Assessment of microplastic transport and distribution in the urban environment of Coimbra municipality

Microplastics (MPs) largely originate from terrestrial environments, especially urban areas, where controlling their spread is difficult. This is concerning due to MPs' impacts on ecosystems and human health. To develop mitigation strategies, understanding how MPs move and spread is essential. This study examined MP transport and distribution in Coimbra municipality, Portugal, at a small watershed scale. Samples were collected from dry and wet atmospheric deposition, runoff, and streams, both before and during rainfall events, across five watersheds with varying urbanization and sizes. MPs were extracted by density separation, counted under a stereo microscope, and identified using μ-FTIR. Results showed higher MP fluxes in wet deposition (235 ± 231 p•m^-2•day^-1) than dry deposition (70 ± 96 p•m^-2•day^-1). Stream concentrations nearly doubled during rainfall (59 ± 83 p•L^-1) compared to before rainfall (39 ± 21 p•L^-1). Wet deposition (41 ± 34 p•L^-1) generally had slightly higher MP levels than runoff (38 ± 29 p•L^-1) or streams before rainfall, though differences were not statistically significant. Correlations indicated that stream MP levels tended to increase in more urbanized, smaller watersheds, while longer dry periods before rainfall were associated with increased MP levels in wet deposition. Land use influenced MP runoff, with higher concentrations from residential areas, and dry deposition was elevated near a hospital. The most common MPs found were polyethylene, polypropylene, and rubber (5-250 μm). Findings highlight MPs' widespread transport through the environment and emphasize the need for monitoring pathways. Urban littering and tire wear should receive particular attention. Overall, urgent measures are needed to control plastic production, usage, and environmental dispersion of MPs.