Occurrence, distribution, and possible sources of microplastics in the surface river water in the Arakawa River watershed

Nowadays, efforts for complementing data concerning microplastics (MPs) in freshwater systems are required as MPs exist in many populated areas. The goal of this study is to investigate the distribution and profiles of riverine MPs along the Arakawa River watershed, which runs through the Tokyo Metropolitan area. The MPs were found in 10 of the 12 sampling sites in the watershed with the mean of 1.8 pieces/m. Also, the spatial distribution of the MPs displayed the accumulation in the downstream and in the tributary areas with high populations, reflecting the levels of the local anthropogenic activities. In contrast to the heterogeneity of the floating concentrations, polymer type compositions were consistent with the predominance of polyethylene compared with polypropylene and polystyrene. Moreover, the size distributions of the particles were consistent among samples with the predominance of the relatively smaller size fractions. These results suggest that the sources of fragmented plastic debris are likely spread over terrestrial areas and that reducing burden from these land-based MPs is necessary for mitigating MPs pollution in urban aquatic environments.