Ground-penetrating radar insight into a coastal aquifer: the freshwater lens of Borkum Island 1Leibniz Institute for Applied Geophysics (LIAG), Hanover, Germany
07 Feb 2013
2Institute for Soil Science, Leibniz University Hannover, Hanover, Germany
Received: 02 March 2012 – Published in Hydrol. Earth Syst. Sci. Discuss.: 16 March 2012 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.
Revised: 21 December 2012 – Accepted: 14 January 2013 – Published: 07 February 2013
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.
Citation: Igel, J., Günther, T., and Kuntzer, M.: Ground-penetrating radar insight into a coastal aquifer: the freshwater lens of Borkum Island, Hydrol. Earth Syst. Sci., 17, 519-531, doi:10.5194/hess-17-519-2013, 2013.