Distribution of Microplastic Contamination in Sapta-Gandaki River System, Nepal

Microplastic (MP) contamination has been reported in many Rivers worldwide. However, there is an increasing concern regarding data quality, particularly in the studies that do not account for positive and negative controls. Additionally, spatiotemporal distribution of MP in transboundary Himalayan River is underexplored. Here, we report spatiotemporal distribution of MP in the second largest river of Nepal; Sapta-Gandaki River system which is 810 km long starting from Himalayan headstream to the Ganges with a catchment area of 46,300 km^2. A total of 120 integrated water samples were collected in pre and post monsoons from 30 sites (2850-140 masl) along three tributaries of Saptagandaki River. The MP data were corrected for procedural blanks (n=23) and positive controls (n=18). We found that the MPs count (cut off size ≥30μm) in pre (dry) monsoon time was significantly higher (61.2±27.8 MP/L, p<0.01) than in post monsoon (winter) time (24.7±10.8 MP/L). High count was observed in the sites near major cities and highways. A gradual increase in MPs count was observed as the River stretches up to downstream (r=-0.6). The shape, size, and color dominance were fragments>pellets>fibers, 30-100>100-250>250-500>500-5000µm, blue>black>transparent; respectively. