Terrestrial surface waters and submarine ground water discharge (SGD) act as a source of dissolved substances for coastal systems. Solute fluxes of SGD depend on the ground water composition and the water-solid-microbe interactions close to the sediment-water interface. Thus, this study aims to characterize and evaluate the hydrogeochemical gradients developing in the fresh-salt water mixing zone of the Wismar Bay (WB), southern Baltic Sea, Germany. Sampling campaigns covering the WB, the fresh-salt water mixing zone at the beach of the WB shoreline, terrestrial surface and ground waters near the WB as well sediments pore water were carried out. In these different waters, the distribution of dissolved inorganic carbon, nutrients, major ions, trace elements, stable isotopes (H, O, C, S), and radium isotopes have been investigated. Enhanced concentrations of radium isotopes together with dissolved manganese, barium in the surface waters of the eastern WB indicated benthic-pelagic coupling via the exchange between pore water and the water column. Salinity, stable isotopes, and major ions in sediment pore water profiles identified the presence of fresh ground water below about 40 cmbsf in the central part of the bay. Geophysical acoustic techniques revealed the local impact of anthropogenic sediment excavation, which reduced the thickness of a sediment layer between the coastal aquifer and the bottom water, causing, therefore, a ground water upward flow close to the top sediments. The fresh impacted pore water stable isotope composition (δ18O, δ2H) plot close to the regional meteoric water line indicating a relatively modern ground water source. The calculated organic matter mineralization rates and the dissolved inorganic carbon sediment-water fluxes were much higher at the fresh impacted site when compared to other unimpacted sediments. Therefore, this study reveals that different fresh water sources contribute to the water balance of WB including a SGD source.

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<p><span>Land-ocean interactions in the coastal zone (LOICZ) are of particular interest regarding the exchange of water and elements, like nutrients, carbon, sulfur, and metals. </span><span>Processes impacting </span><span>groundwater</span><span> fluxes at these boundaries belong to the still unsolved problems in hydrology (Blöschl et al., 2019). </span><span>Stable isotope signatures (H, C, O, S), major and trace element contents in surface waters of a rewetted coastal peatland were investigated to understand the impact of storm-induced flooding by brackish seawater on hydrology and biogeochemical element cycling.</span></p><p><span>The study area is the Hütelmoor, a wetland located at the coastline of the southern Baltic Sea. The area is characterized by a continuous release of fresh water to the Baltic Sea via submarine groundwater discharge (Jurasinski et al., 2018). Surface water is partly drained to a nearby river, but the introduction of brackish waters into the peatland is typically precluded by a small dune and limited to storm-induced flooding events. In the present study, the spatially distributed composition of surface waters was investigated briefly after a flooding event. The results are compared with previous campaigns without actual salt water impact. </span></p><p><span>Conservative elements and water isotopes demonstrate the importance of seasonal variations due to varying evapotranspiration during pre-flood times and allow for a quantification of mixing processes in the post-flood waters. The impact of soil respired CO</span><sub><span>2</span></sub><span>, and/or the mineralization of organic matter or methane on the surface waters is indicated by a shift of the C isotope composition of DIC towards lighter data. The S and O isotopic composition of dissolved sulfate indicates an impact by solutions modified by net microbial sulfate reduction on pre-flood surface waters and a potential oxidation of reduced sulfur species in post-flooding solutions. </span></p><p><span>Previous flooding events already impacted element cycling in the peatland’s past and are also reflected by a sulfidization of peat layers (Fernández-Fernández et al., 2017) and the observation of local areas with enhanced dissolved concentrations in the central part of the peatland.</span></p><p><span>The study is supported by DFG during GK Baltic TRANSCOAST, DAAD, and Leibniz IOW.</span></p><p align="justify"> </p><p><span>References:</span></p><ul><li><span>Blöschl G. et al. (2019) Twenty-three unsolved problems in hydrology (UPH) – a community perspective. Hydrol. Sci. J. 64, 1141-1158.</span></li> <li><span>Jurasinski G. et al. (2018) Understanding the coastal ecocline: Assessing sea-land-interactions at non-tidal, low-lying coasts through interdisciplinary research. Front. Mar. Sci. 5, 1-22</span><span>.</span></li> <li>Fernández-Fernández L.E. et al. <span>(2017) Sulfur isotope biogeochemistry of soils from an episodically flooded coastal wetland, southern Baltic Sea. Geophys. Res. Abs. 19, EGU2017-14335</span><span>.</span></li> </ul>

