More than buried valleys: Do tunnel valleys serve as conduits for preferential freshened groundwater flow within the North Sea?

• Applicant: Dr. Arne Lohrberg
• Subject Area: Geophysics ; Geodesy, Photogrammetry, Remote Sensing, Geoinformatics, Cartography ; Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 524639242

Our society’s steadily growing population leads to increasing demands of freshwater and the abstraction of groundwater is one of its most important sources. Shortages of groundwater supply have led to the search for groundwater offshore and several studies have shown freshened groundwater to exist beneath the seafloor. However, the role of large subsurface landforms produced during glaciations of the Pleistocene is still poorly understood with respect to groundwater flow. In particular, so-called tunnel valleys (TVs) formed beneath ice sheets acted as drainage systems of glacial meltwater. Their dimensions (up to 5 km width, 400 m depth, 100s of km length) reflect the massive amount of meltwater that flushed the subsurface beneath ice sheets. Due to their mechanism of formation, the fill of buried TVs often consists of highly permeable sands and gravels in their lower part and fine-grained deposits at their top. Such a configuration promotes their role as preferential flow pathways for groundwater sealed from the Holocene saltwater above. To understand the potential of TVs as preferential flow pathways of offshore freshened groundwater (OFG) in the southeastern North Sea, this project has the following specific objectives: (O1) Combining electromagnetic and seismic data, we aim to delineate structurally constrained resistivity model for several TVs; (O2) We aim to estimate the corresponding salinity values for different architectures and depths of TVs; (O3) Building on the first two objectives, we aim to extrapolate the findings for the full working area in a detailed 3D lithological model. The resulting salinity distribution in the subsurface will aid to delineate upper and lower limits of OFG volumes and provide the basis for a detailed groundwater model. To fulfil our objectives, we will take the following steps: (S1) Map and characterize the 3D spatial heterogeneity of TVs in the study area using existing seismic data; (S2) Build a lithological model for the subsurface between Amrum and Heligoland from 0 to 400 m depth using existing multi-channel seismic profiles; (S3) Identify promising sites and conduct controlled source electromagnetic (CSEM) measurements to investigate the distribution of electrical resistivities in the subsurface (TVs); (S4) Combine resistivity measurements with MCS to derive pore fluid salinity estimates; (S5) Extrapolate and discuss the results for the full lithological model area. Tunnel valleys exist in most formerly glaciated regions worldwide. Proving offshore freshened groundwater in these subsurface landforms may have substantial implications for as of yet unknown freshwater distributions and hydrological systems. The extensive and dense MCS data available to us provide a unique opportunity for the integration of CSEM and seismic measurements with limited additional effort. The results of the project will provide a new view on offshore glacial landforms and their role in Pleistocene water cycles.

• DFG Programme: Research Grants
• International Connection: Malta
• Co-Investigators: Dr. Amir Haroon; Professor Dr. Sebastian Krastel; Professor Dr. Nils Moosdorf
• Cooperation Partner: Professor Aaron Micallef, Ph.D.