Saltwater intrusion into a coastal aquifer

Fresh groundwater floats on denser sea water. Over-abstraction lowers the freshwater head, and the salt interface moves inland — usually irreversibly on human timescales.

Ghyben-Herzberg relation: Because sea water is about 2.5% denser, roughly forty metres of fresh water sit below sea level for every metre of freshwater head above it. Losing one metre of head can raise the interface by forty.
Why it is hard to reverse: Once salt water occupies the pore space, flushing it out requires far more fresh water and far more time than the abstraction that displaced it.
Management: Reduced abstraction, managed aquifer recharge, and injection barriers along the coast.

Text description

A coastal cross-section. Fresh groundwater flows seaward and floats on a wedge of denser saline water that extends inland beneath it. Under natural conditions the wedge sits offshore or just inland. When a coastal well over-abstracts, the freshwater head falls and the saline wedge advances landward, eventually reaching the well and rendering it unusable.