Distribution and characteristics of microplastics in fluvial sediments from the Koshi River Basin, Nepal

Microplastics (MPs) are emerging contaminants found in various ecosystems including oceans, lakes, rivers, sediment, air, and soil. Mapping of MPs in different deposition zones in fresh water sediment is important to identify their potential sources, sink, and transport mechanism. In this study, MPs were analyzed in sediment samples from Arun, Tamor, and Koshi Rivers in eastern Nepal. A total of 78 samples from 26 sites were collected from three independent deposition regions i.e., recent deposition (R0), recent past deposition (R1), and past deposition (R2) during monsoon season in 2023. All samples were analyzed following standard methods involving drying, peroxidation, density separation, microscopic examination and chemical identification by FTIR. In all the river basins differences in MPs count, color, and morphology were observed in three deposition regions. In Koshi basin the MPs count ranged from 7016–8876 MPKg^–1, 8396–10596 MPKg^–1, and 9416–9816 MPKg^–1 in R2, R1 and R0 regions, respectively. The mean abundance was found to be higher in downstream especially in Koshi River. The predominant shapes, sizes, and colors found in all three river basins were fragment (52.5 %), 20–100 μm (58.86 %), and black (33.76 %). The particles were identified as polyamide, polypropylene, polyvinyl chloride, polysulfone, nylon, and polyether ether ketone. The pollution risk assessment indicated minimal MPs contamination upstream and moderate contamination downstream. Finally, principal component analysis (PCA) and land use and land cover change (LULC) data were utilized to identify the potential sources of MPs. Agricultural and anthropogenic sources were identified as major contributors to the MPs load. This study provides baseline data for MP concentrations and their potential sources in Arun, Tamor, and Koshi river sediments. These insights could be important for future MPs mitigation strategies.