The results of age-dating and quality evolution of intruding Coastal North Sea (CNS) water are presented and discussed, based on 18 monitoring wells in 2 transects, reaching about 9 km inland from the North Sea. There, reclaimed Lake Haarlemmermeer at 5 m below sea level (BSL) is forming a great sink attracting this seawater, which is flowing at 80-180 m BSL underneath a deep, broad, fresh dune water lens. For each sample of intruded CNS a chemical mass balance (CMB) has been drawn up, with a new Excel based code (R+SWi). The radiocarbon dating method is fully linked to this CMB. It yields ages between 0.37 and 6 ka, distributed in 2 main age zones. Old CNS (3-6 ka) intruded during a period of coastal progradation. Young CNS (<2 ka) slowly intruded during a period of coastal erosion (100-1000 AD), followed by slightly accelerated intrusion due to drainage of peat areas as of 1525 AD. Reclamation of Lake Haarlemmermeer and dune groundwater extraction after 1850 AD strongly accelerated the intrusion rate. Sensitivity analysis revealed that the 14C age determination is most sensitive to the assumed 14C activity (age) of reactive organic matter and carbonate minerals in the submarine Holocene deposits under the seafloor.