Processes influencing stormwater purification by roadside biochar-amended sand filter in cold climate conditions

Stormwater in Finland typically lacks substantial treatment despite potentially containing elevated concentrations of nutrients and trace metals that can degrade the water quality of receiving waterbodies. Reactive materials, such as biochar, have been effective in removing pollutants from stormwater. However, knowledge of their performance is primarily based upon laboratory tests as field studies remain scarce and there is little evidence on their long-term field performance. This study evaluated field-scale stormwater pollutant retention by sand and biochar-amended sand filtration systems designed to treat road runoff containing metals and nutrients in southern Finland. Results of field data, and hydrological and geochemical modelling suggest that the filters retain pollutants similarly through a combination of physical filtration, (ad)sorption and/or (co)precipitation additionally affected by leaching of alkali metal cations from biochar media which contributed to the formation of new minerals via PO43- sorption to Fe (oxy)hydroxides or precipitation of Ca phosphate mineral phases (hydroxyapatite).