Saltwater intrusion is the movement of saline water into freshwater aquifers. Most often, it is caused by ground-water pumping from coastal wells, or from construction of navigation channels or oil field canals. Saltwater intrusion occurs in virtually all coastal aquifers, where they are in hydraulic continuity with seawater.

When fresh water is withdrawn at a faster rate than it can be replenished, the water table is drawn down as a result.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.

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.

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. 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 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 ρf and ρs where ρf is the density of freshwater and ρs 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

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.