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'''Saltwater intrusion''' is the movement of [[saline water]] into freshwater [[aquifer]]s, which can lead to contamination of drinking water sources and other consequences. Saltwater intrusion occurs naturally to some degree in most [[coast]]al [[aquifer]]s, owing to the [[hydraulic]] connection between [[groundwater]] and [[seawater]]. Because saltwater has a higher mineral content than freshwater, it is denser and has a higher water pressure. As a result, saltwater can push inland beneath the freshwater.<ref>{{cite web
  |url=http://www.wrd.org/engineering/reports/TB13_Fall07_Seawater_Barriers.pdf
  |title=Battling Seawater Intrusion in the Central & West Coast Basins
  |publisher=Water Replenishment District of Southern California
  |author=Johnson, Ted
  |year=2007
  |accessdate=2012-10-08
}}</ref> Certain human activities, especially groundwater pumping from coastal freshwater wells, have increased saltwater intrusion in many coastal areas. Water extraction drops the level of fresh groundwater, reducing its water pressure and allowing saltwater to flow further inland. Other contributors to saltwater intrusion include navigation channels or agricultural and drainage channels, which provide conduits for saltwater to move inland, and sea level rise.<ref name=Barlow2003>{{cite web
  |url=http://pubs.usgs.gov/circ/2003/circ1262/
  |title=Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast
  |publisher=[[United States Geological Survey|USGS]]
  |author=Barlow, Paul M.
  |year=2003
  |accessdate=2009-03-21
}}</ref> Saltwater intrusion can also be worsened by extreme events like hurricane storm surges.<ref>{{cite web
  |url=http://www.lacoast.gov/WATERMARKS/1996b-fall/6cwptionary/
  |title=CWPtionary Saltwater Intrusion
  |publisher=LaCoast.gov
  |author=
  |year=1996
  |accessdate=2009-03-21
}}</ref>

==Hydrology==
[[image:Salt_water_intrusion_wikipedia4.png|thumb|600px|Cause and Impact of Saltwater Intrusion]]
At the coastal margin, fresh groundwater flowing from inland areas meets with saline groundwater from the ocean. The fresh groundwater flows from inland areas towards the coast where elevation and groundwater levels are lower.<ref name=Barlow2003>{{cite web
  |url=http://pubs.usgs.gov/circ/2003/circ1262/
  |title=Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast
  |publisher=[[United States Geological Survey|USGS]]
  |author=Barlow, Paul M.
  |year=2003
  |accessdate=2009-03-21
}}</ref>  Because saltwater has a higher content of [[Total dissolved solids|dissolved salts and minerals]], it is denser than freshwater, causing it to have higher [[hydraulic head]] than freshwater. Hydraulic head refers to the liquid pressure exerted by a water column: a water column with higher hydraulic head will move into a water column with lower hydraulic head, if the columns are connected.<ref>{{cite web
  |url=http://www.wrd.org/engineering/reports/TB13_Fall07_Seawater_Barriers.pdf
  |title=Battling Seawater Intrusion in the Central & West Coast Basins
  |publisher=Water Replenishment District of Southern California
  |author=Johnson, Ted
  |year=2007
  |accessdate=2012-10-08
}}</ref>

The higher pressure and density of saltwater causes it to move into coastal aquifers in a wedge shape under the freshwater. The saltwater and freshwater meet in a transition zone where mixing occurs through [[dispersion (chemistry)|dispersion]] and [[diffusion]]. Ordinarily the inland extent of the saltwater wedge is limited because fresh groundwater levels, or the height of the freshwater column, increases as land elevation gets higher.<ref name=Barlow2003>{{cite web
  |url=http://pubs.usgs.gov/circ/2003/circ1262/
  |title=Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast
  |publisher=[[United States Geological Survey|USGS]]
  |author=Barlow, Paul M.
  |year=2003
  |accessdate=2009-03-21
}}</ref>

