Combined sewer overflow contribution to microplastic pollution in urban water bodies

Combined Sewer Overflows (CSOs) have recently gained attention as episodic but significant sources of microplastics (MPs) to urban receiving water bodies; however, the occurrence, magnitude and characteristics of MP pollution in this context remains poorly investigated. Through an in-field monitoring campaign in the city of Pisa (Italy), this research aimed to assess the impact of CSOs to microplastic pollution in urban aquatic environment and to predict MP abundance due to hydraulic overflowing event. MP analysis was conducted using Laser Direct InfraRed Chemical Imaging System (LDIR), an innovative technology which enables reliable quantification and characterization of MPs in terms of polymer composition, size and shape, thus supporting source identification. MP abundance ranged between (3.71 ± 0.16) × 10⁶ and (1.28 ± 0.12) × 10⁸ items/m³, with values increasing with the overflow discharge. A positive correlation was observed between MP abundance and the discharged volume, highlighting the influence of precipitation-driver hydrodynamics. In the six monitored events, we observed a first flush effect, with MP concentrations reaching up to an order of magnitude higher than during subsequent stages of the event. Rubber, cellulosic materials and PMMA were identified as the predominant polymers, underlining the mixed sources related to urban runoff and wastewater inputs. Most of the detected MPs had size between 10 and 100 μm (77-93%), with fragment and pellet as predominant shapes (35-81% and 13-48%, respectively). The results highlighted the importance of including microplastics in CSO monitoring programs and provided insights on hydrological and hydraulic factors relevant to understand urban water pollution.