Metal pathways in stormwater treatment systems

Sammanfattning: Heavy metals in urban stormwater runoff affect the ecological status of receiving surface waters. To avoid this, various stormwater treatment systems have been developed and implemented during the last 50 years. One of themost common and continuously popular systems is wet stormwater ponds. Another and relatively new type of system is stormwater biofilters. In wet stormwater ponds the function and design with respect to both waterquantity and water quality has been comprehensively addressed in scientific studies. Concerns today are more related to the fact that they serve as aquatic and wildlife habitats thereby posing a possible ecological risk. Incontrast, for stormwater biofilters, a knowledge gap regarding function and factors affecting their performance still exists.Understanding metal pathways in these two systems can support development of future stormwater management by identifying ways to improve the treatment of stormwater discharges and minimize risks of metals spreadingfrom treatment facilities to the natural environment. Thus, in this thesis metal pathways in wet stormwater ponds and stormwater biofilters were investigated in laboratory studies and field experiments to evaluate metal accu-mulation in different compartments of the two systems. For wet stormwater ponds the primary issue addressed in focus was their risk of affecting the wider environment whereas for stormwater biofilters was about their capaci-ty to improve stormwater runoff discharge. The study was performed by sampling resident fauna in wet stormwater ponds to evaluate bioaccumulation and biomagnification in wet stormwater ponds; sediment was sampled to estimate pollutant loads they receive; and mussel biomarkers were used to assess bioaccumulation over time in them. In addition, a laboratory study wasconducted to assess the reliability of mussels as biomarkers. To assess metal removal pathways and efficiency in stormwater biofilters, heavy metal concentrations were measured in the influent and effluent, as well as in thefilter material and the roots and shoots of stormwater biofilter vegetation.Mussel biomarkers were not confirmed to be a reliable method to assess the ecological state of wet stormwater ponds since they did not reflect the overall bioaccumulation in them. However, the use of resident fauna seemspromising for this purpose since the metal pollution load received by the wet stormwater ponds was reflected in the resident fauna. The results of this study indicated that despite being continuously loaded with heavy metalswet stormwater ponds have the ability to provide high biodiversity and thus ecosystem services without critically contaminating the resident fauna. However, with time wet stormwater ponds will likely end up constituting anecological risk to the surrounding environment.The investigated stormwater biofilters showed an overall efficient metal removal. The retained metals were trapped in the top layer of the filter material and in the roots and shoots of the biofilter vegetation. However, saltwas found to have a negative effect on stormwater biofilter performance inducing leaching of dissolved Cu and Pb and impeding plant metal uptake. Temperature variation did not generally affect metal removal but low tem-peratures caused a higher plant metal uptake than high temperatures. The presence of a submerged zone had a positive effect on stormwater biofilters, improving the overall metal removal from stormwater and plant metaluptake as well as plant health. By drawing parallels between the two studied systems, wet stormwater ponds will most likely be negativelyaffected by wintry conditions, constituting a risk for receiving waters at the same time as they are more likely to pose an ecological risk to the wider environment than stormwater biofilters. Use of stormwater biofilters wherepossible is therefore recommended, to improve future urban stormwater management.