Sedimentary microplastics in the river-to-ocean continuum in Southeast Asia: spatial distribution, particle characteristics, and ecological risk.

This study provides a comprehensive analysis of microplastics (MPs, 13-5000 μm) in superficial sediments from three major Vietnamese river systems - the Red River (RR, 9 stations, 240 km from Hanoi to Ba Lat estuary) in rainy seasons 2023 and 2024; Saigon River (SG, 16 stations, 75 km from Ho Chi Minh city to the East Sea), and Mekong River (MK, 13 stations, 210 km from Can Tho city to the East Sea) in rainy season 2024 - covering downstream to estuarine areas. MPs were quantified and characterized by polymer type and particle size to further evaluate associated chemical hazard. MP abundances ranged from 189 to 8069 items.kg-1 d.w. in RR, 2075 to 32,151 items.kg-1 d.w. in SG, and 1394 to 15,216 items.kg-1 d.w. in MK, corresponding to 1.2 ± 0.4 to 132 ± 42, 2.7 ± 0.9 to 254 ± 81, and 0.3 ± 0.1 to 165 ± 53 mg.kg-1 d.w. in mass, respectively. The highest values were observed near major urban cities (Hanoi, Ho Chi Minh City, and Can Tho), reflecting strong anthropogenic inputs, while elevated loads at estuaries indicate sedimentary sinks. Small particles (13-500 μm) dominated across all sites (>90%), with finer fractions more abundant toward marine zones. The main polymers were polyethylene, polypropylene, polyethylene terephthalate, and polyvinyl chloride, representing 82.4%, 78.3%, and 87.5% of total MPs in RR, SG, and MK, respectively. Risk assessment across the rivers using the Pollution Load Index, Polymer Hazard Index, and Potential Ecological Risk Index indicated screening-level potential chemical-hazard risk ranging from moderate to dangerous. These results establish an initial rainy-season survey for MP contamination in Vietnamese rivers, underline the urgent need for integrated waste management and mitigation targeting hazardous polymers, and provide crucial insights for the Association of Southeast Asian Nations (ASEAN) countries facing accelerating plastic production, consumption, and environmental exposure.