Variable impact of submarine groundwater discharge on the patchiness of seafloor biogeochemical conditions

Submarine groundwater discharge (SGD) can be a significant source of nutrients, carbon and other substances to coastal seas, with detrimental effects on the marine ecosystem, such as eutrophication and acidification. In Hanko, in the northern Baltic Sea off Finland, SGD occurs through several small depressions (pockmarks, <25 m wide, <2.5 m deep) on a sandy seafloor slope ca. 200 m from the shoreline at water depths of ca. 11 m. Sediment porewater profiles of Cl^–, δ²H and δ¹⁸O sampled in September 2019 documented a wide range of discharge rates from the pockmarks – from consistent and relatively strong (0.31 cm/day) to moderate (0.02 cm/day) to cessated discharge. Reactive transport modeling showed that groundwater advection in consistent flow-dominated pockmarks forced the key biogeochemical processes and microbial activity (sulphate reduction, methane production) into a few centimetres thick zone below the sediment surface (Purkamo et al., 2022, Geochim. Cosmochim. Acta).

Here we present results from our extensive revisit to the Hanko pockmarks and onshore groundwater observation wells in June 2025. Pockmark sediment samples were collected for bulk geochemical and grain size analyses. Pockmark porewater, overlying water column and groundwater samples from nearby wells were collected for the analysis of a wide range of parameters such as δ²H, δ¹⁸O, δ¹³C_DIC, major nutrients and ions. Water column and groundwater samples were also analysed for Ra and Rn activity, and groundwater samples were analysed for stable and radioactive noble gases. Preliminary results show significant temporal variability in discharge rates and biogeochemical conditions in the pockmarks.

The authors would like to thank the European Commission and the Research Council of Finland, Swedish Research Council FORMAS, Polish Research Council NCBR, Estonian Research Council ETAG (Mobilitas 3.0 programme), Spanish Research Council AEI, and the Israeli Ministry of Energy and Infrastructure for funding in the frame of the collaborative international consortium SecuCoast financed under the 2023 Joint call of the European Partnership 101060874 — Water4All.