Assessment of Microplastic Contamination in the Surface water of the Bagmati River, Kathmandu Valley

Microplastic (MP) contamination has recently become a growing environmental concern; however, scientific understanding of its occurrence in freshwater systems of Bagmati River remains limited. Recognizing this gap in knowledge, the present study was conducted to assess the abundance, distribution, and characteristics of MP in the Bagmati River and key water quality parameters, including pH, electrical conductivity, total dissolved solids, and turbidity. Fifteen sampling sites were selected across upstream, midstream, and downstream zones based on human activity levels. Water samples were digested using hydrogen peroxide and ferrous sulfate, followed by density separation. MP were identified under a stereomicroscope, and polymer types were confirmed using Fourier Transform Infrared (FTIR) spectroscopy. MP concentrations ranged from approximately 2.0 to 129 MP/L, with an average of 29 MP/L. Blue-colored MP (66%) were most dominant, followed by red (22%), while fibers (94%) overwhelmingly prevailed over fragments (6%). Polyethylene (PE) (70%) and polypropylene (PP) (30%) were the major polymer types identified. Statistical analysis using the Kruskal–Wallis test with post-hoc pairwise comparisons revealed significantly higher MP abundance in the midstream than the upstream segment (p = 0.044), with an increasing trend downstream. The predominance of fibers and the spatial pattern show that urban runoff, untreated wastewater, and mismanaged solid waste are major MP sources. The presence of MP across all sites shows widespread urban-origin contamination and potential ecological and human health risks. Strengthening wastewater treatment, waste segregation, and public awareness is crucial to mitigate MP pollution and safeguard the ecological integrity and socio-cultural values of the Bagmati River system.