Editing Aquifer (section)

====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]]).