Saltwater intrusion

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Saltwater intrusion is the movement of saline water into freshwater aquifers, which can lead to groundwater quality degradation, including drinking water sources, and other consequences. Saltwater intrusion can naturally occur in coastal aquifers, owing to the hydraulic connection between groundwater and seawater. Because saline water 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. [1] In other topologies, submarine groundwater discharge can push fresh water into saltwater.

Contents

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. Sea level rise caused by climate change also contributes to saltwater intrusion. [2] Saltwater intrusion can also be worsened by extreme events like hurricane storm surges. [3]

Hydrology

Cause and impact of saltwater intrusion Salt water intrusion wikipedia2.png
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. [2] Because saltwater has a higher content of dissolved salts and minerals, it is denser than freshwater, causing it to have a 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. [4]

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 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. [2]

Causes

Groundwater extraction

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 freshwater table, reducing the pressure exerted by the freshwater column and allowing the denser saltwater to move inland laterally. [2] 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. [5] [6]

Groundwater extraction can also lead to well contamination by causing upwelling, or upcoming, of saltwater from the depths of the aquifer. [7] 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 Lower Floridan aquifer: though a relatively 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. [6]

Canals and drainage networks

The construction of canals and drainage networks can lead to saltwater intrusion. Canals provide conduits for saltwater to be carried inland, as does the deepening of existing channels for navigation purposes. [2] [8] 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. [9]

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. [6]

Solutions

Two-layer air bubble curtain to prevent seawater intrusion into rivers Air bubble curtain.png
Two-layer air bubble curtain to prevent seawater intrusion into rivers
Workflow of ABC Workflow of ABC.png
Workflow of ABC

The seawater intrusion (SWI) into rivers can lead to many negative consequences, especially on agricultural activities and live ecosystems in upstream areas of rivers. There are many solutions developed to prevent or reduce the negative effects of Seawater intrusion. One of the sustainable solutions for rivers is using air bubble curtains that can completely solve SWI issues in rivers. [10]

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. [11] Many Mediterranean coastal aquifers suffer for seawater intrusion effects. [12] [13] 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. [2] [14] 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. [6]

Ghyben–Herzberg relation

The first physical formulations of saltwater intrusion were made by Willem Badon-Ghijben  [ pt ] in 1888 and 1889 as well as Alexander Herzberg  [ de ] in 1901, thus called the Ghyben–Herzberg relation. [15] 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.

the Ghyben-Herzberg relation Saltwater intrusion en.svg
the Ghyben–Herzberg relation

In the equation,

the thickness of the freshwater zone above sea level is represented as and that below sea level is represented as . The two thicknesses and , are related by and where is the density of freshwater and is the density of saltwater. Freshwater has a density of about 1.000 grams per cubic centimeter (g/cm3) at 20 °C, whereas that of seawater is about 1.025 g/cm3. The equation can be simplified to

. [2]

The Ghyben–Herzberg ratio states that, for every meter of fresh water in an unconfined aquifer above sea level, there will be forty meters of fresh water in the aquifer below sea level.

In the 20th century the vastly increased computing power available allowed the use of numerical methods (usually finite differences or finite elements) that need fewer assumptions and can be applied more generally. [16]

Modeling

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

Mitigation and management

Catfish Point control structure (lock) on the Mermentau River in coastal Louisiana Catfish Pond Control Structure.jpg
Catfish Point control structure (lock) on the Mermentau River in coastal Louisiana

Saltwater is also an issue where a 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. [17]

As groundwater salinization becomes a relevant problem, more complex initiatives should be applied from local technical and engineering solutions to rules or regulatory instruments for whole aquifers or regions. [18]

Areas of occurrence

See also

Related Research Articles

<span class="mw-page-title-main">Brackish water</span> Water with salinity between freshwater and seawater

Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater and fresh water together, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root brak. Certain human activities can produce brackish water, in particular civil engineering projects such as dikes and the flooding of coastal marshland to produce brackish water pools for freshwater prawn farming. Brackish water is also the primary waste product of the salinity gradient power process. Because brackish water is hostile to the growth of most terrestrial plant species, without appropriate management it is damaging to the environment.

<span class="mw-page-title-main">Water table</span> Top of a saturated aquifer, or where the water pressure head is equal to the atmospheric pressure

The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with groundwater, which may be fresh, saline, or brackish, depending on the locality. It can also be simply explained as the depth below which the ground is saturated.

