Saltwater intrusion

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]

Many coastal communities around the United States are experiencing saltwater contamination of water supply wells, and this problem has been seen for decades.[10] Many Mediterranean coastal aquifers suffer for seawater intrusion effects.[11][12] 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][13] 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]

The first physical formulations of saltwater intrusion were made by Willem Badon-Ghijben in 1888 and 1889 as well as Alexander Herzberg in 1901, thus called the Ghyben–Herzberg relation.[14] 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[2]

In the equation,

${\displaystyle z={\frac {\rho _{f}}{(\rho _{s}-\rho _{f})}}h}$

the thickness of the freshwater zone above sea level is represented as ${\displaystyle h}$ and that below sea level is represented as ${\displaystyle z}$. The two thicknesses ${\displaystyle h}$ and ${\displaystyle z}$, are related by ${\displaystyle \rho _{f}}$ and ${\displaystyle \rho _{s}}$ where ${\displaystyle \rho _{f}}$ is the density of freshwater and ${\displaystyle \rho _{s}}$ 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

${\displaystyle z\ =40h}$.[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.[15]

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 and extents 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 taken 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 dissolution of calcium in the mixing zone and changing hydraulic properties.
• The process known as 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 but good results are possible . Model results often depend strongly on sea level and recharge rate. Both are expected to change in the future.

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.[16]

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.[17]

• Benin
• Cyprus
• Bou Regreg (Morocco)
• Pakistan
• Suriname
• Tunisia
• United States
  • ACF River Basin (Florida/Georgia)
  • Environment of Florida
  • Essex County, Massachusetts[18]
  • Hiram M. Chittenden Locks (Washington)
  • Hutchinson Island (Georgia)
  • Lake Lanier (Georgia)
  • Lake Pontchartrain (Louisiana)
  • Miami River (Florida)
  • Mississippi River Delta
  • Oxnard Plain (California)
  • San Leandro (California)
  • Sonoma Creek (California)
  • Western Shore of Lake Superior[19] (Minnesota)
• Mekong Delta
• Italy[11][20][21]

• Groundwater
• Environmental migrant – People forced to leave their home region due to changes to their local environment
• Garald G. Parker – American hydrologistPages displaying wikidata descriptions as a fallback
• Inflatable rubber dam
• Peak water – Concept on the quality and availability of freshwater resources