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=== Terkekang dan bebas ===
{{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 jugatetap 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>
Apabila perbedaan geologis antara akuifer bebas dan terkekang tidak begitu jelas, klasifikasi akuifer dapat ditentukan melalui nilai storativitas yang didapatkan dari uji akuifer. Akuifer terkekang umumnya memilki nilai storativitas kecil (antara {{10^|-5}} hingga 0.001), sementara akuifer bebas memiliki nilai storativitas lebih tinggi (antara 0.01 hingga 0.35). Meskipun demikian, data yang didapatkan dari uji akuifer harus diimbangi dengan data lainnya karena nilai storativitas juga bergantung pada ketebalan akuifer, besar batuan, bentuk pori, dan faktor-faktor lainnya.<ref>{{Cite journal|last=Kumar|first=T. Jeyavel Raja|last2=Balasubramanian|first2=A.|last3=Kumar|first3=R. S.|last4=Dushiyanthan|first4=C.|last5=Thiruneelakandan|first5=B.|last6=Suresh|first6=R.|last7=Karthikeyan|first7=K.|last8=Davidraju|first8=D.|date=2016-06-01|title=Assessment of groundwater potential based on aquifer properties of hard rock terrain in the Chittar–Uppodai watershed, Tamil Nadu, India|url=https://doi.org/10.1007/s13201-014-0216-4|journal=Applied Water Science|language=en|volume=6|issue=2|pages=183|doi=10.1007/s13201-014-0216-4|issn=2190-5495}}</ref>
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]]).
=== 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.
=== Isotropik dan anisotropik ===
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]]).
Akuifer dapat dikategorikan sebagai akuifer [[Isotropi|isotropik]] apabila konduktivitas hidrauliknya (K) sama saat ditinjau dari semua arah. Sebaliknya, apabila konduktivitas hidraulik akuifer berbeda saat ditinjau dari arah yang berbeda, maka akuifer tersebut dikategorikan sebagai akuifer anisotropik. Akuifer semi terkekang dengan satu atau lebih akuitard dikategorikan sebagai sistem anisotropik karena memiliki konduktivitas hidraulik horizontal (Kh) dan vertikal (Kv) yang berbeda meskipun pada kasus tertentu memiliki lapisan-lapisan pembatas isotropik.<ref>{{Cite book|last=Atangana|first=Abdon|date=2018-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780128096703000011|title=Fractional Operators with Constant and Variable Order with Application to Geo-Hydrology|location=|publisher=Academic Press|isbn=978-0-12-809670-3|editor-last=Atangana|editor-first=Abdon|pages=2–3|language=en|doi=10.1016/b978-0-12-809670-3.00001-1|url-status=live}}</ref>[[Berkas:Major US Aquifers by Rock Type.jpg|thumb|right|Map of major US aquifers by rock type]]
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 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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