Reclaimed lakes (surface ≥3 m below sea level) in a coastal plain with formerly more or less stagnant flow conditions, act as huge groundwater discharge areas and convert the bordering land into groundwater recharge zones. The main consequences are: (1) the infiltration of fresh polder and river-Rhine water, both of poor quality; (2) the moving in of fresh groundwater of excellent quality, from relatively remote sandy recharge areas like the coastal dunes and Pleistocene uplands; (3) the encroachment of salt North Sea water along the west coast; and (4) the bleeding-out of relict brackish to salt water.
The sluggish change in the spatial distribution of water types, leads to new possibilities of groundwater pumping for public water supply in and around the deep polders, in future. The chemical consequences are discussed. Salinization hazards for well fields in the coastal dunes and Pleistocene uplands may increase.