Modelling of the effect of a sea-level rise and land subsidence on the evolution of the groundwater density in the subsoil of the northern part of the Netherlands

The Province of Friesland is conducting a study on possible future changes in the surface water and groundwater systems of Friesland. The aim of the study is to assess what changes might be caused by land subsidence and a rise in sea level-focusing in particular on the salinization of the surface water--by modelling the flow of groundwater in the coastal zone of the Province of Friesland. A three-dimensional density distribution of groundwater (based on the chloride distribution in the groundwater) can be projected by using the density dependent version of the MLAEM software; MVAEM (Strack, O.D.L., 1989. Multi-layer aquifer modeling using the analytical element method. Proc. 4th Int. Conf. on the use of models to findworking solutions to groundwater problems, National Water Well Association, Dublin, Ohio, USA, 1995). Construction of a fully three-dimensional density distribution is based on irregularly spaced chloride concentration data points. These are interpolated using a three-dimensional multi-quadric interpolator (in MVAEM). For some parts of the model area, chloride data were not available (e.g. the Wadden Sea); for these regions chloride concentrations were estimated (based on geo-electric and other information) and assigned to artificial data points, which were added to the data set. The techniques used for this process were developed during the construction of the coastal elements in NAGROM--the NAtional GROundwater Model for The Netherlands (De Lange, W.J., van der Meij, J.L., 1995. NAGROM (NAtional GROundwater Model). RIZA, Lelystad, TNO-GG, Delft, The Netherlands; De Lange, W.J., 1996. Groundwater modeling of large domains with analytic elements. Delft University of Technology, Delft, The Netherlands). The influences of a sea-level rise and land subsidence on the groundwater system were simulated by computational modelling of density-driven groundwater flow. The groundwater model that was used is a refined version of the regional model contained in NAGROM, based on the analytic element method (Strack, 1989). NAGROM is a national water management tool for evaluating different water conservation and distribution policies. It was developed by the Institute for Inland Water Management and Waste Water Treatment in co-operation with The Netherlands Institute of Applied Geoscience TNO (NITG-TNO). The effect of a sea-level rise and land subsidence was calculated for a 50-year period. Salinization of the surface waters by discharged groundwater in the Northern part of The Netherlands was compared with the evolution of the region not including sea-level rise and land subsidence. The amount of saline discharge as part of the surface water balance was calculated under special boundary conditions set according to the proposed sea-level rise and land subsidence in a semi-unsteady state.