Freshwater lens formation below juvenile dunes on a barrier island (Spiekeroog, Northwest Germany)

Morphodynamic changes of barrier islands and the consequences for underlying freshwater reservoirs are of major interest for coastal environments and drinking water supply in coastal areas. Here, a geomorphological and hydrological reconstruction of dune formation on a barrier island and their freshwater reservoirs is presented. The aim was to provide an integrated analysis of the co-evolution of dunes and freshwater resources during the development of a barrier island. The study area is the eastern part of Spiekeroog Island, the so-called ‘Ostplate’, at the Northwest German coast, that expanded significantly in the last 360 years due to coastal land reclamation. The first dune growth was initiated in the 1940s at the newly formed sand flat, and dune freshening started around 1970. Pore water measurements in November 2011, March and August 2012 confirm the existence of freshwater below the dune area and indicate a landward movement of the freshwater–saltwater interface in winter and a seaward movement in summer, respectively, in response to storms and corresponding coastal flooding. A density-dependent groundwater flow model was used to estimate the present thickness and shape of the lenses as well as to reconstruct the evolution of the freshwater reservoirs. The simulations suggest an asymmetric lens shape with a freshwater discharge “tube” below the beach to the sea and an extended brackish transition zone below the salt marsh area towards the tidal flat. The simulation results also indicate that the freshwater reservoirs are still in a stage of development (after 40 years of simulation time) and are estimated to reach steady-state after about 300–500 years under the given conditions without taking into account possible impacts of climate change.