Abstract
Abstract. Freshwater lenses, as important resource for drinking water, are sensitive to climate changes and sea level rise. To simulate this impact on the groundwater systems, hydraulic subsurface models have to be designed. Geophysical techniques can provide information for generating realistic models. The aim of our work is to show how ground-penetrating radar (GPR) investigations can contribute to such hydrological simulations. In the pilot area, Borkum island, GPR was used to map the shape of the groundwater table (GWT) and to characterise the aquifer. In total, 20 km of constant offset (CO) profiles were measured with centre frequencies of 80 and 200 MHz. Wave velocities were determined by common midpoint (CMP) measurements and vertical radar profiling (VRP) in a monitoring well. The 80 MHz CO data show a clear reflection at the groundwater table, whereas the reflection is weaker for the 200 MHz data. After correcting the GPR water tables for the capillary rise, they are in good accordance with the pressure heads of the observation wells in the area. In the centre of the island, the groundwater table is found up to 3.5 m above sea level, however it is lower towards the coastline and marshland. Some local depressions are observed in the region of dune valleys and around pumping stations of the local water supplier. GPR also reveals details within the sediments and highly-permeable aeolian sands can be distinguished from less-permeable marine sediments. Further, a silt loam layer below the water table could be mapped on a large area. The reflection characteristics indicates scattered erosion channels in this layer that cause it to be an aquitard with some leakage. GPR provides a high resolution map of the groundwater table and insight into the stratigraphy of the sediments and their hydraulic properties. This is valuable complementary information to the observation of sparsely distributed monitoring wells as input to hydraulic simulation.
Highlights
Freshwater lenses are an important resource for drinking water on islands and coastal areas
The aim of this paper is to evaluate the capability of groundpenetrating radar (GPR) to investigate the near-surface aquifer of Borkum island and to demonstrate the benefit of GPR investigation as a part of geophysical exploration for realistic hydraulic simulation of future developments of an island freshwater lens (Sulzbacher et al, 2012)
Mean radar velocities were determined by Common midpoint (CMP) and Vertical radar profiling (VRP) analysis to 0.124 and 0.065 m ns−1 in the unsaturated and saturated zone corresponding to permittivities of εr = 5.9 and εr = 21.6, respectively (Eq 2)
Summary
Freshwater lenses are an important resource for drinking water on islands and coastal areas. The balance of freshwater and saltwater in those environments is very sensitive to changes of the basic conditions. Within the CLIWAT project (CLImate and WATer, http://www.cliwat.eu), the impact of climate change and sea level rise on freshwater resources of coastal aquifers is investigated by long-term hydraulic simulations (Sulzbacher et al, 2012). Due to its lower density, the freshwater floats on top of the saltwater and forms a freshwater lens analogous to an iceberg floating in the sea. A small part of the freshwater lens emerges above the sea level whereas the larger part is below. The correlation between the thickness of the freshwater lens and the height of the groundwater table above sea level was first observed and described by Ghyben and Herzberg (Herzberg, 1901):
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
