Freshwater and Salt Water Interface Flow in GFLOW

In coastal areas groundwater quality may be degraded by intrusion of salt sea water into freshwater aquifers. The salinity of the groundwater changes its physical properties: an increase in salinity increases both the density and viscosity of the groundwater. These density differences are the primary driving force that allows salt water to move into the coastal aquifer, forming a salt water “wedge” underneath the freshwater that moves toward the sea. Viscosity differences play only a minor role in this process, see e.g. Haitjema (1991), and will be ignored. Under steady state conditions the salt water will be stationary, while the freshwater will move. Under these circumstances it was found that the mixing zone between freshwater and salt water tends to be rather thin; the brackish water is being flushed away to the sea, e.g. Verruijt (1971). Assuming an abrupt interface its position may be estimated using the “Badon Ghyben Herzberg” principle (Badon Ghyben, 1888 and Herzberg, 1901). This principle states that the depth d of the interface below sea level is: