Late and postglacial sea-level changes in The Kattegat: The consequences to different coastal aquifers (on the islands Zealand and Anholt)

Fluctuations in sea level influence the condition of coastal groundwater aquifers. In this paper the palaeo-hydrology of Anholt and part of Zealand and its dependence on the sea level history of Kattegat will be analyzed. The Late and Postglacial Sea-Level Changes in Kattegat are outlined and the consequences analyzed for Late and Postglacial ice movement and for two different coastal aquifers: a simple unconfined aquifer at the island Anholt, and a complex confined Limestone aquifers around Roskilde on Zealand. Data are presented which provide a description of how sea level changes have influenced and formed the island Anholt. This geological history can be used to provide information on the development of the salt-fresh groundwater interface under a sand island. The center of Anholt was covered by the sea at 6 ka and has now an ideal Ghyben-Herzberg freshwater lens. This means that the present equilibrium between the saltwater and freshwater has been established in less than 6000 years. Limestone aquifers on Zealand show no ideal Ghyben- Herzberg freshwater lens. Detailed 3-D mapping of a study area, around Roskilde- and Isefjord, has revealed how low sea-levels (around 50–30 ka) cause development of karst structures and deep erosion channels in the limestone (later filled with high permeable sand and gravel). These channels drain the groundwater to subsea springs, working as natural pumps, resulting in a freshwater “lens” much thinner than the equilibrium.