ZusammenfassungDie zukünftige Trinkwasserversorgung in Mecklenburg-Vorpommern steht vor verschiedenen Herausforderungen, wie z. B. möglichen klimatischen Änderungen, dem demographischen Wandel oder einem stetig steigenden Wasserbedarf in touristischen Zentren. Im Rahmen einer vom Ministerium für Klimaschutz, Landwirtschaft, ländliche Räume und Umwelt Mecklenburg-Vorpommern beauftragten landesweiten Trinkwasserversorgungskonzeption wurde untersucht, welche Faktoren eine besondere Gefährdung für die Trinkwasserbereitstellung in den kommenden Jahren und Jahrzehnten darstellen. Dabei zeigte sich eindeutig, dass die größte Herausforderung in einem Interessenausgleich zwischen der Trinkwasserversorgung und der Landwirtschaft besteht. Potenzielle Konflikte sind sowohl in quantitativer als auch in qualitativer Hinsicht zu verzeichnen und können durch klimatische Änderungen noch verstärkt werden. Im Ergebnis der quantitativen und qualitativen Zustandsbewertungen wurden Handlungserfordernisse herausgearbeitet und mögliche Präventions- und Anpassungsmaßnahmen diskutiert.

<p>The biogeochemistry of sulfur and carbon in groundwater of a Quaternary porous aquifer system and associated surface (lake) waters was investigated to identify processes of water mixing and the sources of dissolved sulfate and dissolved inorganic carbon (DIC). The study area is situated in North-Eastern Germany (Mecklenburg-Western Pomerania) close to the Baltic Sea coastline. The area is under impact by agricultural activity on a regional scale. A major goal was to identify the natural and anthropogenic key hydrobiogeochemical processes controlling the coupled element cycles upon groundwater development. Besides major and minor elements, redox-sensitive trace elements, nutrients, and stable mulit-isotope signatures (H, C, O, S) were considered.</p><p>While water isotopes of most groundwaters are positioned on the meteoric water line, surface waters are affected by an evaporation-induced enrichment of heavy isotopes. These shifts allow for a quantification of mixing proportions in influenced groundwater wells between direct precipitation-derived groundwater and  infiltrating lake water born fractions.</p><p>Major element hydrochemical and the carbon isotope composition of DIC indicate soil CO<sub>2</sub> and the subterrestrial dissolution of carbonate minerals within the aquifer matrix as primary sources for DIC. Furthermore, contributions from oxidized dissolved organic carbon (DOC) under water-saturated conditions are found.</p><p>The coupled sulfur and oxygen isotope composition of dissolved sulfate indicates an origin dominatly  from the subterrestrial oxidation of iron sulfides, mainly pyrite. These iron sulfides are found in the sediments making the modern porous aquifer, in the study area with a deduced sulfur isotope composition of about -12 per mil vs. VCDT. These findings coupled to enhanced loads in dissolved iron and manganese, but low nutrient concentrations indicate nitrate as an important driver for lithoautothrophic pyrite oxidation. At several sites, the enhanced sulfate loads led to dissimilatory sulfate reduction and, thereby, to in-situ transformation of DOC (and/or Methane) to DIC. The enhancements of sulfate and DIC seems to be a typical feature in North German younger groundwaters and strongly (in)directly impacted by anthropogenic forces.</p>