The Belgian coastline extends for some 67 km along the North Sea. The coastal region has an almost continuous dune belt of 1 to 2 km width, separating inland lying polders from the beach slope. The North Sea itself shows strong tidal fluctuations, which also penetrate into the coastal aquifer system. This coastal aquifer is on average around 30 m thick and forms an important source for water supply in this part of the country. For this purpose it has already been used for decades. In natural conditions the dune belt acts as a groundwater divide which separates two groundwater flow systems, one to the sea and one inland to the polder region.Water catchment well fields have changed this flow pattern locally. A groundwater flow model of the western coastal region has been used to calculate groundwater fluxes from under the dune belt to the sea under different conditions: for average seasonal variations with and without pumping in the dune belt, and under increased (+10 %) and decreased (-10 %) recharge rates, and during a three decade long period (1970-2000) with real recharge rates. The results give an indication of the discharge rates to the sea and how they change from season to season, their interannual variation, and how they are affected by groundwater pumping in the dunes. The impact of small changes in average recharge rates, both increasing and decreasing, have been quantified.