Saltwater intrusion is the movement of saline water into freshwater aquifers. Most often, it is caused by ground-water pumping from coastal wells,^[1] or from construction of navigation channels or oil field canals. The channels and canals provide conduits for salt water to be brought into fresh water marshes. But salt water intrusion can also occur as the result of a natural process like a storm surge from a hurricane.^[2] Saltwater intrusion occurs in virtually all coastal aquifers, where they are in hydraulic continuity with seawater.

[edit] Effect on drinking water

When fresh water is withdrawn at a faster rate than it can be replenished, the water table is drawn down as a result. This draw-down also reduces the hydrostatic pressure. When this happens near an ocean coastal area, salt water from the ocean is pulled into the fresh water aquifer. The result is that the aquifer becomes contaminated with salt water. This is happening to many coastal communities.^[3][4]

[edit] Hydrology

Saltwater intrusion happens when saltwater is drawn-in (from the sea) into freshwater aquifers. This behavior is caused because sea water has a higher density (which is because it carries more solutes) than freshwater. This difference in density causes the pressure under a column of saltwater to be greater than the pressure under a column of the same height of freshwater. If these two columns are connected at the bottom, then the pressure difference would cause a flow of saltwater column to the freshwater column until the pressure equalizes.

The flow of saltwater inland is limited to coastal areas. Further inland, the freshwater column is higher due to the increasing altitude of the land and is able equalize the pressure from the salt water, stopping the saltwater intrusion. The higher water levels inland have another effect: the freshwater flows seaward. This completes the picture: at the sea-land boundary, at the high part of the aquifer freshwater flows out and in the lower part, saltwater flows in. The saltwater intrusion forms a wedge.

Pumping of fresh water from an aquifer reduces the water pressure and intensifies the effect, drawing salt water into new areas. When freshwater levels drop, saltwater intrusion can proceed inland, reaching the pumped well. Then saltwater, unfit for drinking or irrigation, is produced by the pump. To prevent this, more and more countries adopt extensive monitoring schemes and numerical models to assess how much water can be pumped without causing such effects.

[edit] 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.^[5] 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.

^[1]

The figure shows the Ghyben-Herzberg relation. In the equation,

the thickness of the freshwater zone above sea level is represented as h and that below sea level is represented as z. The two thicknesses h and z, are related by ρ_f and ρ_s where ρ_f is the density of freshwater and ρ_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

.^[1]

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 computing power allowed the use of numerical methods (usually finite differences or finite elements) that need less assumptions and can be applied more generally.^{[citation needed]}

[edit] 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 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. Model results often depend strongly on sea level and recharge rate. Both are expected to change in the future.

[edit] Mitigation

Catfish Point control structure (lock) on the Mermentau River in coastal Louisiana

Saltwater intrusion is also an issue where a lock separates salt water from fresh water (for example the Hiram M. Chittenden Locks in Washington). In this case a collection basin was built from which the salt water can be pumped back to the sea. Some of the intruding salt water is also pumped to the fish ladder to make it more attractive to migrating fish.^[6]

[edit] See also

• Aquifer
• 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)

[edit] References