Evolution of the fresh water lens of Veermansplaat: A field and modelling investigation of the influence of a restricted tide on the evolution of a fresh water lens using Hydrogeosphere

Abstract

The Veermansplaat, located at the middle of saltwater lake Grevelingen in the Netherlands, has formed a fresh water lens since the embankment of lake Grevelingen in 1971. Rijksstructuurvisie Grevelingen en Volkerak-Zoommeer (RSV) has suggested a mitigation plan to stop the increasing anoxic concentrations in the former deep tidal channels by introducing a restricted tide of 50 cm in lake Grevelingen. This research investigates the historical and the future evolution of the fresh groundwater lens below Veermansplaat in a non-tidal and tidal system from 1971 to 2025 by conducting a field and model study using the fully integrated, physically based hydrological model Hydrogeosphere. The purpose of this research is to indicate the effect of a restricted tide on the evolution of the fresh water lens of Veermansplaat compared to the evolution without a tidal system. The first step of this research involves the analysis of infiltration rate, depth of the fresh-saline groundwater interface, chemical composition of the groundwater, the hydraulic head an historical data by using single ring infiltration, electrical resistivity tomography, groundwater sampling and automated groundwater level registration. The second step involves modelling the evolution of the fresh water lens for a tidal and non-tidal system using Hydrogeosphere. The fieldwork and historical data show a fresh water infiltration depth of -12 m NAP in 2019 and a varying infiltration depth along the island due to surface infiltration rate, vegetation and elevation variability. The models show a future lens shaped evolution of the fresh water lens between 2019 and 2025 for a non-tidal boundary condition. The introduction of a restricted tide will increase the mixing zones of the fresh water lens and will start a salinization of the coastline. The RSV is advised to take caution and to conduct further research of the effects on a restricted tide on the evolution of the fresh water lens of Veermansplaat using other modelling programs.