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Coastal groundwater reserves often reflect a complex evolution of marinetransgressions and regressions, and are only rarely in equilibrium withcurrent boundary conditions. Understanding and managing the present-daydistribution and future development of these reserves and their hydrochemicalcharacteristics therefore requires insight into their complex evolutionhistory. In this paper, we construct a paleo-hydrogeological model, togetherwith groundwater age and origin calculations, to simulate, study and evaluatethe evolution of groundwater salinity in the coastal area of the Netherlandsthroughout the last 8.5 kyr of the Holocene. While intended as a conceptualtool, confidence in our model results is warranted by a good correspondencewith a hydrochemical characterization of groundwater origin. Throughout themodeled period, coastal groundwater distribution never reached equilibriumwith contemporaneous boundary conditions. This result highlights theimportance of historically changing boundary conditions in shaping thepresent-day distribution of groundwater and its chemical composition. Assuch, it acts as a warning against the common use of a steady-state situationgiven present-day boundary conditions to initialize groundwater transportmodeling in complex coastal aquifers or, more general, against explainingexisting groundwater composition patterns from the currently existing flowsituation. The importance of historical boundary conditions not only holdstrue for the effects of the large-scale marine transgression around5 kyr BC that thoroughly reworked groundwater composition, but also for themore local effects of a temporary gaining river system  still recognizable today. Model results further attest to the impact ofgroundwater density differences on coastal groundwater flow on millennialtimescales and highlight their importance in shaping today's groundwatersalinity distribution. We found free convection to drive large-scale fingeredinfiltration of seawater to depths of 200 m within decades after a marinetransgression, displacing the originally present groundwater upwards.Subsequent infiltration of fresh meteoric water was, in contrast, hampered bythe existing density gradient. We observed discontinuous aquitards to exert asignificant control on infiltration patterns and the resulting evolution ofgroundwater salinity. Finally, adding to a long-term scientific debate on theorigins of groundwater salinity in Dutch coastal aquifers, our modelingresults suggest a more significant role of pre-Holocene groundwater in thepresent-day groundwater salinity distribution in the Netherlands thanpreviously recognized. Though conceptual, comprehensively modeling theHolocene evolution of groundwater salinity, age and origin offered a uniqueview on the complex processes shaping groundwater in coastal aquifers overmillennial timescales

Paleo-modeling of coastal saltwater intrusion during the Holocene: an application to the Netherlands