The Westhoek nature reserve is a dune area situated along the French-Belgian border. Below the dunes a fresh-water lens is found. A particular distribution of salt-water occurring above fresh-water is found under the adjacent shore. This less known water quality distribution is in dynamic equilibrium. A 2D density dependent groundwater flow model was made using the MOCDENS3D code. First, the groundwater flow and water quality evolution under the shore and in the dunes are modeled. Then the possible impact of sea level rise is simulated for a number of different scenarios. These scenarios reflect different reactions of coastal morphology and human intervention on the sea level rise. Depending on the scenario, the extent of the shore's salt-water lens can increase, decrease or even completely disappear. Simultaneously, the extent of the dune's fresh-water lens can significantly be altered. The simulations illustrate also, besides the effects of sea level rise, that changes in boundary conditions (drainage levels, shore morphology, sea water level), either natural or human induced, can importantly alter the water quality distribution. Because of the high ecological value of the area and the dune's importance for drinking water production, these changes should be well studied beforehand.