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== Klasifikasi ==
[[File:Ekman layer.jpg|thumb|350px|Transpor Ekman adalah pergerakan fluida akibat keseimbangan antara [[efek Coriolis]] dan [[gaya hambat]] turbulen. Pada gambar di atas, angin yang bergerak ke utara pada belahan Bumi utara menyebabkan terbentuknya tegangan permukaan dan [[spiral Ekman]] pada [[kolom air]] di bawahnya.]]
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 ===
'''Transpor Ekman''' adalah bagian dari teori pergerakan Ekman yang pertama kali diteliti pada tahun 1902 oleh [[Vagn Walfrid Ekman]]. Angin merupakan sumber energi utama bagi terbentuknya sirkulasi laut, termasuk transpor Ekman.<ref name="Ocean biogeochemical dynamics">{{cite book |last1=Sarmiento |first1=Jorge L. |last2=Gruber |first2=Nicolas |title=Ocean biogeochemical dynamics |date=2006 |publisher=Princeton University Press |isbn=978-0-691-01707-5}}</ref> Transpor Ekman terjadi ketika permukaan laut dipengaruhi oleh gaya gesek angin di atasnya. Hembusan angin menimbulkan gaya gesek pada permukaan laut dan turut mempengaruhi kolom air sedalam 10-100 meter di bawahnya.<ref name="Cambridge University Press">{{cite book |last1=Emerson |first1=Steven R. |last2=Hedges |first2=John I. |title=Chemical Oceanography and the Marine Carbon Cycle |date=2017 |publisher=Cambridge University Press |location=New York, United States of America |isbn=978-0-521-83313-4}}</ref> Meskipun demikian, [[efek Coriolis]] menyebabkan air tidak bergerak searah dengan arah angin, melainkan bergerak membentuk sudut 90° terhadap arah hembusan angin di permukaan.<ref name="Cambridge University Press"/> Arah transpor tergantung pada belahan Bumi terjadinya peristiwa tersebut. Pada belahan Bumi utara, transpor memiliki arah 90° searah jarum jam terhadap arah angin. Sementara itu, transpor pada belahan Bumi selatan memiliki arah 90° berlawanan arah jarum jam terhadap arah angin.<ref name=Colling42-44>Colling, pp 42-44</ref> Fenomena ini pertama kali dicatat oleh [[Fridtjof Nansen]] ketika Ia menjalani sebuah [[ekspedisi]] pada tahun 1890an. Ketika itu, Ia mengamati es bergerak dengan sudut tertentu terhadap arah angin.<ref>Pond & Pickard, p 101</ref>
{{see also|Kadar air}}
Air tanah dapat ditemui di hampir setiap titik di permukaan Bumi, meskipun tidak semua akuifer 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, kondisi tak jenuh merupakan kondisi area di atas muka air tanah dengan pressure head negatif dan
Transpor Ekman berdampak signifikan terhadap properti biogeokimia laut dunia. This is because they lead to [[upwelling]] (Ekman suction) and [[downwelling]] (Ekman pumping) in order to obey mass conservation laws. Mass conservation, in reference to Ekman transfer, requires that any water displaced within an area must be replenished. This can be done by either Ekman suction or Ekman pumping depending on wind patterns.<ref name="Ocean biogeochemical dynamics"/>
 
''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>
==Theory==
Ekman theory explains the theoretical state of circulation if water currents were driven only by the transfer of momentum from the wind. In the physical world, this is difficult to observe because of the influences of many simultaneous [[Ocean current|current]] driving forces (for example, [[pressure]] and [[density gradient]]s). Though the following theory technically applies to the idealized situation involving only wind forces, Ekman motion describes the wind-driven portion of circulation seen in the surface layer.<ref>Colling p 44</ref><ref>Sverdrup p 228</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.
Surface currents flow at a 45° angle to the wind due to a balance between the Coriolis force and the [[Drag (physics)|drags]] generated by the wind and the water.<ref>Mann & Lazier p 169</ref> If the ocean is divided vertically into thin layers, the magnitude of the velocity (the speed) decreases from a maximum at the surface until it dissipates. The direction also shifts slightly across each subsequent layer (right in the northern hemisphere and left in the southern hemisphere). This is called the [[Ekman Spiral|Ekman spiral]].<ref>Knauss p 124.</ref> The layer of water from the surface to the point of dissipation of this spiral is known as the [[Ekman layer]]. If all flow over the Ekman layer is integrated, the net transportation is at 90° to the right (left) of the surface wind in the northern (southern) hemisphere.<ref name=Colling42-44/>
 
=== 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. 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.
 
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|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#Forms of pollution|contamination]]. Where and how much water enters the groundwater from rainfall and snowmelt? How fast and what direction does the groundwater travel? How much water leaves the ground as springs? How much water can be sustainably pumped out? How quickly will a contamination incident reach a well or spring? [[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 flows through open conduits where water flows as underground streams]]
[[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 create 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 [[geologic 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 |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 |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 create 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 }}</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.
 
===Transboundary aquifer===
 
When an aquifer transcends international boundaries, the term ''transboundary aquifer'' applies.<ref>{{cite web |title=International Waters |website=United Nations Development Programme |url=http://www.undp.org/gef/05/portfolio/iw.html |url-status=dead |archive-date=27 January 2009 |archive-url=https://web.archive.org/web/20090127055412/http://www.undp.org/gef/05/portfolio/iw.html }}</ref>
 
Transboundariness is a concept, a measure and an approach first introduced in 2017.<ref>{{cite journal |last1=Sanchez |first1=Rosario |last2=Eckstein |first2=Gabriel |author-link2=Gabriel Eckstein|date=2017 |title=Aquifers Shared Between Mexico and the United States: Management Perspectives and Their Transboundary Nature |journal=Groundwater |volume=55 |number=4 |pages=495–505 |doi=10.1111/gwat.12533 |pmid=28493280 |url=https://transboundary.tamu.edu/media/1368/sanchez_et_al-2017-groundwater.pdf }}</ref> The relevance of this approach is that the physical features of the aquifers become just additional variables among the broad spectrum of considerations of the transboundary nature of an aquifer:
 
* social (population);
* economic (groundwater productivity);
* political (as transboundary);
* available research or data;
* water quality and quantity;
* other issues governing the agenda (security, trade, immigration and so on).
 
The discussion changes from the traditional question of “is the aquifer transboundary?” to “how transboundary is the aquifer?”.
 
The socio-economic and political contexts effectively overwhelm the aquifer's physical features adding its corresponding geostrategic value (its transboundariness)<ref>{{cite journal|url = https://transboundary.tamu.edu/media/1385/2018_awras_impact.pdf|title = Transboundary Groundwater|journal = Water Resources Impact|date = May 2018|volume = 20|number = 3}}</ref>
 
The criteria proposed by this approach attempt to encapsulate and measure all potential variables that play a role in defining the transboundary nature of an aquifer and its multidimensional boundaries.
[[Berkas:Major US Aquifers by Rock Type.jpg|thumb|right|Map of major US aquifers by rock type]]
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