Terrestrial inputs can alter total alkalinity (TA) and dissolved inorganic carbon (DIC) of the coastal ocean and modify seawater pH. Here, we first characterize the carbonate system in river and groundwater draining to the Baltic Sea using observations of TA, DIC, δ 13 C-DIC, and major ions across 6 countries and 17 beaches. We then assess whether submarine groundwater discharge (SGD) may impact coastal acidification. TA and D...