Editing Aquifer (section)

== Classification ==
An ''[[wikt:aquitard|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]].

=== Saturated versus unsaturated ===
{{See also|Water content|Soil moisture}}
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 ''[[water content|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 contain some water, but can be filled with more water.

''Saturated'' means the pressure head of the water is greater than [[atmospheric pressure]] (it has a gauge pressure &gt; 0). The definition of the water table is the surface where the [[Hydraulic head|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 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>

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.

=== Aquifers versus aquitards ===
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.<ref>{{Cite web |title=Understanding Aquitards and Aquicludes {{!}} UNSW Connected Waters Initiative |url=https://www.connectedwaters.unsw.edu.au/schools-resources/fact-sheets/understanding-aquitards-and-aquicludes |access-date=2022-12-07 |website=www.connectedwaters.unsw.edu.au}}</ref> A completely impermeable aquitard is called an ''aquiclude'' or ''aquifuge''. Aquitards contain 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 ===
An unconfined aquifer has no impermeable barrier immediately above it, such that the water level can rise in response to recharge. A confined aquifer has an overlying impermeable barrier that prevents the water level in the aquifer from rising any higher. An aquifer in the same geologic unit may be confined in one area and unconfined in another. ''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 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.

Semi-confined aquifers with one or more aquitards work as an anisotropic system, even when the separate layers are isotropic, because the compound Kh and Kv values are different (see [[Transmissibility (fluid)|hydraulic transmissivity]] and [[hydraulic conductivity#Resistance|hydraulic resistance]]).

When calculating [[drainage equation|flow to drains]]<ref>''The energy balance of groundwater flow applied to subsurface drainage in anisotropic soils by pipes or ditches with entrance resistance''. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. On line : [http://www.waterlog.info/pdf/enerart.pdf] {{Webarchive|url=https://web.archive.org/web/20090219221547/http://waterlog.info/pdf/enerart.pdf|archive-url=https://web.archive.org/web/20060522173736/http://www.waterlog.info/pdf/enerart.pdf|archive-date=2006-05-22|url-status=live|date=2009-02-19}} . Paper based on: R.J. Oosterbaan, J. Boonstra and K.V.G.K. Rao, 1996, "The energy balance of groundwater flow". Published in V.P.Singh and B.Kumar (eds.), Subsurface-Water Hydrology, pp. 153–60, Vol. 2 of Proceedings of the International Conference on Hydrology and Water Resources, New Delhi, India, 1993. Kluwer Academic Publishers, Dordrecht, The Netherlands. {{ISBN|978-0-7923-3651-8}} . On line : [http://www.waterlog.info/pdf/enerbal.pdf] . The corresponding "EnDrain" software can be downloaded from : [http://www.waterlog.info/software.htm], or from : [http://www.waterlog.info/endrain.htm]</ref> or [[drainage by wells|flow to wells]]<ref>ILRI (2000), ''Subsurface drainage by (tube)wells: Well spacing equations for fully and partially penetrating wells in uniform or layered aquifers with or without anisotropy and entrance resistance'', 9 pp. Principles used in the "WellDrain" model. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. On line : [http://www.waterlog.info/pdf/wellspac.pdf] . Download "WellDrain" software from : [http://www.waterlog.info/software.htm], or from : [http://www.waterlog.info/weldrain.htm]</ref> in an aquifer, the anisotropy is to be taken into account lest the resulting design of the drainage system may be faulty.

===Porous, karst, or fractured===

To properly manage an aquifer its properties must be understood. Many properties must be known to predict how an aquifer will respond to rainfall, drought, pumping, and [[Pollution#Definitions and types|contamination]]. Considerations include where and how much water enters the groundwater from rainfall and snowmelt, how fast and in what direction the groundwater travels, and how much water leaves the ground as springs. [[Groundwater model|Computer models]] can be used to test how accurately the understanding of the aquifer properties matches the actual aquifer performance.<ref name="FieldMethodsGeoHydrogeo">{{cite book|last1= Assaad |first1= Fakhry |last2=LaMoreaux |first2=Philip |last3=Hughes |first3=Travis |date=2004 |title=Field methods for geologists and hydrogeologists |location=Berlin, Germany |publisher= Springer-Verlag Berlin Heidelberg |isbn= 978-3-540-40882-6 |doi=10.1007/978-3-662-05438-3}}</ref>{{rp|192–193, 233–237}} Environmental regulations require sites with potential sources of contamination to demonstrate that the [[Hydrology#Groundwater|hydrology]] has been [[Environmental monitoring|characterized]].<ref name="FieldMethodsGeoHydrogeo" />{{rp|3}}

