Treating urban stormwater in constructed wetlands in Kalmar, SE Sweden, for improved water quality and biodiversity

Stormwaters’ heavy loads of nutrients, metals and oil etc, via urban ditches / streams, are increasingly treated in engineered shallow wetlands/ponds. Such approaches are still only remedying a minor part of affected storm waters, urban streams, and freshwater and marine recipients. In Kalmar, a middle-sized Swedish town at the Baltic Sea, > 10 such wetlands / ponds are established for reduction of such heavily polluted waters. Aiming at determining the effectiveness in reduction of nutrients and metals, and the facilitation of the aquatic biodiversity (primarily in the wetlands), monitoring programs were initiated. Regular in/outflow water sampling, water flow and biology, were followed for 1-2-3 years (varied with parameter) after construction in two of these wetlands. Skälby Dämme, constructed 14 years ago (2000), showed ca 40 % nutrient reduction figures, less for TSS and metals, while Hagbygärde Dämme, constructed two years ago (2012), attained only much lower results. Reasons for this difference might be that Hagbygärde is less surrounded by forested land, also that less than one year had elapsed before evaluation commenced, while two-three years in Skälby. More recent monitoring in Skälby Dämme is also compared with. Moreover, this kind of wetlands/ponds is shown to favour the biodiversity of benthic invertebrates, (aquatic) plants and birds, although with trends to decline in species richness some years after the initial colonization period