Occurrence and distribution of plastic particles (10-25,000 μm) and microfibers in the surface water of an urban river network in Japan

Abstract

Urban rivers remain the key conduits conveying land-sourced plastics into the ocean. However, detailed information is limited on the concurrent evaluation over a wide array of particle size-specific abundances, characteristics, and distribution patterns of plastics in riverine environments. Therefore, this study provides a comprehensive assessment of plastic pollution in an urban river network in Japan by analyzing mesoplastics (5000-25,000 μm), large microplastics (300-5000 μm), small microplastics (SMPs, 10-300 μm), and microplastic-fibers (MPFs, 10-5000 μm) concurrently, for the first time. Sampling was conducted at seven stations in the Kamo and Katsura Rivers flowing across metropolitan Kyoto City. The analytical procedures involved infrared spectroscopy and fluorescence-staining microscopy. The concentrations of plastics were moderate compared to the global reports and gradually increased along the river flow (3550-15,840 items/m³; 180-13,180 μg/m³), mostly due to urban discharges via non-point sources. The number concentrations increased with decreasing particle size, marking 99.94% of SMPs, including 50% smaller than 40 μm. Conversely, mass concentrations decreased, exhibiting 96% larger than 1000 μm (64% mesoplastics including 20% around 5000 μm), along with 2% SMPs. Polyethylene (PE) and polyvinyl alcohol were distinct among SMPs, with PE indicating higher susceptibility to fragmentation compared to polypropylene and other polymer types. MPF concentrations were homogeneous throughout the watershed (1470-3600 items/m³; 520-1060 μg/m³), with a higher proportion of fibers smaller than 1000 μm (86%), apparently originating from polyethylene terephthalate/nylon/acrylic-like textile fibers. The proportion of MPFs surpassed particles within 100-3000 μm and was considerably high around 300 μm (> 98%). The river network of Kyoto conveys billions of tiny microplastics to the Yodo River, the primary water resource downstream, within a dry day.

Keywords: Japanese rivers; Microplastic pollution; Particle size; Spatial dynamics; Synthetic fibers; Urban watershed.