Assessing the impact of climate change and anthropogenic factors on future salinization of a low-lying coastal groundwater system (Northwestern Germany)

Coastal fresh groundwater reservoirs are often threatened by different salinization processes. Anthropogenic drivers for salinization include groundwater abstraction, land cultivation and artificial drainage. Global climate change, which modifies groundwater recharge patterns and results in sea-level rise, presents another important process enhancing groundwater salinization.

A good understanding regarding the physical behaviour of coastal groundwater systems is needed for sustainable management, including the development of effective counter measures to antagonize future groundwater salinization. At present, however, such knowledge is incomplete.

The aim of this study was to clarify the role of important factors expected to affect future groundwater salinization of low-lying coastal groundwater systems, namely sea-level rise, varying groundwater recharge and abstraction rates, changing drainage and river levels as well as land subsidence. A novel numerical 3-D variable-density groundwater flow and salt transport modeling approach was developed for this purpose, using Northwestern Germany as case study. To systematically evaluate the role of the individual salinization factors, separate model variants were employed.

We found that sea-level rise causes strongest salinization, particularly close to the coastline. Lifting of drainage levels results in freshening of the marsh areas, while a decrease of drainage levels amplifies salinization. Variation of groundwater recharge patterns and abstraction rates have least impact on regional groundwater salinization. As the system is not at steady-state, autonomous salinization will continue into the future and contribute a significant portion of salt loads until the end of the 21^st century. Findings and implications of this study may be relevant to similar low-lying coastal groundwater systems around the world.