Fresh water sources for submarine groundwater discharge to the southern Baltic Sea 

The impact of freshwater sources like surface river and submarine groundwater discharge (SGD) on the coastal water, the element balance therein, and the associated biogeochemical transformations within the subterranean estuary is currently a matter of intense debate and investigation. A quantification of freshwater mixing in coastal areas has been found to be challenging. In this sense the combination of stable water isotopes with further (isotope) hydro(bio)geochemical tracers provides a fundamental valuable tool to identify different freshwater sources found in the mixing zone with seawater. Here, we report the geochemical and isotopic composition of porewaters of permeable sediments in front of a coastal peatland, the Huetelmoor (southern Baltic Sea). Gradients in pore water measurements from 5 m long stationary porewater lances are used to calculate the zero-salinity (ZS) component. The application of binary mixing approaches on water isotopes and conservative elements on the compositional gradients yields temporarily relatively stable ZS compositions but with substantial isotope differences for spatially distant lances. This indicates a subterranean estuary under steady-state conditions with different fresh waters entering the coastal area. At least two freshwater sources can be identified for sediments impacted by SGD without substantial impact of short-term hydrological or meteorologic processes. These results are compared to the composition of potential endmembers, such as the local surface and groundwaters, the local meteoric water line, and the open brackish Baltic Sea. Besides stable isotopes, also dissolved major and minor elements were used to characterize the biogeochemical processes leading to the non-conservative behavior of nutrients, the carbon system, and trace elements. In addition, tritium-noble gas dating of the pore waters allows for an estimate of the fresh water residence time before mixing with the brackish Baltic Sea water. Results will be discussed in the context of other SGD sites along the northern German coast.   Acknowledgement for support by DFG RTG Baltic TRANSCOAST, DFG-KiSNet, BMBF COOLSTYLE/CARBOSTORE, DAAD, and Leibniz IOW