Saltwater intrusion is the movement of seawater or other saline water into a freshwater aquifer, usually caused by pumping.
Saltwater intrusion happens when saline water moves into an aquifer that holds fresh water. In coastal aquifers, fresh water naturally floats on denser seawater and flows out to the sea. Pumping lowers the fresh water level, and the seawater wedge moves inland. The Ghyben-Herzberg relation gives a simple estimate of the position of the boundary. The depth of the interface below sea level, z, is about 40 times the height of the fresh water table above sea level, h. So a 1 m fall in the water table can raise the interface by about 40 m. Real interfaces are mixing zones rather than sharp lines, but the relation shows why small falls in level matter. Beneath a pumped well, saline water can also rise in a cone, called upconing. Inland, it can come from deep brines or from irrigation return flow. Intrusion usually shows up first as rising chloride and electrical conductivity in wells. Chloride above about 250 mg/L gives water a salty taste. Remedies include cutting or moving pumping, recharge from wells or ponds to form a barrier, and regular monitoring at different depths. Recovery can take decades.