====Porous====
[[File:Water seep from sandstone in Hanging Garden SE Utah.jpg|thumb|left|alt=Water slowly seeping from tan porous sandstone at contact with impermeable gray shale creates a refreshing growth of green vegetation in the desert. |Water in porous aquifers slowly seeps through pore spaces between sand grains]]

Porous aquifers typically occur in sand and [[sandstone]]. Porous aquifer properties depend on the [[depositional environment|depositional sedimentary environment]] and later natural cementation of the sand grains. The environment where a sand body was deposited controls the orientation of the sand grains, the horizontal and vertical variations, and the distribution of shale layers. Even thin shale layers are important barriers to groundwater flow. All these factors affect the [[porosity]] and [[Permeability (earth sciences)|permeability]] of sandy aquifers.<ref name="SandSandstone">{{cite book|last1= Pettijohn |first1= Francis |last2=Potter |first2=Paul |last3=Siever |first3=Raymond |date=1987 |title=Sand and Sandstone |location=New York |publisher= Springer Science+Business Media |isbn= 978-0-387-96350-1 |doi=10.1007/978-1-4612-1066-5 }}</ref>{{rp|413}}

Sandy deposits formed in [[Shallow water marine environment|shallow marine environments]] and in [[aeolian processes|windblown sand dune environments]] have moderate to high permeability while sandy deposits formed in [[Fluvial processes|river environments]] have low to moderate permeability.<ref name="SandSandstone" />{{rp|418}} Rainfall and snowmelt enter the groundwater where the aquifer is near the surface. Groundwater flow directions can be determined from [[potentiometric surface]] maps of water levels in wells and springs. [[Aquifer test]]s and [[well test]]s can be used with [[Darcy's law]] flow equations to determine the ability of a porous aquifer to convey water.<ref name="FieldMethodsGeoHydrogeo" />{{rp|177–184}}

Analyzing this type of information over an area gives an indication how much water can be pumped without [[overdrafting]] and how contamination will travel.<ref name="FieldMethodsGeoHydrogeo" />{{rp|233}} In porous aquifers groundwater flows as slow seepage in pores between sand grains. A groundwater flow rate of 1 foot per day (0.3 m/d) is considered to be a high rate for porous aquifers,<ref>{{cite book |title=Sustainability of ground-water resources. |publisher=U.S. Geological Survey |location=Denver, Colorado |series=Circular 1186 |url=https://archive.org/details/sustainabilityof00alle/page/8 |last1=Alley |first1=William |last2=Reilly |first2=Thomas |last3=Franke |first3=O. |page=[https://archive.org/details/sustainabilityof00alle/page/8 8] |date=1999 |isbn=978-0-607-93040-5 |doi=10.3133/cir1186 |url-access=registration }}</ref> as illustrated by the water slowly seeping from sandstone in the accompanying image to the left.

Porosity is important, but, ''alone'', it does not determine a rock's ability to act as an aquifer. Areas of the [[Deccan Traps]] (a [[basalt]]ic lava) in west central India are good examples of rock formations with high porosity but low permeability, which makes them poor aquifers. Similarly, the micro-porous (Upper [[Cretaceous]]) [[Chalk Group]] of south east England, although having a reasonably high porosity, has a low grain-to-grain permeability, with its good water-yielding characteristics mostly due to micro-fracturing and fissuring.

====Karst====
[[File:MammothCaveNPS.jpg|thumb|left |alt=Several people in a jon boat on a river inside a cave. |Water in karst aquifers can form [[Subterranean river|subterranean rivers]].]]
[[Karst]] aquifers typically develop in [[limestone]]. Surface water containing natural [[carbonic acid]] moves down into small fissures in limestone. This carbonic acid gradually dissolves limestone thereby enlarging the fissures. The enlarged fissures allow a larger quantity of water to enter which leads to a progressive enlargement of openings. Abundant small openings store a large quantity of water. The larger openings form a conduit system that drains the aquifer to springs.<ref>{{cite book |last=Dreybrodt |first=Wolfgang |date=1988 |title=Processes in karst systems: physics, chemistry, and geology |volume=4 |location=Berlin |publisher=Springer |pages=2–3 |isbn=978-3-642-83354-0 |doi=10.1007/978-3-642-83352-6 |series=Springer Series in Physical Environment }}</ref>

