EGU General Assembly 2021
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Controls on submarine groundwater discharge in an urbanized bay of the southern Baltic Sea: An isotope and trace metal perspective.

Catia Milene Ehlert von Ahn1, Jan Scholten2, Christoph Malik3, Peter Feldens4, Bo Liu5, Olaf Dellwig1, Anna-Kathrina Jenner1, Svenja Papenmeier4, Iris Schmiedinger1, Mary Zeller1, and Michael Böttcher1,6,7
Catia Milene Ehlert von Ahn et al.
  • 1Geochemistry & Isotope Biogeochemistry Group, Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Rostock, Germany
  • 2Institute of Geosciences, Kiel University, Kiel, Germany.
  • 3UmweltPlan GmbH Stralsund, Department of Hydrogeology, Stralsund, Germany.
  • 4Marine Geophysics Group, Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Rostock, Germany.
  • 5Section of Marine Geochemistry, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany.
  • 6Marine Geochemistry, University of Greifswald, Greifswald, Germany.
  • 7Interdisciplinary Faculty, University of Rostock, Rostock, Germany.

Submarine groundwater discharge (SGD) acts as a source of fresh water and dissolved substances for coastal ecosystems. Evaluation of the actual controls on SGD and corresponding chemical fluxes require a closer understanding of the processes that take place in the mixing zone between SGD and the coastal waters. It is hypothesized that artificial infrastructures, like sediment channeling, may ease the hydrological connection between coastal aquifer and coastal bottom water. The resultant, increase of SGD, changes the residence time in the mixing zone, and thereby, reduces the impact of early diagenesis. The present study focuses on the distribution of SGD, including the characterization of different mixing zones in the urbanized Wismar Bay (WB), southern Baltic Sea. Short sediment cores were retrieved for geochemical porewaters and sediment analyses. Surface sea water samples were collected along across-shore transects in the WB.  Besides major ions, Ba, Fe, and Mn, the water samples were analyzed for nutrients, dissolved inorganic carbon (DIC), stable isotopes (H, O, C, S), and Ra isotopes. Sediments were analyzed for C, N, S, Hg contents as well as reactive components (e.g. Fe, Mn, P) by HCl extractions. Organic matter mineralization rates, DIC, and SO4 fluxes for the sediment-water interface were modeled from porewater profiles. Shallow seismic techniques were applied to identify potential litho-morphological controls on SGD. Geochemical porewater data allow identification of active SGD sites in the WB. In the central part, the freshening of porewaters in the top surface sediments indicates the upward flow of SGD originating from a coastal aquifer. The acoustic profiles show that the bottom sediments in the central bay are under local impact of excavation, reducing the sediment thickness above the coastal aquifer. Overall, the impact of SGD on the coastal water body of the WB is diffuse and promoted by local anthropogenic activity. The water isotope composition of porewaters at this site are close to the local meteoric water line at Warnemünde (located 50 km east of the WB), suggesting a discharge of relatively modern fresh waters. The (isotope) hydrochemical composition of the fresh water discharging is controlled by water-rock interactions in the aquifer and modulated by intense diagenesis in the brackish surface sediments. Furthermore, the SGD facilitates the upward migration of elements and enhances their fluxes across the sediment-water interface, e.g. DIC concentrations in the fresh groundwater are further enhanced in the mixing zone, indicating that SGD is a potential source of excess CO2 in the investigated coastal waters.

The investigations are supported by the DAAD, DFG RTS Baltic TRANSCOAST, KiSnet project, BONUS SEAMOUNT, FP7 EU Marie Curie career integration grant, DAM-MFG, and IOW.

How to cite: Ehlert von Ahn, C. M., Scholten, J., Malik, C., Feldens, P., Liu, B., Dellwig, O., Jenner, A.-K., Papenmeier, S., Schmiedinger, I., Zeller, M., and Böttcher, M.: Controls on submarine groundwater discharge in an urbanized bay of the southern Baltic Sea: An isotope and trace metal perspective., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2470,, 2021.