This study demonstrates the integration of diverse data sources to map the dynamic interface between fresh and saline groundwater in the coastal dune system of the Amsterdam Water Supply Dunes (AWD) in The Netherlands. We showcase the value of long-term direct groundwater salinity monitoring and explore the complementary benefits of indirect salinity measurements, including groundwater electrical conductivity (EC), borehole geophysics, and airborne electromagnetic surveys. The Amsterdam Water Supply Dunes (AWD) have been used for drinking water production for more than 170 years, initially mining the natural freshwater lens. To meet the growing demand, Managed Aquifer Recharge (MAR) with pre-treated river water was implemented approximately 70 years ago. A dense network of monitoring wells, established over time, allows for regular sampling and chloride concentration measurements at various depths. The wells are regularly sampled and the chloride concentrations at different depths are determined. These long-term datasets, some of them spanning over a century, enable the reconstruction of the fresh/saline groundwater interface's behavior, which has been significantly influenced by the dynamic history of groundwater abstraction and recharge. We demonstrate that indirect salinity measurements provide valuable complementary data to direct chloride measurements: - Borehole geophysical measurements, using permanent electrode cables and electromagnetic downhole probes, offer high-resolution vertical salinity distribution profiles. - A single airborne electromagnetic survey facilitates spatial interpolation of the fresh/saline interface between groundwater point measurements. - We evaluate the reliability of using simple electrical conductivity (EC) measurements of groundwater samples as a cost-effective alternative to direct chloride analyses for determining groundwater salinity. Furthermore, we present a validated methodology for mapping the extent of the artificially infiltrated river water and distinguishing it from the native dune freshwater using natural tracers chloride, and Br/Cl and 18O/16O ratios: