The water-bearing strata in the area under study consist mainly of fluviatile Pleistocene. The base is formed by the marine Plio-Pleistocene, the top by the Holocene clay and peat deposits. The chemical composition of the ground-water in this aquifer depends on processes related to the geological history, not on the type of sediment. The most important of these processes is cation exchange, which occurs in two ways. When fresh water replaces salt water in the aquifer, an exchange takes place between the Ca2+ ions from the ground-water and the exchangeable Na+ ions from the sediments. The reverse process takes place when sea-water infiltrates a sediment with fresh ground-water: the Na+ ions from the sea-water are exchanged for Ca2+ ions from the sediments. A classification of ground-water according to 8 types was made. These types are indicated on a map and on sections. In the western part of the area, intrusion of salt water originating from the Eemian transgression can be recognized. During the Calais transgression salt water infiltrated the aquifer; this salt water has gradually been replaced by fresh water. In part of the area salt water also infiltrated during the Duinkerke transgression. In an east-west zone, where the ground-water flows from the high-lying areas in the north and the Pleistocene outcrops in the south converge, the fresh/salt water boundary rises. In this zone high Cl¯-concentrations occur at the surface locally, due to increased upward seepage through the Holocene clay and peat layers at places where the vertical resistance is low. In some bore-holes the concentration of the minor constituents I¯ and Br¯ and of the isotopes of oxygen and carbon was measured. The Cl¯: Br¯ concentration ratio proved to be indicative of polluted ground-water. High 13C concentrations are an indication for an upward flow of ground-water.