Saltwater intrusion is a natural process that occurs in virtually all coastal aquifers. It consists in salt water (from the sea) flowing inland in freshwater aquifers. This behaviour is caused by the fact that sea water has a higher density (which is because it carries more solutes) than freshwater. This higher density has the effect that the pressure beneath a column of saltwater is larger than that beneath a column of the same height of freshwater. If these columns were connected at the bottom, then the pressure difference would trigger a flow from the saltwater column to the freshwater column. This is precisely what happens in saltwater intrusion cases.

The flow of saltwater inland is limited to coastal areas. Inland the freshwater column gets higher and the pressure at the bottom also gets higher. This compensates for the higher density of the saltwater column. Where this happens, saltwater intrusion stops.

The higher water levels inland have another effect: they trigger flow of freshwater seaward. This completes the picture: at the sea-land boundary, we have in the high part of the aquifer outflow of freshwater and in the lower part, inflow of saltwater. The saltwater intrusion gets a sort of cone shape.

Saltwater intrusion is natural process, but it becomes an environmental problem when excessive 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, the intrusion can proceed further inland until reaching the pumped well. Then you may get saltwater out of the pump, unfit for drinking or irrigation. To prevent this, more and more countries adopt extensive monitoring schemes and numerical models to assess how much can be pumped without causing such efects.

The first physical formulations of saltwater intrusion were made by Badon and Ghyben-Herzberg. They derived analytical solutions to aproximate the intrusion behaviour, which are based on a number of assumptions that do not hold in all field cases. 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.

Even so the 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 unkwnown 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 pretty 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 you would expect based on the sea level, pumping scheme etc.

• for long term models, the future climate change forms a big unknown. Model results often depend strongly on sea level and recharge rate. Both are expected to change the coming century.

Saltwater intrusion is also an issue where a lock separates salt water from fresh water (for example the Hiram M. Chittenden Locks). 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.