Microplastic pollution in urban drainage systems discharging into the alpine lakes of Como and Lugano, Italy

Urban stormwater outfalls and combined sewer overflows (CSOs) represent major sources of MPs pollution in receiving water bodies. This study presents an experimental and numerical investigation on MPs pollution from combined sewerage systems and wastewater treatment plants (WWTPs) discharging into two alpine lakes in northern Italy. Spatial and temporal trends of MPs concentrations and loads are investigated in the sewerage under dry-weather conditions and during representative rainfall events in the period September 2022-January 2023. The study allows the identification of key factors which contribute to higher MPs emissions. The acquired waste- and storm-water samples (2 L each) were dried and subjected to chemical oxidation with H₂O₂ before optical microscopy and infrared spectroscopy analysis. Dry-weather sewerage MPs concentration exhibits an average of 17.6 mg/L and standard deviation of 2.1 mg/L in dry-weather, with slightly higher values for the system with the lower per capita water supply. The daily mass of MPs directed to the treatment in the tourist season (i.e., September) is slightly above 50 % of the average daily mass characterizing the subsequent non-tourist months. Wet-weather MPs concentration is relatively lower (average of 12.9 mg/L, standard deviation of 2.0 mg/L), these values depending on the relative importance of the stormwater flow from the directly drained catchment compared to the flow contribution coming from the upstream pumping stations. The outcomes have implications for assessing spatial-temporal exposure of aquatic ecosystems to MPs pollution in urban areas, providing indications on effective management practices and control measures of urban drainage systems against environmental contamination.