==Causes==
===Groundwater extraction===
[[Water abstraction|Groundwater extraction]] is the primary cause of saltwater intrusion. Groundwater is the main source of drinking water in many coastal areas of the United States, and extraction has increased over time. Under baseline conditions, the inland extent of saltwater is limited by higher pressure exerted by the freshwater column, owing to its higher elevation. Groundwater extraction can lower the level of the [[water table|freshwater table]], reducing the pressure exerted by the freshwater column and allowing the denser saltwater to move inland laterally.<ref name=Barlow2003>{{cite web
  |url=http://pubs.usgs.gov/circ/2003/circ1262/
  |title=Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast
  |publisher=[[United States Geological Survey|USGS]]
  |author=Barlow, Paul M.
  |year=2003
  |accessdate=2009-03-21
}}</ref> In [[Cape May, New Jersey]], since the 1940s water withdrawals have lowered groundwater levels by up to 30 meters, reducing the water table to below sea level and causing widespread intrusion and contamination of [[water supply]] wells.<ref name=Lacombe&Carleton2002>{{cite web
  |url=http://pubs.usgs.gov/wri/wri014246/pdf/wrir01-4246.pdf
  |title=Hydrogeologic Framework, Availability of Water Supplies, and Saltwater Intrusion, Cape May County, New Jersey
  |publisher=[[United States Geological Survey|USGS]]
  |author=Lacombe, Pierre J. and Carleton, Glen B.
  |year=2002
  |accessdate=2012-12-10
}}</ref><ref name=Barlow&Reichard2010>{{cite web
  |url=http://pubs.er.usgs.gov/publication/70003372
  |title=Saltwater intrusion in coastal regions of North America
  |publisher=[[United States Geological Survey|USGS]]
  |author=Barlow, Paul M. and Reichard, Eric G.
  |year=2010
  |accessdate=2012-12-10
}}</ref>

Groundwater extraction can also lead to well contamination by causing upwelling, or upcoming, of saltwater from the depths of the aquifer.<ref name=Reilly&Goodman1987>{{cite journal |author=Reilly, T.E. and Goodman, A.S. |title=Analysis of saltwater upconing beneath a pumping well |journal=Journal of Hydrology |volume=89 |year=1987 |pages=169–204}}</ref> Under baseline conditions, a saltwater wedge extends inland, underneath the freshwater because of its higher density. Water supply wells located over or near the saltwater wedge can draw the saltwater upward, creating a saltwater cone that might reach and contaminate the well. Some aquifers are predisposed towards this type of intrusion, such as the [[Floridan aquifer|Lower Floridan aquifer]]: though a relatively [[Permeability (earth sciences)|impermeable]] rock or clay layer separates fresh groundwater from saltwater, isolated cracks breach the confining layer, promoting upward movement of saltwater. Pumping of groundwater strengthens this effect by lowering the water table, reducing the downward push of freshwater.<ref name=Barlow&Reichard2010>{{cite web
  |url=http://pubs.er.usgs.gov/publication/70003372
  |title=Saltwater intrusion in coastal regions of North America
  |publisher=[[United States Geological Survey|USGS]]
  |author=Barlow, Paul M. and Reichard, Eric G.
  |year=2010
  |accessdate=2012-12-10
}}</ref>

===Canals and drainage networks===
The construction of [[canal]]s and [[drainage]] networks can lead to saltwater intrusion. Canals provide conduits for saltwater to be carried inland, as does the deepening of existing [[Channel (geography)|channels]] for [[navigation]] purposes.<ref name=Barlow2003>{{cite web
  |url=http://pubs.usgs.gov/circ/2003/circ1262/
  |title=Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast
  |publisher=[[United States Geological Survey|USGS]]
  |author=Barlow, Paul M.
  |year=2003
  |accessdate=2009-03-21
}}</ref><ref name=Good_etal1995>{{cite web |format=pdf 
  |url=http://sonris-www.dnr.state.la.us/dnrservices/redirectUrl.jsp?dID=3903961
  |title=Louisiana's Major Coastal Navigation Channels
  |publisher=[[Louisiana Department of Natural Resources]]
  |author=Good, B. J., Buchtel, J., Meffert, D.J., Radford, J., Rhinehart, W., Wilson, R.
  |year=1995
 |deadurl=no |accessdate=2013-09-14
}}</ref> In [[Sabine Lake]] Estuary in the Gulf of Mexico, large-scale waterways have allowed saltwater to move into the lake, and upstream into the rivers feeding the lake. Additionally, channel dredging in the surrounding wetlands to facilitate oil and gas drilling has caused land [[subsidence]], further promoting inland saltwater movement.<ref name=SRAofTexas>{{cite web
  |url=ftp://ftp.sratx.org/pub/BBEST/Library/BBEST_020.pdf
  |title=Preliminary Investigation: Saltwater Barrier - Lower Sabine River
  |publisher=[[Sabine River Authority of Texas]]
  |author=Barlow, Paul M.
  |year=2008
  |accessdate=2012-12-09
}}</ref>

