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GPR investigations of an island aquifer as a contribution to groundwater modelling

The freshwater resources of the North Sea Islands are fed only by precipitation. In the subsurface of the islands, a freshwater lens floats on the saltwater that fills the pore space in this coastal region. The aim of the GPR investigation was to map the shape of the groundwater table as well as sedimentary structures as input parameters for hydrogeological simulation. In total, 20 km of constant offset (CO) radar 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 data show a clear reflection at the groundwater table whereas the reflection is blurry and shifted to lower frequencies for the 200 MHz data. This is caused by the gradual increase of water content above the capillary fringe. The GPR results are in good accordance to the observation wells in the area. In the centre of the island the groundwater table is found to be up to 3.5 m above sea level and it sinks down to sea level towards the coast line. Local depressions are observed in the region of the pumping stations of the local water supplier. A sharp horizontal reflection below the water table can be seen on most of the profiles and was identified as a silt loam layer by hand drillings. However, GPR data indicate some scattered gaps in this layer so that hydraulically it acts as an aquitard with leakage. GPR is useful to map the shape of the groundwater table on large areas of the island as addition to point information at observation wells. It gives valuable complementary information that enables an improved model for hydrogeological simulations. These simulations show that with predicted climate change until the year 2100, the salinity of the freshwater in the deeper aquifer near the pumping wells of the local water supplier will increase and thus a relocation of the pumping wells is recommended.