Ground-penetrating radar insight into a coastal aquifer: The freshwater lens of Borkum Island
Creators: name: Igel, J.
name: Günther, T.
name: Kuntzer, M.
Publisher: name: Göttingen : Copernicus GmbH
Year: 2013
Subjects: subject: Distributed monitoring
subject: Geophysical techniques
subject: Ground Penetrating Radar
subject: Ground penetrating radars
subject: Hydraulic simulation
subject: Hydrological simulations
subject: Reflection characteristics
subject: Vertical radar profiling
subject: Aquifers
subject: Climate change
subject: Groundwater resources
subject: Radar
subject: Radar measurement
subject: Sea level
subject: Silt
subject: Stratigraphy
subject: Submarine geology
subject: Water
subject: Wells
subject: Ground penetrating radar systems
subject: biostratigraphy
subject: capillarity
subject: carbon monoxide
subject: climate change
subject: coast
subject: coastal aquifer
subject: data set
subject: drinking water
subject: ground penetrating radar
subject: hydraulic property
subject: hydrogeology
subject: marine sediment
subject: monitoring
subject: numerical model
subject: pressure effect
subject: pumping
subject: scattering
subject: sea level change
subject: silty loam
subject: water table
subject: Borkum
subject: East Frisian Islands
subject: Frisian Islands
subject: Germany
subject: Lower Saxony
subject: Dewey Decimal Classification::500 | Naturwissenschaften::550 | Geowissenschaften
subject: Dewey Decimal Classification::500 | Naturwissenschaften::551 | Geologie, Hydrologie, Meteorologie

description: 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 groundpenetrating 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 80MHz CO data show a clear reflection at the groundwater table, whereas the reflection is weaker for the 200MHz 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 3.5m 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.