🧂 Saltwater Intrusion

The Ghyben-Herzberg relation states that the depth of the freshwater-saltwater interface below sea level is about 40 times the height of the freshwater table above sea level. It follows from balancing hydrostatic pressure between a freshwater column and a slightly denser seawater column meeting at the interface, first described independently by W. Badon Ghyben in 1888 and Alexander Herzberg in 1901.

Pumping lowers the freshwater table near the well. Because interface depth scales with roughly 40 times the table height, even a small drop in the table pulls the underlying saltwater interface up by a much larger amount, bringing saline water closer to the well screen and risking contamination.

Upconing is a localized, exaggerated rise of the saltwater interface directly beneath a pumping well, on top of the broader regional Ghyben-Herzberg trend. Past a critical pumping rate the upconed cone becomes unstable and saline water is drawn straight up into the well screen.

Yes. Raising the ocean boundary condition reduces the effective freshwater head throughout the aquifer, pushing the saltwater wedge further inland and shallower even without any increase in pumping — a real, compounding effect of climate change on coastal water supplies.

Miami-Dade and other South Florida well fields, Jakarta, Bangkok, coastal California and parts of the Nile Delta and Mediterranean coast are all documented, ongoing cases where over-pumping and rising seas have pushed saline water into drinking-water aquifers.

Water utilities reduce pumping rates near the coast, relocate wells further inland, inject treated freshwater to build a hydraulic barrier against the wedge, and continuously monitor water levels and salinity to keep the interface a safe distance below well screens.