Stormwater bioretention: Pollutant occurrence and accumulation in filter materials and forebays

Urban areas are affected by anthropogenic activities and produce pollutants that are transported to recipients and receiving waters during precipitation. Untreated stormwater runoff is a main driver of environmental degradation, and the interest in stormwater quality treatment has increased with the awareness of stormwater pollution. Several pollutants, including metals PAHs, PCBs, phthalates, and phenols, pose a hazard to aquatic life and are listed among the 45 priority substances in The European Water Framework Directive list (Directive 2013/39/EU). Stormwater bioretention technology is employed to efficiently remove pollutants during stormwater treatment in urban areas. However, the resulting accumulation of pollutants in bioretention facilities could ultimately create a pollutant depot. Hence, it is important to understand the occurrence, availability, and mobility of pollutants in bioretention facilities, as well as the processes that control their accumulation and mobility over time. The aim of the work in this licentiate thesis was to investigate the occurrence, accumulation, distribution, and concentration of organic micropollutants and metals inbioretention facilities after long-term stormwater treatment in urban areas of Ohio, Michigan, and Kentucky in November 2019. Samples were collected from 29 bioretention facilities, 20 of which were equipped with forebays. A total of 269 samples were analysed for metals commonly found in stormwater (Cr, Cu, Ni, Pb, and Zn), and a five-step sequential extraction method was used to assess the metal mobility in the filter material. Additionally, 116 samples from 12 sites were analysed for 38 organic micropollutants (OMPs), including 16 PAHs, 7 PCBs, 13 phthalates, and 2 alkylphenols. All studied metals were found in all samples, except for Cd, which was detected in 245 samples (91%). For the OMPs, 32 of 38 analytes were detected in at least one sample. PAHs and PCBs were the most frequently detected pollutants, and were found in 12 and 10 sites, respectively. The bioretention sites showed large variations in the concentrations of pollutants. The concentration of OMPs was highest in the upper 10 cm of the filter material and decreased with increasing depth, while a similar, but less obvious trend was observed for Cr, Cu, Pb, and Zn. A trend of decreasing concentration with increasing distance from the inlet was observed for OMPs but was less clear for the metals. Thefore bays, which contain sediment but no filter material, had the highest concentrations of OMPs. A strong correlation was observed between the pollutant concentrations and the ratio between the filter area and catchment area in a Principle Component Analysis. The accumulation and pathways of particle-bound OMPs and particle-bound metals showed similarities. These results will assist with improving the methods used for stormwater management. Importantly, regular replacement of the top filter layer and regular forebay maintenance may prevent pollutant accumulation and clogging, thereby extending the filters’ treatment function