Hydrodynamics of terrestrial nano- and microplastics: simulating seasonal retention and first-flush emissions

Several estimations of global plastic emissions exist; however, the photodegradation of plastic litter and the complex hydrological processes on land have not been adequately integrated into these models. This study simulates the generation and emission of nano- and microplastic (NMP) particles using the Catchment-based Macro-scale Floodplain (CaMa-Flood) model to elucidate their hydrodynamics.First, a photodegradation model for plastic litter was established through accelerated ultraviolet (UV) weathering tests under dry and wet conditions. The mass of plastic litter decreased linearly with the cumulative UV irradiation dose, with wet conditions exhibiting a lower mass decay rate than dry conditions. By incorporating these linear relationships and the ratio of rainy days, we estimated the NMP generation rate across Japan. Subsequently, NMP emissions were diagnosed using a new scheme implemented within the CaMa-Flood model and validated against microplastic observation data from 177 sites. The simulated concentrations showed strong consistency with observed data.Notably, our simulation focuses on the seasonal variations in the amount of plastic retained on land. We identified a distinct "first flush" effect during the rising stages of floods and observed how spatial distributions of surface runoff influence NMP transport. These results demonstrate that the CaMa-Flood model is a robust tool for understanding the terrestrial hydrodynamics of NMP particles and estimating plastic fluxes. This framework provides a basis for future global-scale estimations to identify NMP accumulation hotspots driven by hydrodynamic processes.