A large-scale lysimeter study of stormwater biofiltration under cold climatic conditions

Stormwater biofiltration was studied at a large-scale lysimeter facility in southern Finland. Biofiltration systems, constructed within eight lysimeters, consisted of an organic topsoil, a sand layer and three gravel layers of varying gravel size underneath. Six of the lysimeter systems were vegetated with Phalaris arundinacea, Salix glauca var. callicarpaea and Lythrum salicaria, and two systems were left unplanted. The effect of vegetation on the functioning of the systems was studied by comparing the vegetated with the non-vegetated systems. The systems were irrigated during three seasons with artificial stormwater containing nitrate, phosphate, zinc, copper and aluminium to study pollutant retention and water infiltration during warm and cold temperatures. Half of the systems also received road salt (NaCl) to study its effect on the functioning of the systems. Phosphate, zinc and copper were well retained (81-98%) during each season in each system. Nitrate retention improved (from 0 to 47%) with time in the vegetated system but negative retention was also observed during some irrigation events, indicating that nitrate also leached from the parent soil. The retention of aluminium in the unsalted biofilters decreased (from ca. 80 to 0%) during the snowmelt season in spring and was highly negative in the salted systems during spring and summer. Salt also slightly diminished (ca. 8%) the retention of Cu. Vegetation only improved the retention of nitrate. However, the non-vegetated systems infiltrated water poorly while the vegetated systems infiltrated large volumes and intensities of water one year after biofilter establishment. Hence, vegetation was necessary for operational infiltration. The study showed positive results for stormwater biofiltration in cold climates. Nonetheless, long-term studies with varying soil types and pollutants are recommended.