Microplastic discharge and other anthropogenic pollution in urban runoff

Runoff waters pose a significant pathway for plastics and other anthropogenic material to the local aquatic systems. We studied microplastic fluxes of the runoff systems in Kuopio, Finland, using fraction filtration and imaging FPA-FTIR spectroscopy. We also studied metals, nutrients, solid matter, and flowrate in the runoff. We studied in total of four sites around the city including snow dump site, city center, suburb and a combination of park, city center and suburb. Sampled volumes for microplastics were 50 L in two replicates when possible. Size of analyzed microplastics ranged from 13 to 4164 μm, with an average ± standard deviation size of 185 ± 215 μm. Microplastic concentrations varied greatly depending on rain conditions, ranging from 0.6 to 46 MPs L^-1 per replicate. Average ± standard deviation of microplastic in all samples combined was 9.6 ± 12.1 MPs L^-1. Correspondingly, we estimated microplastic discharge to the environment to be as high as 23.4 × 10⁶ particles and 62.5 g per rain event. The most common polymer types in runoff were polypropylene, polyethylene and polystyrene and the distribution of these polymers varied considerably. Microplastic particle concentrations showed strong Spearman correlation with nickel, and moderate correlation with zinc and lead. Furthermore, microplastic mass concentration had strong correlation with solid matter, total phosphorus, nickel, copper, zinc and lead. This study shows that urban runoff has a relatively high concentration of microplastics, and that plastics are released alongside other matter from the catchment areas. Also, snow dump site acts as a constant source of microplastics throughout the summer regardless of rain events. Our results highlight the importance of runoff water management in urban areas, as the discharge of anthropogenic litter to nearby aquatic environments is prominent.