.|pages=61–62|language=en|doi=10.1016/b978-0-12-394626-3.00004-1|editor-last2=Gerba|editor-first2=Charles P.|editor-last3=Gentry|editor-first3=Terry J.|url-status=live}}</ref> Sebaliknya, zona vados adalah area dalam tanah yang masih memiliki ruang untuk diisi oleh lebih banyak air.<ref>{{Cite web|last=|first=|date=|title=Unsaturated Zone|url=https://water.usgs.gov/ogw/unsaturated.html|website=USGS Groundwater Information|access-date=30-12-2020}}</ref>
 
Kondisi jenuh merupakan kondisi ketika [[pressure head]] air lebih besar daripada [[tekanan atmosfer]]. Dengan demikian, tekanan ukur untuk kondisi tersebut lebih dari nol. Pada muka air tanah, pressure head air sama dengan tekanan atmosfer, sehingga tekanan ukur pada kondisi tersebut sama dengan nol. Sementara itu, kondisiKondisi tak jenuh merupakan kondisi area di atas muka air tanah dengan pressure head negatif.<ref>{{Cite book|last=Atangana|first=Abdon|date=2018-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780128096703000126|title=Fractional Operators with Constant and Variable Order with Application to Geo-Hydrology|location=|publisher=Academic danPress|isbn=978-0-12-809670-3|editor-last=Atangana|editor-first=Abdon|pages=299|language=en|doi=10.1016/b978-0-12-809670-3.00012-6|url-status=live}}</ref>
 
=== Akuifer, akuitard, dan akuiklud ===
''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 that incompletely fills the pores of the aquifer material is under [[suction]]. The [[Hydrogeology#Water content|water content]] in the unsaturated zone is held in place by surface [[Adhesion|adhesive forces]] and it rises above the water table (the zero-[[Hydrogeology#Hydraulic head|gauge-pressure]] [[Contour line#Barometric 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 [[sand]]y soils with larger pores, the head will be less than in clay soils with very small pores. The normal capillary rise in a clayey soil is less than {{convert|1.8|m|ft|0|abbr=on}} but can range between {{convert|0.3|and|10|m|ft|0|abbr=on}}.<ref>{{cite web |url=http://www.ces.ncsu.edu/plymouth/programs/vepras.html |title=Morphological Features of Soil Wetness |publisher=Ces.ncsu.edu |access-date=6 September 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100809084433/http://www.ces.ncsu.edu/plymouth/programs/vepras.html |archive-date=9 August 2010 }}</ref>
Akuifer merupakan daerah di bawah tanah yang mampu mengalirkan air dalam jumlah yang cukup untuk menyediakan air bagi [[sumur]] dan [[mata air]]. Zona yang mengalirkan air lebih lambat dibandingkan akuifer disebut akuitard. Akuitard mampu mengalirkan air dalam jumlah yang kecil sehingga tidak dapat memenuhi kebutuhan air sumur. Sementara itu, akuiklud adalah formasi geologis jenuh yang mampu menyimpan air, tetapi sulit untuk mengalirkannya.<ref name=":0">{{Cite book|last=Şen|first=Zekâi|date=2015-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780128000755000029|title=Practical and Applied Hydrogeology|location=Oxford|publisher=Elsevier|isbn=978-0-12-800075-5|editor-last=Şen|editor-first=Zekâi|pages=47|language=en|doi=10.1016/b978-0-12-800075-5.00002-9|url-status=live}}</ref>
 
Akuifer dapat berupa formasi batuan seperti kerikil, [[Batu pasir|paras]], batu kapur, dan sebagainya. Akuitard dapat berupa formasi pasir berlumpur yang memiliki [[permeabilitas]] rendah. Tanah lempung dan serpih dapat dikategorikan sebagai akuiklud karena tidak mampu mengalirkan air dalam jumlah yang berarti.<ref name=":0" />
The capillary rise of water in a small-[[diameter]] tube involves the same physical process. The water table is the level to which water will rise in a large-diameter pipe (e.g., a well) that goes down into the aquifer and is open to the atmosphere.
 
=== AquifersTerkekang versusdan aquitardsbebas ===
{{see also|Porositas}}Akuifer juga dapat dibagi menjadi akuifer terkekang dan akuifer bebas. Akuifer bebas, juga disebut sebagai akuifer muka air atau akuifer freatik, adalah akuifer yang batas atasnya berupa muka air atau permukaan freatik. Akuifer bebas tidak memiliki lapisan pembatas seperti akuiklud dan akuitard di atasnya. Umumnya, akuifer yang berada paling dekat dengan permukaan Bumi merupakan akuifer bebas. Sebaliknya, akuifer terkekang merupakan akuifer yang dibatasi oleh lapisan pembatas seperti tanah lempung di atasnya.<ref>{{Cite web|title=What is the difference between a confined and an unconfined (water-table) aquifer?|url=https://www.usgs.gov/faqs/what-difference-between-a-confined-and-unconfined-water-table-aquifer?qt-news_science_products=0#qt-news_science_products|website=www.usgs.gov|access-date=2020-12-31}}</ref> Lapisan pembatas ini dapat melindungi akuifer terkekang terhadap kontaminasi dari permukaan Bumi. Meskipun demikian, akuifer terkekang juga dapat terkontaminasi akibat aktivitas pertambangan.<ref>{{Cite journal|last=Teng|first=Yanguo|last2=Feng|first2=Dan|last3=Song|first3=Liuting|last4=Wang|first4=Jinsheng|last5=Li|first5=Jian|date=2013-11-01|title=Total petroleum hydrocarbon distribution in soils and groundwater in Songyuan oilfield, Northeast China|url=https://doi.org/10.1007/s10661-013-3274-4|journal=Environmental Monitoring and Assessment|language=en|volume=185|issue=11|pages=9567|doi=10.1007/s10661-013-3274-4|issn=1573-2959}} "In the unconfined aquifer, the highest concentration of TPH in groundwater was mainly in farmland, while for the confined aquifer, higher concentrations of TPH in groundwater were mainly in the oil exploitation area."</ref>
Aquifers are typically saturated regions of the subsurface that produce an economically feasible quantity of water to a well or [[spring (hydrosphere)|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. A completely impermeable aquitard is called an ''aquiclude'' or ''aquifuge''. Aquitards comprise layers of either clay or non-porous rock with low [[hydraulic conductivity]].
 
In mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated [[alluvium]], 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), whereas 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.
 
{{see also|Hydraulic conductivity|Storativity}}
 
=== 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. (See [[Biscayne Aquifer]].) 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. The term "perched" refers to ground water accumulating above a low-permeability unit or strata, such as a clay layer. 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). Confined aquifers are aquifers that are overlain by a confining layer, often made up of clay. The confining layer might offer some protection from surface contamination.
 
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 [[Specific storage|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 storage|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 [[Hydrogeology#Porosity|porosity]] of the aquifer material, or the minimum volumetric [[water content]]).
{{see also|Porosity|Storativity}}
 
=== Isotropic versus anisotropic ===
In [[Isotropy|isotropic]] aquifers or aquifer layers the hydraulic conductivity (K) is equal for flow in all directions, while in [[Anisotropy|anisotropic]] conditions it differs, notably in horizontal (Kh) and vertical (Kv) sense.
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