Submarine groundwater discharge into a temperate tidal basin: Mapping and characterization by a multi-method and multi-scale approach

Abstract

Submarine groundwater discharge (SGD) is considered as an important source of chemical substances to the oceans. Investigations of SGD need to consider varying spatial and temporal scales. They remain a challenge and require the application of different complementary detection and quantification methods. Our study focuses on the identification of fresh SGD (FSGD) in intertidal zones using the Königshafen Bay on the island Sylt, Germany, as an example case. We applied geophysical, remote sensing and in-situ pore water measurements to develop a suitable combination of survey methods for an efficient detection of FSGD. Our results provide detailed information on the occurrence and spreading of FSGD at scales ranging from meters to kilometers. We propose a stepwise approach to identify FSGD in intertidal zones as follows: First, orthophotos from unmanned aerial vehicles (UAVs) can highlight distinct tidal flat features characterized by brighter surrounding sediments and typical plant growth at possible FSGD sites. These can be mapped by large-scale electromagnetic induction (EMI), where possible FSGD patches appear as areas of higher electrical resistivity. Ground penetrating radar (GPR) can then be used to check the possible FSGD patches directly on the surface with high lateral resolution. The deep structure of FSGD can then be explored with Electrical Resistivity Tomography (ERT) on targeted profiles to detect fresh water in the sediment through the zone of increased electric resistivity. Additional Nuclear Magnetic Resonance (NMR) measurements from the surface will help to estimate the water content of the sediments and to distinguish between clay layers and salt water containing sand layers. Finally, the geophysical results are verified by in-situ measurements of pore water salinity. Using this approach, 17 significant areas with FSGD sites in the Königshafen bay could be efficiently identified. These areas have a diameter of 30–60 m and are characterized by increased electrical resistivities of 5 Ωm to 30 Ωm and decreased salinity of 0–10 PSU with respect to the surrounding saltwater-saturated areas.