Abstract
Abstract. Geophysical techniques are increasingly being 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 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 which gives a total of about 3200 line km. About 38 km of seismic lines have been collected. Faults bordering a graben structure, buried tunnel valleys, glaciotectonic thrust complexes, marine clay units, and sand aquifers are all examples of geological structures 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 and thus provide important information on the salt/freshwater 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 are strongly influenced by the increased electrical conductivity there, buried valleys and other geological elements are still revealed. The mapped salinity distribution indicates preferential flow paths through and along specific geological structures within the area. The effects of a future sea level rise on the groundwater system and groundwater chemistry are discussed with special emphasis on the importance of knowing the existence, distribution and geometry of the mapped geological elements, and their control on the groundwater salinity distribution is assessed.
Highlights
The westernmost part of the study area along the North Sea coast is saturated with saline water and the transient electromagnetic (TEM) data are strongly influenced by the increased electrical conductivity there, buried valleys and other geological elements are still revealed
The sea level is predicted to rise in response to climate change (IPCC, 2007), which will result in saltwater intrusion into coastal aquifer systems according to theory (Bear et al, 1999) as well as observation (e.g. Custodio and Bruggeman, 1987; Iribar et al, 1997; Edmunds et al, 2001; Oude Essink, 2001; Oude Essink et al, 2010)
Most borehole data originate from water well construction, and a few investigative drillings contribute to ground-truthing of the geophysical data
Summary
The sea level is predicted to rise in response to climate change (IPCC, 2007), which will result in saltwater intrusion into coastal aquifer systems according to theory (Bear et al, 1999) as well as observation (e.g. Custodio and Bruggeman, 1987; Iribar et al, 1997; Edmunds et al, 2001; Oude Essink, 2001; Oude Essink et al, 2010). Chang et al (2011) show that a sea level rise has no long-term impact on the saltwater wedge in confined systems where the ambient recharge to the aquifer remains constant. For head-controlled systems where the inland hydraulic head remains unchanged during sea-level changes, the toe of the saltwater wedge is predicted to migrate hundreds of metres to several kilometres inland in a realistic sea level rise. These theoretical studies show the potential impact of a sea level rise for idealized, homogeneous systems
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