Salt water intrusion

The term salt water intrusion ( intrusion , noun from the Latin verb intrudere , dt literally pushing in ) describes the penetration of salt water into freshwater aquifers near the coast . Due to the difference in density between salt and fresh water, this process takes place naturally on almost all coasts that border on sea water . Storm surges can also naturally cause the intrusion of salt water into coastal areas. In many cases, this process is now also generated by pumping groundwater from wells near the coast or caused by the construction of shipping canals . The channels offer the salt water an opportunity to come into contact with fresh water in coastal plains.

Since 1996, attention has been drawn to the phenomenon in many places that - conversely - groundwater can have a major influence on the water quality of coastal waters. It is less the discharge of rare, point sources than the diffuse infiltration of nitrates and ammonium into the coastal waters that makes empirical verification more difficult, but: "... the penetration of nutrient-rich groundwater into coastal waters can significantly contribute to eutrophication and harmful algal blooms ."

Effects on drinking water

If groundwater is removed faster than it can flow in , the groundwater level is lowered. This lowering reduces the hydrostatic pressure . If this happens near a seashore, the underground inflow of water from the sea becomes possible and the groundwater is contaminated with salt water. This process takes place today in numerous coastal locations, for example in the coastal states of the United States .

Hydrology

Saltwater intrusion is a normal process in many groundwater bodies near the ocean, even if the state is not disturbed by groundwater abstraction. The salt water has a higher density than the fresh water, so that the pressure under a salt water column is slightly higher than under a fresh water column of the same height. If the groundwater body and the saltwater body are connected to one another, this leads to a water flow at depth from the denser (salt water) to the less dense (freshwater) medium until the pressure conditions are balanced.

This inland balancing flow of salt water is restricted to areas near the coast. Further inland, the level of the groundwater is higher because the land surface is usually higher there, so that the higher pressure of the freshwater there is sufficient to withstand the inland pressure of the salt water. The higher level of freshwater further inland also creates an oceanward current in the upper part of the aquifer. At the land-sea boundary, fresh water flows towards the sea in the higher part of the aquifer; in the lower part, sea water is in hydrostatic equilibrium with the fresh water above. The salt water intrusion is thus wedge-shaped.

Withdrawal of fresh water from the aquifer upsets this balance by reducing the pressure of the fresh water so that salt water moves inland. This can lead to the salt water reaching the extraction points, so that they pump brackish water and can no longer be used for drinking water or irrigation purposes. To prevent such effects, the groundwater close to the coast is monitored intensively in many countries, and the flow of the groundwater is estimated using numerical models .

The Ghijben-Herzberg equation

The figure shows the Ghyben-Herzberg relationship (see also formula in the text), where h is the thickness of the freshwater zone above sea level and Z is the thickness below sea level.

After the French Joseph Du Commun had described the principle for the first time in an essay in 1828, the physical formula for calculating the saltwater intrusion was developed independently in 1888/89 by the two Dutch military men J. Drabbe and Willem Badon Ghyben (1845–1907, also Willem Badon Ghijben written) and published in 1901 by the German civil engineer Alexander Herzberg (1841–1912). They developed analytical solutions to describe the behavior of intruding salt water as closely as possible, based on a number of assumptions, which, however, do not apply in all cases.

The formula they found is referred to as the Ghijben-Herzberg equation , sometimes also as the DGH effect . The effect follows the Archimedean principle .

The figure clarifies the formula :; stand by ${\ displaystyle z = {\ frac {\ rho f} {(\ rho s- \ rho f)}} h}$

${\ displaystyle h}$ : for the height of fresh water above sea level
${\ displaystyle z}$ : for the height of fresh water below sea level

The two values and are linked via their respective densities (fresh water: 1.0 gram per cubic centimeter (g / cm ³ ) at 20 ° C) and (salt water: 1.025 g / cm ³ ). The equation can be simplified to the expression . ${\ displaystyle h}$ ${\ displaystyle z}$ ${\ displaystyle \ rho f}$ ${\ displaystyle \ rho s}$ ${\ displaystyle z \ = 40h}$

In this form, the Ghijben-Herzberg equation shows that in an unlimited aquifer, every meter of fresh water above sea level corresponds to a water column 40 m below it - similar to the appearance of an iceberg floating in salt water , where only a fraction of its mass is visible is.

Today, computer modeling allows the use of numerical methods (generally the finite difference or finite element method ) that are better adapted to the specific conditions of a location.

Modeling of salt water intrusion

Modeling saltwater intrusion is difficult for several reasons. Typical problems with modeling are:

the occurrence of crevices and crevices in the water-bearing rock. Their presence or absence has a major impact on saltwater penetration, but their size and location are not precisely known.

the occurrence of different hydraulic properties on a small scale. They may also have a major influence on the behavior of the aquifer, but cannot be captured by the model due to their small size.

the change in hydraulic properties due to salt water intrusion. A mixture of salt water and fresh water is often undersaturated with regard to calcium , triggers the calcium solution in the mixing zone and thus changes the hydraulic properties.

the process known as cation exchange , which slows the advance and retreat of salt water and makes accurate calculations difficult.

the fact that a saltwater intrusion is normally in motion, i.e. not in equilibrium, makes it difficult to control the modeling sets with data on the water level or the pumping rates.

with long-term modeling, the long-term development of the climate is not known. However, the models are sensitive, for example, to changes in sea level and the rate of groundwater recharge , which changes cannot be precisely foreseen based on current knowledge.

