Hydrology
At the coastal margin, fresh groundwater flowing from inland areas meets with saline groundwater from the ocean. The fresh groundwater flows away from inland areas with higher elevation towards the coast where elevation and groundwater levels are lower. Because saltwater has a higher content of dissolved salts and minerals, it is denser than freshwater, causing it to have higher hydraulic head than freshwater. Hydraulic head refers to the liquid pressure exerted by a water column: a water column with higher hydraulic head will move into a water column with lower hydraulic head, if the columns are connected.
As a result of pressure and density differences, saltwater moves into coastal aquifers in a wedge shape under the freshwater, at the lowest point of the aquifer. The saltwater and freshwater meet in a so-called transition zone where mixing occurs through dispersion and diffusion. Ordinarily the inland extent of the saltwater wedge is limited because fresh groundwater levels, or the height of the freshwater column, increases as land altitude gets higher.
Read more about this topic: Saltwater Intrusion