Distribution of microplastic contamination in Sapta-Gandaki river system, Nepal

Microplastic (MP) contamination has been reported in many rivers worldwide. However, the distribution of MP in transboundary Himalayan river is underexplored. This study reports the MP contamination in the Sapta-Gandaki river system of Nepal; a river with an 810 km stretch from the Himalayan headstream to the Ganges. A total of 120 integrated and independent fresh water samples from 30 sites ranging from 2850 to 140 m above sea level along the length of river were collected. MPs larger than 30 μm were extracted from water samples using standard methods of organic digestion, density separation, and filtration. They were then viewed in stereomicroscope and identified using FTIR spectroscopy. The average MP count in pre- and post-monsoon was found to be 61.2 ± 27.8 and 24.7 ± 10.8 particles per liter (MPL⁻¹); respectively. Interestingly, a gradual increase in MPs count was observed across the sampling sites from upstream to downstream. The shape, size, and color dominance were also determined. The MP particles were identified to be polyethylene, poly-ethylene terephthalate, polyvinyl chloride, and cellophane. The average annual flux rate for MP was estimated to be 0.7 × 10⁸ microplastics per second (MP/s). The findings of this study provide baseline data for MPs contamination in one of the major Himalayan river water systems of Nepal that could be useful to identify potential control measures of microplastic pollution in the river water systems.