Sources and fate of dissolved sulphate, carbonate, and nitrate in groundwater of the temperate climate zone: a high-resolution multi-isotope (H, C, O, S) study in north-eastern Germany**

Different natural and anthropogenic drivers impact the groundwater in the catchment area of the southern Baltic Sea, north-eastern Germany. To understand the sources and fate of dissolved sulphate, carbonate, and nitrate on a regional scale, in the present study, the hydrogeochemical and multi-stable isotope (H, C, O, S) composition of groundwater samples from up to more than 300 sites (depths from near-surface down to 291 m) was studied. To investigate the element sources and the water–rock–microbe interaction processes that took place along the groundwater flow path, a mass balance approach is combined with physico-chemical modelling. Microbial oxidation of pyrite using nitrate as electron acceptor and a superimposition by dissimilatory sulphate reduction at depth is shown in a drilled vertical profile at one site. This trend frames the behaviour of sulphate at many investigated groundwater wells. Dissolved inorganic carbon (DIC) in the groundwater was found to be controlled by the uptake of biogenic carbon dioxide, the dissolution of carbonate minerals, the <i>in situ</i> oxidation of DOC and, at a few sites, the formation and/or oxidation of biogenic methane. Enhanced groundwater DIC loads may potentially increase future CO₂ degassing to the atmosphere upon release of groundwaters to the surface. These results form a comprehensive base for understanding the present situation and for future investigations.