Microplastic Emissions and Retention in Urban Catchments and Stormwater Ponds

The rapid growth in plastic production and mismanagement of plastic waste streams have raised environmental concerns, with microplastics (MPs) emerging as pervasive pollutants. This study quantifies stormwater MP exports from urban areas by examining five stormwater management ponds (SWPs) and their representative catchments in Kitchener, Ontario, Canada, using field sampling, laboratory extraction, and modeling approaches. Using Laser Direct Infrared (LDIR) spectroscopy, MP concentrations were determined for different MP shapes and polymer compositions, enabling the calculation of both particle- and mass-based fluxes. A hydrology model coupled with a mass balance approach was employed to estimate MP emission factors (i.e. export coefficients) and retention efficiencies in both particle- and mass-based units. Land use impacts were examined by classifying stormwater catchments through machine learning-aided analysis of aerial imagery. Sediment emissions were also quantified through surveys and samplings to explore potential correlations with MP exports. Industrial catchments showed the highest MP emission factor at 8.7×10¹¹ particles ha^-1 year^-1 (19.6 kg ha^-1 year^-1), whereas residential areas exhibited the lowest emissions at 1.7×10¹¹ particles ha^-1 year^-1 (2.3 kg ha^-1 year^-1). Fibrous MPs accounted for 2–6% of particle-based emissions but 10–24% by mass, highlighting differences in composition across land use types. Parking lots and traffic were key contributors to MP pollution, consistent with polymer composition analysis. SWP retention efficiencies ranged from 73–97% for total loads but varied for specific polymers, from minimal to complete retention. Retention performance was influenced by SWP design features such as inlet and outlet configurations, catchment wash-off dynamics, and hydraulic residence time. These findings emphasize the critical role of land use and SWP design in urban stormwater MP mitigation, with industrial and high-traffic areas contributing significantly to pollution. Understanding these dynamics provides actionable insights for mitigating MP emissions and optimizing SWP retention performance to protect aquatic ecosystems.