<span class="mw-page-title-main">Water extraction</span> Process of taking water from any source

Water extraction is the process of taking water from any source, either temporarily or permanently, for flood control or to obtain water for, for example, irrigation. The extracted water could also be used as drinking water after suitable treatment.

<span class="mw-page-title-main">Groundwater</span> Water located beneath the ground surface

Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. Groundwater is recharged from the surface; it may discharge from the surface naturally at springs and seeps, and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology.

Spiciness (τ) is a term in oceanography that defines the salinity and potential temperature variation, often at constant density. Here, a temperature change offsets a salinity change; an increase in temperature decreases density whereas an increase in salinity increases density. Warmer and more saline water is spicier whereas cooler and less saline water is mintier.

In science and engineering, hydraulic conductivity, is a property of porous materials, soils and rocks, that describes the ease with which a fluid can move through the pore space, or fractures network. It depends on the intrinsic permeability of the material, the degree of saturation, and on the density and viscosity of the fluid. Saturated hydraulic conductivity, Ksat, describes water movement through saturated media. By definition, hydraulic conductivity is the ratio of volume flux to hydraulic gradient yielding a quantitative measure of a saturated soil's ability to transmit water when subjected to a hydraulic gradient.

<span class="mw-page-title-main">Hydraulic head</span> Specific measurement of liquid pressure above a vertical datum

Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum.

The Floridan aquifer system, composed of the Upper and Lower Floridan aquifers, is a sequence of Paleogene carbonate rock which spans an area of about 100,000 square miles (260,000 km2) in the southeastern United States. It underlies the entire state of Florida and parts of Alabama, Georgia, Mississippi, and South Carolina.

Submarine groundwater discharge (SGD) is a hydrological process which commonly occurs in coastal areas. It is described as submarine inflow of fresh-, and brackish groundwater from land into the sea. Submarine Groundwater Discharge is controlled by several forcing mechanisms, which cause a hydraulic gradient between land and sea. Considering the different regional settings the discharge occurs either as (1) a focused flow along fractures in karst and rocky areas, (2) a dispersed flow in soft sediments, or (3) a recirculation of seawater within marine sediments. Submarine Groundwater Discharge plays an important role in coastal biogeochemical processes and hydrological cycles such as the formation of offshore plankton blooms, hydrological cycles, and the release of nutrients, trace elements and gases. It affects coastal ecosystems and has been used as a freshwater resource by some local communities for millennia.

Surficial aquifers are shallow aquifers typically less than 50 feet (15 m) thick, but larger surficial aquifers of about 60 feet (18 m) have been mapped. They mostly consist of unconsolidated sand enclosed by layers of limestone, sandstone or clay and the water is commonly extracted for urban use. The aquifers are replenished by streams and from precipitation and can vary in volume considerably as the water table fluctuates. Being shallow, they are susceptible to contamination by fuel spills, industrial discharge, landfills, and saltwater. Parts of southeastern United States are dependent on surficial aquifers for their water supplies.

<span class="mw-page-title-main">Overdrafting</span> Unsustainable extraction of groundwater

Overdrafting is the process of extracting groundwater beyond the equilibrium yield of an aquifer. Groundwater is one of the largest sources of fresh water and is found underground. The primary cause of groundwater depletion is the excessive pumping of groundwater up from underground aquifers.

<span class="mw-page-title-main">Well</span> Excavation or structure to provide access to groundwater

A well is an excavation or structure created in the ground by digging, driving, or drilling to access liquid resources, usually water. The oldest and most common kind of well is a water well, to access groundwater in underground aquifers. The well water is drawn up by a pump, or using containers, such as buckets or large water bags that are raised mechanically or by hand. Water can also be injected back into the aquifer through the well. Wells were first constructed at least eight thousand years ago and historically vary in construction from a simple scoop in the sediment of a dry watercourse to the qanats of Iran, and the stepwells and sakiehs of India. Placing a lining in the well shaft helps create stability, and linings of wood or wickerwork date back at least as far as the Iron Age.

Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure the "accessible void", the total amount of void space accessible from the surface.

<span class="mw-page-title-main">Lens (hydrology)</span> Layer of fresh groundwater

In hydrology, a lens, also called freshwater lens or Ghyben-Herzberg lens, is a convex-shaped layer of fresh groundwater that floats above the denser saltwater and is usually found on small coral or limestone islands and atolls. This aquifer of fresh water is recharged through precipitation that infiltrates the top layer of soil and percolates downward until it reaches the saturated zone. The recharge rate of the lens can be summarized by the following equation:

The State of California enforces several methodologies through technical innovation and scientific approach to combat saltwater intrusion in areas vulnerable to saltwater intrusion. Seawater intrusion is either caused by groundwater extraction or increased in sea level. For every 1-foot of freshwater depression (0.30 m), sea-salty waters rises 40 feet (12 m) as the cone of depression forms. Salinization of groundwater is one of the main water pollution ever produced by mankind or from natural processes. It degrades water quality to the point it passes acceptable drink water and irrigation standards.

