Saltwater intrusion is a process that occurs in virtually all coastal aquifers, where they are in hydraulic continuity with seawater.
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.
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, 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.