Boreal peatland forestry dominated catchment iron leaching and the possibilities for mitigating it

Iron (Fe) concentrations in boreal surface waters have increased since the 1990s. This phenomenon is driven by land use, particularly in black schist, acid sulfate soil and peatland areas. This increase has impaired water quality. We developed a catchment scale approach to improve identification of areas where Fe leaching occurs. High-resolution surface runoff modelling combined with spatial sampling proved to be a good method for identifying the main Fe leaching areas. In the Jäälinjärvi catchment, Finland, spatial analysis showed that Fe leaching is linked to peatland drainage and black schist zones. Undrained peatlands areas had measurably smaller concentrations, indicating their potential to buffer Fe leaching. We also assessed Fe retention efficiency in various nature-based water treatment solutions (constructed wetland (CW), settling basin (SB), vortex settling basin (VSB) and wood bundles (WB)) and examined how water properties influence Fe processes. Fe retention was generally weak. Only 3.5 ha SB showed significant removal (total 14 %, dissolved 33 %). Weak Fe removal may be due to short retention and contact time in water treatment solutions as most Fe was in a dissolved form. Additionally, water chemistry can have a major impact in sites with a low pH (5.90–6.16), which inhibits Fe oxidation. Low redox potential (ORP < 100 mV) indicated a possible microbial reduction of Fe(III). Current water treatment solutions offer limited Fe retention under low-pH and humus-rich conditions, which highlights the importance of preventing iron leaching by minimizing land use activities such as peatland drainage. • Peatland drainage and the presence of black schist promote iron leaching. • Nature-based water treatment solutions may be ineffective at removing iron. • Iron removal from humus-rich waters requires a long residence time. • Unnecessary soil disturbance should be avoided in high Fe-mobilization risk areas.