Modelling nationwide climate change effects on coastal groundwater in the Netherlands

The coastal zone of the Netherlands is the densely populated economic heartland of the Netherlands. This low-lying area is predominantly located below current mean sea level. Groundwater in large parts of the Dutch coastal zone is saline, having infiltrated during Holocene transgressions. This saline groundwater is now slowly moving upward, driven by artificially lowered drainage levels and resulting land subsidence. Coastal groundwater in the Netherlands is vulnerable to climate change and rising sea levels, as groundwater levels rise, fresh groundwater reserves decrease, and surface water is salinized by exfiltrating saline groundwater.

We developed a high-resolution nationwide 3D fresh-salt groundwater flow and transport model to assess effects of climate change and sea level rise on groundwater salinization in the Netherlands. The fully scripted modelling workflow includes a 3D multiple indicator kriging interpolation of all available salinity measurements, that accounted for uncertainty in both measurements and interpolation. The developed model used a parallellized version of the SEAWAT model code to allow otherwise time-consuming calculations. It links to the existing national hydrological modelling framework to allow calculation of climate change effects on surface water supply and demand and agricultural damage. We used the resulting modelling framework to calculate groundwater effects of different climate change and sea level rise scenarios up to 2100.

Results show significant effects of climate change and especially sea level rise on coastal groundwater. Significant head increase (> 5% of SLR) is experienced in shallow aquifers between 2 to 10 km inland, dependent on the varying hydrogeological settings along the Dutch coast. In deeper aquifers, head increase generally propagates further, to up to 15 km inland. Through the combined effects of head increase and the inward movement of saline groundwater, salt loads to surface water increase over a significantly larger zone, extending to 25 km inward. Results signify the importance of including the long-term displacement of brackish and saline groundwater when assessing coastal groundwater effects of climate change and sea level rise.