On of the most important factors in modelling chloride concentrations are the speed at which the salt water moves and the initial chloride concentration. There are usually little measurements available to get an accurate estimate of shifting chloride concentrations over time. In order to gain better insights in the changing chloride concentrations we used an approach in which we model the movement of chloride over a very long time period (>100.000 years). Such a paleo reconstruction helps us to get a better understanding of the origin of the current salt concentrations and an idea of the speed at which it changes over time. For the modelling we use a groundwater flow model combined with a groundwater transport model, both from the MODFLOW-6 framework. The models are set up with publicly available data of the geology, hydraulic conductivity, chloride concentrations, surface water, surface elevation and extraction wells. We apply this approach to a well field near the land reclamation of Southern Flevoland. The Dutch drinking water company Vitens assesses possibilities to expand this well field. The expansion may lead to an increased risk of salinization. Measurements from 1989 and 2024 near this well field show that the depth of the freshwater/saltwater interface has shifted towards the well field. The paleo modelling illustrates the natural displacement of salt water from the aquifer as a results of long-term freshwater recharge. Over time the aquifer becomes fresh while pockets of saltwater remain where the groundwater flow is nearly stagnant. These saltwater pockets correspond to zones where higher salinity levels have been measured. Thus the model results help explain the presence of saltwater in an otherwise freshwater aquifer. Simulations over shorter time periods demonstrate the influence of the pumping rates and land reclamation on the advective transport of salt water. Additionally the model’s sensitivity to some of the hydrological parameters is evaluated, such as hydraulic conductivity, initial chloride concentration and interaction with surface water.