Hydrogeochemical Signature of Groundwater-Influenced Sloping Fen in the Calcareous Rich Northern Boreal Environment, Finland

Groundwater-influenced, nutrient-rich sloping fens are characteristic of northeastern Finland, where they are affected by both groundwater discharge and surface water inputs, including spring floods. This study examines the geochemical composition of peat pore water in Puukkosuo, a sloping fen underlain by carbonate bedrock, with two objectives: (1) to evaluate the influence of groundwater discharge on peat hydrogeochemistry, and (2) to compare hydrochemical and geochemical signatures with those of northern boreal peatlands underlain by different bedrock types.

The 3D structure of the peatland was modeled to understand flow paths within the Peatland basin (Åberg et al. 2025). Four peat cores were collected to characterize peat composition, humification, and geochemistry, alongside pore water samples from the same locations (Erhovaara 2023). Porewater samples from three locations from different depths were analyzed for trace elements, major ion composition, pH, EC, TOC, TC, IC, NO₃+NO₂, NH₄, Tot N, Tot P, stable isotopic composition of water and δ¹³C (DIC ) to assess groundwater and bedrock influence. Additional surface and groundwater samples were analysed for trace elements and majos ion composition, pH and EC as a reference.  

High Ca and Mg concentrations, as well as high pH and EC in pore water indicate strong groundwater input, while elevated nutrient levels observed at the western edge of Puukkosuo primarily reflect surface runoff. Although the broader catchment lacks extensive carbonate deposits, the presence of carbonate bedrock beneath Puukkosuo, combined with its sloping morphology, demonstrates that groundwater flow paths significantly shape the hydrogeochemical conditions of the fen.

Erhovaara, S., 2023. Carbon accumulation and peat geochemistry in the Puukkosuo fen during the Holocene. MSc. thesis, University of Helsinki. 45 p.

Åberg, A., Nurmilaukas, O., Korkka-Niemi, K. & Kultti, S., 2025. Kuusamossa sijaitsevan Puukkosuon maatutkaluotaukseen perustuva 3D-mallinnus. Geological Survey of Finland. GTK Open File Work Report 40/2025. 24 p.