Transboundary geophysical mapping of geological elements and salinity distribution critical for the assessment of future sea water intrusion in response to sea level rise

Abstract

Abstract. Geophysical techniques are increasingly used as tools for characterising the subsurface and they are generally required to develop subsurface models that properly delineate the distribution of aquifers and aquitards, salt/freshwater interfaces and geological structures that affect groundwater flow. In a study area covering 730 km2 across the border between Germany and Denmark a combination of an airborne transient electromagnetic survey (performed with the SkyTEM system), a high-resolution seismic survey and borehole logging has been used in an integrated mapping of important geological, physical and chemical features of the subsurface. The spacing between flight lines is 200–250 m giving a total of about 3200 line km. About 38 km of seismic lines have been collected. Faults bordering a graben structure, deep and shallow buried tunnel valleys, glaciotectonic thrust complexes, marine clay units, and sand aquifers are all examples of geological elements mapped by the geophysical data that control groundwater flow and to some extent hydrochemistry. Additionally, the data provide an excellent picture of the salinity distribution in the area thus providing important information on the fresh-saltwater boundary and the chemical status of groundwater. Although, the westernmost part of the study area along the North Sea coast is saturated with saline water and the TEM data therefore is strongly influenced by the increased electrical conductivity here, buried valleys and other geological elements are still revealed. The salinity distribution indicates preferential flow paths through and along specific geological elements within the area. The effects of future sea level rise on the groundwater system and chemical status are discussed with special emphasis on the importance of knowing the existence, distribution and geometry of the mapped geological elements, and assessing their control on the groundwater salinity distribution.