Simultaneous outflow of fresh water and inflow of sea water in a coastal spring

The ability of sea water to flow into coastal or submarine cavities is surprising, in view of the fact that sea level is, for water, a level of minimum gravitational potential where it can perform no work. To achieve a lower potential requires energy, the provision of which is an example of a completely original aspect of the interaction between subterranean hydrodynamics and marine hydraulics. We have studied a spectacular example of the simultaneous outflow of fresh water and inflow of salt water at a karstic spring running into a salt–water lagoon on the French Mediterranean coast, and we suggest here a hypothesis to account for the surprising double–layered, two–directional flow observed.