The study area is situated at the landward border of the Belgian western coastal plain at the right bank of the river Ijzer. The phreatic aquifer is of quaternary age. Until about the 12th century, the hydrology of the area was influenced by the North Sea. The area was criss-crossed by tidal channels and gullies and the aquifer was mainly filled with salt water. From about 1100 AD, the area was gradually reclaimed, halting the influence of the sea. The channels silted up. Fresh water started to infiltrate replacing the salt water. This leads to the actual distribution of fresh and salt water. Fresh water lenses are found below the creek ridges, the former tidal channels where the aquifer consists mainly of sandy sediments. In the adjacent area the phreatic aquifer consists of less permeable sediments (clay, silt and peat) and is still mainly filled with salt water. In these areas where the water table is slightly lower than in the creek ridges, only a thin lens of fresh water occurs above the salt water. A preliminary field survey was performed to compliment the existing data. In these field survey a number of drillings were executed and observation wells were installed. The variation of the salt content with depth was inferred from electrical conductivities measured with the focused electromagnetic induction method. A pumping test was performed to derive the hydraulic conductivity of the channel sediments. All these data were integrated in a 3D density dependent model, using the MOCDENS3D code to simulate the evolution of the water quality distribution from 1100 AD until a few centuries in the future. The simulations demonstrate that the fresh water lenses under the channels are formed in about 400 years. These fresh water lenses can only be considered as a limited water resource in this area. The influence of this drainage on the groundwater flow and water quality distribution is illustrated by the model. The mean drainage level is below mean sea level. In the creek ridges there is an important upward flow of ...