What does the interface on the fresh-saltwater distribution map of the Belgian coastal plain represent?

Knowing the distribution between fresh and saline groundwater is imperative for a sustainable and integrated management of water resources in coastal areas. The Belgian coastal plain forms no exception. Based on vertical electrical soundings (VES) and available geological data maps were published in 1974 and 1989 that show the depth to the interface between the freshwater and the underlying saltwater. This interface was defined as the 1.5 g/L total dissolved solids surface and interface depth was depicted using classes based on depth intervals. Recent developments in geophysical prospection make it possible to collect data with a high spatial resolution. The comparison with 1974 and 1989 maps is thereby not so straightforward as it seems. Therefore, this paper elucidates the nature of the mapped interface and its relation with salinity. The interface, as determined by VES, is the boundary between a freshwater and saltwater electrical distinct layer. The resistivity of this interface can vary between 1 and 75 Omega.m. However, this translates in a narrow range of salinities for which 1.5 g/L represents a mean. Comparing new data with the old maps thus takes more than simply comparing the depth to a certain resistivity value