Distribution and Characteristics of Microplastics in Fluvial Sediments from the Koshi River Basin, Nepal

Abstract

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 Tamor basin the MPs count ranged from 3140-9500 MPKg^-1, 3800-9420 MPKg^-1, and 3180-9700 MPKg^-1 in R2, R1 and R0 regions, respectively. The mean abundance was found 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 provided baseline data for MP concentrations and their potential sources in Arun, Tamor, and Koshi river sediments. This information could be important for future MPs mitigation strategies.