CO2 agility in hardwater creeks discharging to the southern Baltic Sea: Seasonal source and fate controls

Mineral formation from hard water creeks is sensitive to variations of physico-chemical boundary conditions, biological factors, and climate change. Groundwaters are often equilibrated with aquifer CaCO₃, but supersaturated in CO₂ wrt. to the atmosphere emerge from springs near the German coastline. Upon contact with the Earth’s atmosphere, CO₂ starts to degas, absorbs O₂, and after exceeding a critical threshold carbonate starts to precipitate. Ferruginous groundwaters may be dominated by the precipitation of iron oxyhydroxides. Trace element and stable isotope partitioning upon crystallization are controlled by non-equilibrium processes. The spring composition characterizes element sources and subterrestrial water-microbe-rock interaction related to subterrestrial weathering. The spatial gradients display exchange with the atmosphere and mineral precipitation, controlled by morphology and season. The hardwater creeks form a source for carbon and will finally contribute to the coastal waters, modulating their buffer capacity, and, therefore, impact surface water acidification and their CO₂ release potential.

Examples for hard water systems were chosen from different aquifer lithologies at Gespensterwald, Rügen Island, and Meschendorf beach.

Besides in-situ parameters, the hydrogeochemistry (major and trace elements) and multiple stable isotope partitioning between dissolved and solid phase were analyzed. The aqueous solution was subjected to physicochemical PHREEQC analysis. Carbonate or iron oxyhydroxide precipitation and CO₂ degassing rates, are estimated based on different methods:

            - Hydrochemical gradients along the flow path

            - Direct measurements using a new in-situ placer technique (Peters, MSc thesis 2025)

- Comparison of field findings with lab-based trace element partitioning into the solid