Understanding Holocene coastal development and the resulting evolution of groundwater chloride distribution in the North-West coastal region of the Netherlands. A historical modelling approach

Abstract

The origin and evolution of chloride distribution in the Dutch coastal area has been the subject of many studies. Most of these studies formulated their conclusions concerning the evolution of groundwater chloride distribution by reconstructing and interpolating present day data to earlier times. This research analyzes the evolution of the current groundwater chloride distribution using forward modelling integrated with hypothesis on palaeogeographical development and groundwater salinization. The analysis is conducted using a 2D-profile from Zandvoort till Hilversum, modelled with a density dependent groundwater flow model, MODFLOW and SEAWAT. Results show that the hypotheses represented the dune area correctly and that the extensive salinization of the subsurface in the west was indeed caused by Holocene transgression. However, amelioration of boundaries, such as surface level,s and taking account of inflow from northern and eastern regions is needed to get a better understanding of differences between simulated results and measured or interpolated data. We conclud that Holocene transgression, lithology and landscape change were important controls on the evolution of the chloride distribution during de Holocene coastal development. Holocene transgression proved to be the main source of salinization, while lithology caused retardation of salinization in areas with low permeable sediments and landscape change caused hydraulic gradients which resulted in repositioning of the fresh-brackish groundwater interface.