Expectations versus reality: flood-driven microplastic distribution in water and sediments of the Arno River (Italy)

Microplastic contamination in riverine environments has become a global concern. Yet, the fate of microplastics associated with flood events, in terms of microplastic particles suspended in water and those accumulated in riverbed deposits, remains poorly constrained. This study examines microplastic dynamics in the Arno River (Italy) during a flood in early 2024, combining high-frequency water sampling throughout the event with systematic post-flood sediment analyses from the river spring to its mouth. Microplastics were detected in all water samples, with concentrations ranging from 46 to 410 (mean ± SD: 201 ± 105) items/L, values comparable to or slightly higher than those reported for similar rivers worldwide. Microplastic concentration in sediments accumulated along the river was highly heterogeneous, ranging from 0.68 to 11.50 (2.15 ± 2.01) items/g, and showed no consistent downstream increase or correlation with urban areas, tributaries, or population density. Temporal variations in waterborne microplastics also exhibited irregular peaks, only partly related to variations in discharge and suspended sediment load that occurred during the flood. These results suggest that spatial and temporal microplastic distributions are strongly influenced by local hydrodynamics and sediment transport processes. Microplastic mobilisation, deposition, and downstream transport during river floods are, therefore, highly dynamic and site-specific, indicating that downstream accumulation does not necessarily occur as a gradual and predictable process. Overall, our findings underscore the importance of event-based monitoring and extensive sampling for better prediction of microplastic fluxes and their ecological implications in fluvial systems.