Prevention of salt water intrusions at locks

Catfish Pond Control Structure (sluice) locks on the Mermentau River in coastal Louisiana

Saltwater intrusion can be a problem in the area of locks where saltwater meets freshwater. Special locks such as the Hiram M. Chittenden Locks in Washington are equipped with a collecting basin from which the salt water pumped out of the locks is collected and can be pumped back into the salt-bearing part of the water. At the Hiram M. Chittenden Locks, a small part of the salt water is also pumped to the fish ladder to make it more attractive for fish migrating upstream.

Areas with active saltwater intrusion

Active saltwater intrusion takes place in many coastal areas around the world. Examples of problem areas with regard to water supply can be found in Benin , Cyprus , Morocco , Pakistan , Tunisia or the Gulf of Bohai in China .

In the United States of America , coastlines of Florida and Georgia with z. B. the Chicot Aquifer , part of the Gulf Coast Aquifer , the area around San Leandro in California and the area of Lake Pontchartrain in Louisiana .

literature

Sascha Wisser, Wolfgang Korthals, Heiko Gerdes, Yunshe Dong, Fulin Li, Rolf-Dieter Wilken: Saltwater intrusion in the coastal area around the Bohai Sea, China. In: GWF Wasser Abwasser. 147, No. 7/8, 2006, ISSN 0016-3651 , pp. 496–500 ( short version )
Submarine ground-water discharge and its role in coastal processes and ecosystems, US Geological Survey, Reston Virginia, 2004. US Department of the Interior. http://sofia.usgs.gov/publications/ofr/2004-1226/OFR_2004-1226.pdf . With 2 photos, 3 graphics.
Moore, WS, 1996, Large groundwater inputs to coastal waters revealed by 226Ra enrichments: Nature, v. 380, p. 612-614.

Web links

M. Schipek: Exercise salt water intrusion. Hydrogeology II seminar - summer semester 2007. TU Bergakademie Freiberg, Chair of Hydrogeology (PDF, 410 kB)
Sebastian Bauer: Modeling in Applied Geology II: Geohydromodeling. Institute for Geosciences, Christian Albrechts University Kiel (PDF; 3.1 MB)

Individual evidence

↑ CWPtionary Saltwater intrusion . LaCoast.gov. 1996. Retrieved March 21, 2009.

^ ^A ^b ^c Paul M. Barlow: Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast . USGS . 2003. Retrieved March 21, 2009.

↑ see literature: Moore, WS 1996

↑ see literature: USGS 2004 and the NASA-orbital photo: https://commons.wikimedia.org/wiki/File:Caspian_Sea_from_orbit.jpg

^ David K. Todd: Salt water intrusion of coastal aquifers in the United States . In: IAHS Publ. (Ed.): Subterranean Water . No. 52, 1960, pp. 452-461. Retrieved June 22, 2009.

↑ Jacques Willy Delleur: The handbook of groundwater engineering Second Edition . 2nd edition. CRC Press , Boca Raton FL et al. 2007 ,, ISBN 0-8493-4316-X ..

↑ Georg Mattheß (Ed.): Textbook of Hydrogeology. Volume 1: Georg Mattheß, Károly Ubell: General hydrogeology, groundwater balance . 2nd revised and expanded edition. Borntraeger Brothers, Berlin et al. 2003, ISBN 3-443-01049-0 , pp. 245–246.

↑ bienenwaage.de, lecture, April 17, 2010, Dannenberg, Dieter Ortlam: Pleistocene Rinnen and the DGH Effect - Why “Gorleben” was the wrong choice, 2.3. The Pleistocene Gullies (July 29, 2011).

↑ bienenwaage.de, lecture, April 17, 2010, Dannenberg, Dieter Ortlam: Pleistocene Rinnen and the DGH Effect - Why “Gorleben” was the wrong choice, 2.1. The DGH Effect (July 29, 2011).

↑ Sherrill Mausshardt, Glen Singleton: Mitigating Salt-Water Intrusion through Hiram M. Chittenden Locks . In: Journal of Waterway, Port, Coastal and Ocean Engineering . 121, No. 4, 1995, ISSN 0733-950X , pp. 224-227. doi : 10.1061 / (ASCE) 0733-950X (1995) 121: 4 (224) .

↑ deutschlandfunk.de , Research News , November 4, 2015, Dagmar Röhrlich : Strategies against Shrinkage (November 4, 2015)

↑ academic.emporia.edu: Gulf Coast Aquifer, Texas (November 4, 2015)

[1] CWPtionary Saltwater intrusion . LaCoast.gov. 1996. Retrieved March 21, 2009.

[Barlow2003-2] A ^b ^c Paul M. Barlow: Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast . USGS . 2003. Retrieved March 21, 2009.

[3] see literature: Moore, WS 1996

[4] see literature: USGS 2004 and the NASA-orbital photo: https://commons.wikimedia.org/wiki/File:Caspian_Sea_from_orbit.jpg

[Todd1960-5] David K. Todd: Salt water intrusion of coastal aquifers in the United States . In: IAHS Publ. (Ed.): Subterranean Water . No. 52, 1960, pp. 452-461. Retrieved June 22, 2009.

[Delleur2006-6] Jacques Willy Delleur: The handbook of groundwater engineering Second Edition . 2nd edition. CRC Press , Boca Raton FL et al. 2007 ,, ISBN 0-8493-4316-X ..