In coastal areas the groundwater and seawater flows are driven by a variety of factors. Both types of water can circulate in marine sediments due to tidal pumping, waves, bottom currents or density driven transport processes. Meteoric freshwaters can discharge along confined and unconfined aquifers into the sea or the oppositional process of seawater intruding into groundwater charged aquifers can take place. The flow of both fresh and sea water is primarily controlled by the hydraulic gradients between land and sea and differences in the densities between both waters and the permeabilities of the sediments.
According to Drabbe and Badon-Ghijben (1888) and Herzberg (1901), the thickness of a freshwater lens below sea level (z) corresponds with the thickness of the freshwater level above sea level (h) as:
z= ρf/((ρs-ρf))*h
With z being the thickness between the saltwater-freshwater interface and the sea level, h being the thickness between the top of the freshwater lens and the sea level, ρf being the density of freshwater and ρs being the density of saltwater. Including the densities of freshwater (ρf = 1.00 g •cm-3) and seawater (ρs = 1.025 g •cm-3) equation (2) simplifies to:
z=40*h
Together with Darcy's Law, the length of a salt wedge from the shoreline into the hinterland can be calculated:
L= ((ρs-ρf)Kf m)/(ρf Q)
With Kf being the hydraulic conductivity, m the aquifer thickness and Q the discharge rate. Assuming an isotropic aquifer system the length of a salt wedge solely depends on the hydraulic conductivity, the aquifer thickness and is inversely related to the discharge rate. These assumptions are only valid under hydrostatic conditions in the aquifer system. In general the interface between fresh and saline water forms a zone of transition due to diffusion/dispersion or local anisotropy.