<span class="mw-page-title-main">Fresh water</span> Naturally occurring water with low amounts of dissolved salts

Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include non-salty mineral-rich waters such as chalybeate springs. Fresh water may encompass frozen and meltwater in ice sheets, ice caps, glaciers, snowfields and icebergs, natural precipitations such as rainfall, snowfall, hail/sleet and graupel, and surface runoffs that form inland bodies of water such as wetlands, ponds, lakes, rivers, streams, as well as groundwater contained in aquifers, subterranean rivers and lakes. Fresh water is the water resource that is of the most and immediate use to humans.

The Haline contraction coefficient, abbreviated as β, is a coefficient that describes the change in ocean density due to a salinity change, while the potential temperature and the pressure are kept constant. It is a parameter in the Equation Of State (EOS) of the ocean. β is also described as the saline contraction coefficient and is measured in [kg]/[g] in the EOS that describes the ocean. An example is TEOS-10. This is the thermodynamic equation of state.

Oceanic freshwater fluxes are defined as the transport of non saline water between the oceans and the other components of the Earth's system. These fluxes have an impact on the local ocean properties, as well as on the large scale circulation patterns.

<span class="mw-page-title-main">Coastal hydrogeology</span> Branch of hydrogeology

Coastal Hydrogeology is a branch of Hydrogeology that focuses on the movement and the chemical properties of groundwater in coastal areas. Coastal Hydrogeology studies the interaction between fresh groundwater and seawater, including seawater intrusion, sea level induced groundwater level fluctuation, submarine groundwater discharge, human activities and groundwater management in coastal areas.

An anchialine system is a landlocked body of water with a subterranean connection to the ocean. Depending on its formation, these systems can exist in one of two primary forms: pools or caves. The primary differentiating characteristics between pools and caves is the availability of light; cave systems are generally aphotic while pools are euphotic. The difference in light availability has a large influence on the biology of a given system. Anchialine systems are a feature of coastal aquifers which are density stratified, with water near the surface being fresh or brackish, and saline water intruding from the coast at depth. Depending on the site, it is sometimes possible to access the deeper saline water directly in the anchialine pool, or sometimes it may be accessible by cave diving.

References

  1. ↑ Johnson, Teddy (2007). "Battling Seawater Intrusion in the Central & West Coast Basins" (PDF). Water Replenishment District of Southern California. Archived from the original (PDF) on 2012-09-08. Retrieved 2012-10-08.
  2. 1 2 3 4 5 6 7 8 Barlow, Paul M. (2003). "Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast". USGS . Retrieved 2009-03-21.
  3. ↑ "CWPtionary Saltwater Intrusion yes". LaCoast.gov. 1996. Retrieved 2009-03-21.
  4. ↑ Johnson, Ted (2007). "Battling Seawater Intrusion in the Central & West Coast Basins" (PDF). Water Replenishment District of Southern California. Archived from the original (PDF) on 2012-09-08. Retrieved 2012-10-08.
  5. ↑ Lacombe, Pierre J. & Carleton, Glen B. (2002). "Hydrogeologic Framework, Availability of Water Supplies, and Saltwater Intrusion, Cape May County, New Jersey" (PDF). USGS . Retrieved 2012-12-10.
  6. 1 2 3 4 Barlow, Paul M. & Reichard, Eric G. (2010). "Saltwater intrusion in coastal regions of North America". Hydrogeology Journal. USGS. 18 (1): 247–260. Bibcode:2010HydJ...18..247B. doi:10.1007/s10040-009-0514-3. S2CID   128870219 . Retrieved 2012-12-10.
  7. ↑ Reilly, T.E. & Goodman, A.S. (1987). "Analysis of saltwater upconing beneath a pumping well". Journal of Hydrology. 89 (3–4): 169–204. Bibcode:1987JHyd...89..169R. doi:10.1016/0022-1694(87)90179-x.
  8. ↑ Good, B. J., Buchtel, J., Meffert, D.J., Radford, J., Rhinehart, W., Wilson, R. (1995). "Louisiana's Major Coastal Navigation Channels" (pdf). Louisiana Department of Natural Resources . Retrieved 2013-09-14.{{cite web}}: CS1 maint: multiple names: authors list (link)
  9. ↑ Barlow, Paul M. (2008). "Preliminary Investigation: Saltwater Barrier - Lower Sabine River" (PDF). Sabine River Authority of Texas . Retrieved 2012-12-09.[ permanent dead link ]
  10. ↑ Kahrizi, Ehsan; et al. (2023). "Experimental evaluation of two-layer air bubble curtains to prevent seawater intrusion into rivers". Journal of Water and Climate Change. 14 (2): 543–558. doi: 10.2166/wcc.2023.384 . S2CID   255924963.
  11. ↑ Todd, David K. (1960). "Salt water intrusion of coastal aquifers in the United States" (PDF). Subterranean Water. IAHS Publ. (52): 452–461. Archived from the original (PDF) on 2005-10-25. Retrieved 2009-03-22.
  12. 1 2 Polemio, Maurizio (2016-04-01). "Monitoring and Management of Karstic Coastal Groundwater in a Changing Environment (Southern Italy): A Review of a Regional Experience". Water. 8 (4): 148. doi: 10.3390/w8040148 .
  13. ↑ Polemio, Maurizio; Pambuku, Arben; Limoni, Pier Paolo; Petrucci, Olga (2011-01-01). "Carbonate Coastal Aquifer of Vlora Bay and Groundwater Submarine Discharge (Southwestern Albania)". Journal of Coastal Research. 270: 26–34. doi:10.2112/SI_58_4. ISSN   0749-0208. S2CID   54861536.
  14. ↑ Romanazzi A, Polemio M. "Modelling of coastal karst aquifers for management support: Study of Salento (Apulia, Italy)" (PDF). Italian Journal of Engineering Geology and Environment. 13, 1: 65–83.
  15. ↑ Verrjuit, Arnold (1968). "A note on the Ghyben-Herzberg formula" (PDF). Bulletin of the International Association of Scientific Hydrology. Delft, Netherlands: Technological University. 13 (4): 43–46. doi:10.1080/02626666809493624 . Retrieved 2009-03-21.[ permanent dead link ]
  16. ↑ Romanazzi, A.; Gentile, F.; Polemio, M. (2015-07-01). "Modelling and management of a Mediterranean karstic coastal aquifer under the effects of seawater intrusion and climate change". Environmental Earth Sciences. 74 (1): 115–128. Bibcode:2015EES....74..115R. doi:10.1007/s12665-015-4423-6. ISSN   1866-6299. S2CID   56376966.
  17. ↑ Mausshardt, Sherrill; Singleton, Glen (1995). "Mitigating Salt-Water Intrusion through Hiram M. Chittenden Locks". Journal of Waterway, Port, Coastal, and Ocean Engineering. 121 (4): 224–227. doi:10.1061/(ASCE)0733-950X(1995)121:4(224).
  18. ↑ Polemio, Maurizio; Zuffianò, Livia Emanuela (2020). "Review of Utilization Management of Groundwater at Risk of Salinization". Journal of Water Resources Planning and Management. 146 (9): 03120002. doi:10.1061/(ASCE)WR.1943-5452.0001278. ISSN   0733-9496. S2CID   225224426.
  19. ↑ "Case Studies of Various Water Quality Problems | H2O Care".
  20. ↑ "In a Pickle: The Mystery of the North Shore's Salty Well Water". www.seagrant.umn.edu. Retrieved 2018-09-27.
  21. ↑ Vespasiano, Giovanni; Cianflone, Giuseppe; Romanazzi, Andrea; Apollaro, Carmine; Dominici, Rocco; Polemio, Maurizio; De Rosa, Rosanna (2019-11-01). "A multidisciplinary approach for sustainable management of a complex coastal plain: The case of Sibari Plain (Southern Italy)". Marine and Petroleum Geology. 109: 740–759. Bibcode:2019MarPG.109..740V. doi:10.1016/j.marpetgeo.2019.06.031. ISSN   0264-8172. S2CID   197580624.
  22. ↑ Zuffianò, L. E.; Basso, A.; Casarano, D.; Dragone, V.; Limoni, P. P.; Romanazzi, A.; Santaloia, F.; Polemio, M. (2016-07-01). "Coastal hydrogeological system of Mar Piccolo (Taranto, Italy)". Environmental Science and Pollution Research. 23 (13): 12502–12514. doi:10.1007/s11356-015-4932-6. ISSN   1614-7499. PMID   26201653. S2CID   9262421.
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