The areas where there is groundwater salinisation are shown. On the coast, the heavy seawater pushes back the groundwater, even behind the coastline. If the sea level rises, the seawater will penetrate further inland and the groundwater there will become increasingly saline, making it impossible to use as drinking water.

Source:

https://www.lbeg.niedersachsen.de/HUEK200Versalzung/versalzung-des-grundwassers-1200-000-641.html last accessed on 08/05/2023

Further information on groundwater salinisation worldwide: IPCC, 2021 p. 1092

IPCC. (2021): Climate Change 2021 The Physical Science Basis WGI. In Bulletin of the Chinese Academy of Sciences (vol. 34, number 2) p. 1092