Drainage networks constructed to drain flat coastal areas can lead to intrusion by lowering the freshwater table, reducing the water pressure exerted by the freshwater column. Saltwater intrusion in southeast Florida has occurred largely as a result of drainage canals built between 1903 into the 1980s to drain the [[Everglades]] for agricultural and urban development. The main cause of intrusion was the lowering of the water table, though the canals also conveyed seawater inland until the construction of water control gates.<ref name=Barlow&Reichard2010>{{cite web
  |url=http://pubs.er.usgs.gov/publication/70003372
  |title=Saltwater intrusion in coastal regions of North America
  |publisher=[[United States Geological Survey|USGS]]
  |author=Barlow, Paul M. and Reichard, Eric G.
  |year=2010
  |accessdate=2012-12-10
}}</ref>

==Effect on water supply==
Many coastal communities around the United States are experiencing saltwater contamination of water supply wells, and this problem has been seen for decades.<ref name=Todd1960>{{cite journal
  | last = Todd
  | first = David K.
  | authorlink = 
  | title = Salt water intrusion of coastal aquifers in the United States
  | journal = Subterranean Water
  | volume = 
  | issue = 52
  | pages = pp. 452–461
  | publisher = IAHS Publ.
  | location = 
  | year = 1960
  | url = http://www.cig.ensmp.fr/~iahs/redbooks/a052/052043.pdf
  | accessdate = 2009-03-22
}}</ref> The consequences of saltwater intrusion for supply wells vary widely, depending on extent of the intrusion, the intended use of the water, and whether the salinity exceeds standards for the intended use.<ref name=Barlow2003>{{cite web
  |url=http://pubs.usgs.gov/circ/2003/circ1262/
  |title=Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast
  |publisher=[[United States Geological Survey|USGS]]
  |author=Barlow, Paul M.
  |year=2003
  |accessdate=2009-03-21
}}</ref> In some areas such as Washington State, intrusion only reaches portions of the aquifer, affecting only certain water supply wells. Other aquifers have faced more widespread salinity contamination, significantly affecting groundwater supplies for the region. For instance, in [[Cape May, New Jersey]], where groundwater extraction has lowered water tables by up to 30 meters, saltwater intrusion has caused closure of over 120 water supply wells since the 1940s.<ref name=Barlow&Reichard2010>{{cite web
  |url=http://pubs.er.usgs.gov/publication/70003372
  |title=Saltwater intrusion in coastal regions of North America
  |publisher=[[United States Geological Survey|USGS]]
  |author=Barlow, Paul M. and Reichard, Eric G.
  |year=2010
  |accessdate=2012-12-10
}}</ref>

==Ghyben-Herzberg relation==
The first physical formulations of saltwater intrusion were made by W. Badon-Ghijben (1888, 1889) and  A. Herzberg (1901), thus called the Ghyben-Herzberg relation.<ref name=Verrjuit1968>{{cite journal
  | last = Verrjuit
  | first = Arnold
  | authorlink = 
  | coauthors = 
  | title = A note on the Ghyben-Herzberg formula
  | journal = Bulletin of the International Association of Scientific Hydrology
  | volume = 13
  | issue = 4
  | pages = pp. 43–46
  | publisher = Technological University
  | location = Delft, Netherlands
  | year = 1968
  | url = http://www.cig.ensmp.fr/~iahs/hsj/134/134004.pdf
  | issn = 
  | doi = 
  | id = 
  | accessdate = 2009-03-21
}}</ref>  They derived analytical solutions to approximate the intrusion behavior, which are based on a number of assumptions that do not hold in all field cases.

[[Image:Saltwater Intrusion.gif]]<ref name=Barlow2003/>

The figure shows the Ghyben-Herzberg relation. In the equation, <blockquote><math> z = \frac{ \rho_f} {(\rho_s-\rho_f)} h</math></blockquote> the thickness of the freshwater zone above sea level is represented as <math>h</math> and that below sea level is represented as <math>z</math>. The two thicknesses <math>h</math> and <math>z</math>, are related by <math>\rho_f</math> and <math>\rho_s</math> where <math>\rho_f</math> is the density of freshwater and <math>\rho_s </math> is the density of saltwater. Freshwater has a density of about 1.000 grams per cubic centimeter (g/cm<sup>3</sup>) at 20 °C, whereas that of seawater is about 1.025 g/cm<sup>3</sup>. The equation can be simplified to <blockquote><math>z\ = 40 h</math>.<ref name=Barlow2003/></blockquote>

The Ghyben-Herzberg ratio states, for every foot of fresh water in an unconfined aquifer above sea level, there will be forty feet of fresh water in the aquifer below sea level.

