The municipal public water-supply company of Knokke-Heist is producing drinking-water from the dunes aquifer. However, the available resources are limited and overexploitation of the ecologically valuable dune aquifer system should be avoided. Therefore, the company has developed a new strategy for the future. An important component of this new strategy consists in the treatment of surface water by means of reverse osmosis, and remineralisation of the produced filtrate by means of groundwater taken from a new catchment around the “De Cloedt” pit, a former sand quarry, which is now inundated and recharged by surface water. The “De Cloedt” pit is situated in the polder area, just to the south of the dune border. In the polder area, saline groundwater occurs in depth, which may limit the groundwater exploitation potential. This paper deals with the hydrogeological investigations that have been performed of the area. This includes geophysical measurements, piezometric and surface water discharge measurements, groundwater sampling and analysis and modeling.