The coastal area of the Netherlands has experienced multiple phases of seawater intrusion, starting with the predominantly vertical salinization driven by density differences that occurred when the formerly exposed land surface became flooded by the sea at around 6000 years before present (BP). Small freshwater lenses developed when the coastline stabilized and beach ridges formed at approximately 3500 BP. These lenses coalesced and deepened after the dune belt became established about 1000 years ago. The mixing and outflow of brackish water in the transition zone drove circulation of seawater beneath the lenses. This natural flow pattern became disrupted when land drainage resulted in lower water levels in the hinterland. Then, during the first half of the twentieth century, over-abstraction caused seawater intrusion and up-coning until the adverse effects were mitigated by artificial recharge starting in 1957. In this contribution we simulate these processes in a reactive transport model in the Amsterdam Water Supply area. The model is based on an earlier SEAWAT model by Nienhuis et al (2014) and takes into account variable-density flow and solute transport in a cross-section across the freshwater lens. The simulation starts 3500 BP and considers the changing boundary conditions as well as the main geochemical processes, being mineral equilibria, cation exchange and organic matter transformations. The latter were included to assess the fate of radiocarbon (14C). Using this tracer, Stuyfzand et al. (2025) identified bodies of intruded seawater of different ages along the modelled transect. By comparing the model results to the measured radiocarbon ages, as well as to the chemical characteristics of both the fresh and saline groundwater in the area, we were able to provide new insights into the paleohydrological developments of the area. We found that, while remnants of brackish groundwater dating back millennia have also been preserved, the main thrust for seawater intrusion was the reclamation of the Haarlemmermeer Lake in 1850 and the over-exploitation of the freshwater lens in the first half of the 20th century. The model results also indicate that the development of the freshwater lens starting 1000 BP caused a temporary reversal of the flow direction in the saltwater wedge, turning seawater intrusion into submarine groundwater discharge. The simulated age distribution generally matches the pattern of the radiocarbon ages of the groundwater samples.