In the 20th century the higher [[computer|computing]] power allowed the use of numerical methods (usually [[finite difference]]s or [[finite elements]]) that need fewer assumptions and can be applied more generally.{{Citation needed|date=March 2009}}

==Modeling==
Modeling of saltwater intrusion is considered difficult. Some typical difficulties that arise are:

* The possible presence of [[fissures]] and cracks and fractures in the aquifer, whose precise positions are unknown but which have great influence on the development of the saltwater intrusion

* The possible presence of small scale heterogeneities in the hydraulic properties of the aquifer, which are too small to be take into account by the model but which may also have great influence on the development of the saltwater intrusion

* The change of hydraulic properties by the saltwater intrusion. A mixture of saltwater and freshwater is often undersaturated with respect to calcium, triggering [[solvation|dissolution]] of calcium in the mixing zone and changing hydraulic properties.

* The process known as [[Ion exchange|cation exchange]], which slows the advance of a saltwater intrusion and also slows the retreat of a saltwater intrusion.

* The fact that saltwater intrusions are often not in equilibrium makes it harder to model. Aquifer dynamics tend to be slow and it takes the intrusion cone a long time to adapt to changes in pumping schemes, rainfall, etc. So the situation in the field can be significantly different from what would be expected based on the sea level, pumping scheme etc.

* For long-term models, the future [[climate change]] forms a large unknown. Model results often depend strongly on sea level and recharge rate. Both are expected to change in the future.

==Mitigation==
[[Image:Catfish Pond Control Structure.jpg|thumb|300px|Catfish Point control structure (lock) on the Mermentau River in coastal Louisiana]]

Saltwater intrusion is also an issue where a [[Canal lock|lock]] separates saltwater from freshwater (for example the [[Hiram M. Chittenden Locks]] in Washington). In this case a collection basin was built from which the saltwater can be pumped back to the sea. Some of the intruding saltwater is also pumped to the [[fish ladder]] to make it more attractive to migrating [[fish]].<ref name=Mausshardt1995>{{cite journal
  | last = Mausshardt
  | first = Sherrill
  | authorlink = 
  | coauthors = Singleton, Glen
  | title = Mitigating Salt-Water Intrusion through Hiram M. Chittenden Locks
  | journal = Journal of Waterway, Port, Coastal and Ocean Engineering
  | volume = 121
  | issue = 4
  | pages = pp. 224–227
  | publisher = 
  | location = 
  | year = 1995
  | url = http://cedb.asce.org/cgi/WWWdisplay.cgi?9503505
  | issn = 
  | doi = 10.1061/(ASCE)0733-950X(1995)121:4(224)
  | accessdate = 2009-03-20
}}</ref>

{{clear}}

==See also==
*[[Aquifer]]
*[[Environmental migrant]]
*[[Groundwater]]
*[[Garald G. Parker]]
*[[Inflatable rubber dam]]
*[[Living Waters for the World]]
*[[Peak water]]
*[[Sea level rise]]
*[[Water well]]

;Areas where water intrusion is occurring
*[[Water supply and sanitation in Benin]]
*[[Geography of Cyprus]]
*[[Bou Regreg]] (Morocco)
*[[Water resources management in Pakistan]]
*[[Water supply and sanitation in Tunisia]]
*United States
**[[ACF River Basin]] (Florida/Georgia)
**[[Environment of Florida]]
**[[Hiram M. Chittenden Locks]] (Washington)
**[[Hutchinson Island (Georgia)]]
**[[Lake Lanier]] (Georgia)
**[[Lake Pontchartrain]] (Louisiana)
**[[Miami River (Florida)]]
**[[San Leandro, California]]
**[[Sonoma Creek]] (California)

==References==
{{Reflist|30em}}

[[Category:Aquifers]]
[[Category:Geology]]
[[Category:Hydrology]]
[[Category:Aquatic ecology]]
[[Category:Environmental issues]]
[[Category:Water and the environment]]