Yields and stormwater loads of microplastics in five urban stormwater catchments (City of Kitchener, Ontario, Canada)

Urban stormwater runoff is a major pathway for microplastic (MP) pollution to aquatic environments. In this study, MPs from five stormwater catchments (SWCs) in the City of Kitchener, Ontario, Canada, were analyzed in water and sediment samples collected at their receiving stormwater ponds (SWPs). We integrated MP characterization targeting particles between 20 and 500 µm using a Laser Direct Infrared (LDIR) system with hydrological modeling, measured sediment accumulation rates, and particle to mass conversion calculations to quantify MP concentrations and loads. The LDIR data included polymer type, size class, and particle shape. Inflow concentrations to the SWPs ranged from 4.9 × 10⁶ to 4 × 10⁷ particles m^-3 and from 5.8 × 10^-2 to 0.9 g m^-3, yielding annual loads of 9.7 × 10¹¹ to 3.5 × 10¹² particles yr^-1 and 1.1 × 10⁴ to 8 × 10⁴ g yr^-1. Per unit impervious surface area MP yields fell in the range 3.6 × 10¹⁰ to 2.1 × 10¹¹ particles ha^-1 yr^-1 and 4.2 × 10² to 4.9 × 10³ g ha^-1 yr^-1, while normalized to the total SWC area produced 1.6 × 10¹⁰-1.4 × 10¹¹ particles ha^-1 yr^-1 and 1.9 × 10²-3.2 × 10³ g ha^-1 yr^-1. Fragments were the dominant morphology (94-98% by count; 83-91% by mass). MPs smaller than 100 µm comprised 92.6-97.3% of particle counts but only 32.1-60.8% of mass, whereas MPs larger than 500 µm accounted for 0.2-0.8% of particle counts but 5.9-33.9% of mass. Yields showed no systematic differences across broad land-use categories and no correlation with total population. Instead, positive associations were observed with residential population density and with SWC imperviousness, including strong correlations with rubber and several polymer-specific yields. Annual average daily traffic was not related to total MP loads from SWCs but aligned with the relative abundances of specific polymers, particularly polystyrene and polycarbonate. Yields of rubber particles correlated with overall MP yields, reinforcing the role of road traffic as a source. Road and mixed-impervious cover considered independently were not significant predictors, but their ratio explained the relative yields of fragments, fibers, and selected polymers.