Characterization of karst aquifers requires field exploration to locate [[sinkhole|sinkholes, swallets]], [[Losing stream|sinking streams]], and [[Spring (hydrology)|springs]] in addition to studying [[geological map]]s.<ref name="DelineationGrdwtrBasinsTaylor">{{cite book |last=Taylor |first=Charles |date=1997 |title=Delineation of ground-water basins and recharge areas for municipal water-supply springs in a karst aquifer system in the Elizabethtown area, Northern Kentucky |url=https://pubs.usgs.gov/wri/1996/4254/report.pdf |location=Denver, Colorado |publisher=U.S. Geological Survey |series=Water-Resources Investigations Report 96-4254 |doi=10.3133/wri964254 }}</ref>{{rp|4}} Conventional hydrogeologic methods such as aquifer tests and potentiometric mapping are insufficient to characterize the complexity of karst aquifers. These conventional investigation methods need to be supplemented with [[Dye tracing|dye traces]], measurement of spring discharges, and analysis of water chemistry.<ref>{{cite book |last1=Taylor |first1=Charles |last2=Greene |first2=Earl |date=2008 |title=Field Techniques for Estimating Water Fluxes Between Surface Water and Ground Water |chapter=Hydrogeologic characterization and methods used in the investigation of karst hydrology. |chapter-url=https://pubs.usgs.gov/tm/04d02/pdf/TM4-D2-chap3.pdf |archive-url=https://web.archive.org/web/20081102202902/http://pubs.usgs.gov/tm/04d02/pdf/TM4-D2-chap3.pdf |archive-date=2008-11-02 |url-status=live |series=Techniques and Methods 4–D2 |publisher=U.S. Geological Survey |page=107 }}</ref> U.S. Geological Survey dye tracing has determined that conventional groundwater models that assume a uniform distribution of porosity are not applicable for karst aquifers.<ref>{{cite journal |last1=Renken |first1=R. |last2=Cunningham |first2=K. |last3=Zygnerski |first3=M. |last4=Wacker |first4=M. |last5=Shapiro |first5=A. |last6=Harvey |first6=R. |last7=Metge |first7=D. |last8=Osborn |first8=C. |last9=Ryan |first9=J. |date=November 2005 |title=Assessing the Vulnerability of a Municipal Well Field to Contamination in a Karst Aquifer |journal= Environmental and Engineering Geoscience |publisher=GeoScienceWorld|volume=11 |number=4 |page=320 |doi=10.2113/11.4.319 |bibcode=2005EEGeo..11..319R |citeseerx=10.1.1.372.1559 }}</ref>

Linear alignment of surface features such as straight stream segments and sinkholes develop along [[Fracture (geology)|fracture traces]]. Locating a well in a fracture trace or intersection of fracture traces increases the likelihood to encounter good water production.<ref>{{cite book |last=Fetter |first=Charles |date=1988 |title=Applied Hydrology |location=Columbus, Ohio |publisher=Merrill |pages=294–295 |isbn=978-0-675-20887-1 }}</ref> Voids in karst aquifers can be large enough to cause destructive collapse or [[subsidence]] of the ground surface that can initiate a catastrophic release of contaminants.<ref name="FieldMethodsGeoHydrogeo" />{{rp|3–4}} Groundwater flow rate in karst aquifers is much more rapid than in porous aquifers as shown in the accompanying image to the left. For example, in the Barton Springs Edwards aquifer, dye traces measured the karst groundwater flow rates from 0.5 to 7 miles per day (0.8 to 11.3&nbsp;km/d).<ref>{{cite journal |last1=Scanlon |first1=Bridget|author1-link= Bridget Scanlon |last2=Mace |first2=Robert |last3=Barrett |first3=Michael |last4=Smith |first4=Brian |date=2003  |title= Can we simulate regional groundwater flow in a karst system using equivalent porous media models? Case study, Barton Springs Edwards aquifer, USA |journal= Journal of Hydrology |publisher=Elsevier Science |volume=276 |issue= 1–4|page=142 |doi= 10.1016/S0022-1694(03)00064-7 |bibcode=2003JHyd..276..137S|s2cid=16046040 }}</ref> The rapid groundwater flow rates make [[Karst#Hydrology|karst aquifers much more sensitive]] to groundwater contamination than porous aquifers.<ref name="DelineationGrdwtrBasinsTaylor" />{{rp|1}}

In the extreme case, groundwater may exist in ''underground rivers'' (e.g., [[cave]]s underlying [[karst topography]]).

====Fractured====
If a rock unit of low [[porosity]] is highly fractured, it can also make a good aquifer (via [[Fracture (geology)|fissure]] flow), provided the rock has a hydraulic conductivity sufficient to facilitate movement of water.

[[File:Major US Aquifers by Rock Type.jpg|thumb|right|Map of major US aquifers by rock type]]