A study of the characteristics and the hydrochemistry of the water table aquifer in northwest Belgium has been conducted between August 1965 and January 1968.Two areas of about 450 km² each have been studied in detail: the Eastern Coastal Area north of Bruges and the Flemish Valley north of Ghent. The topography only shows a microrelief and the water table is usually found at a depth of 1 to 2 m below the surface. The aquifer is being supplied by precipitation and run-off from the hinterland. In both areas the reservoir rock is formed mainly by Quaternary sands and sometimes reaches a thickness of more than 35 m. These sands cover an erosion relief on top of the Tertiary strata, which are dipping gently in a NNE direction. The reservoir is reaching a thickness of more than 60 m where the Tertiary substratum consists of sand. In the main part of the Flemish Valley Weichselian sands are found at the surface and Holocene alluvium along the water courses. In the polder land in the northern part of the Flemish Valley and in the Coastal Area the surface sediments are made up by Subboreal peat and clayey and sandy Dunkerquian sediments. The study is founded upon a survey of 1500 resistivity soundings, in Wenner arrangement up to an electrode spacing of 100 m. The interpretation of the resistivity data has provided a map showing the depth of the main electrical contact surface. By further interpretation of the resistivities and the contact depths several hydrological units have been outlined. In two of these units, where brackish water is present in the subsoil, the fresh-/brackish-water boundary has been mapped. To control the geo-electrical survey 90 dry borings ranging in depth from 7 to 35 m have been performed. In every drilling site observation wells have been constructed. One-meter-long screens have been placed at several depths. Thus unmixed water samples have been taken at known depths. Chemical analysis of 150 samples has been made. Waters have been classified after their total dissolved-solids content, their ...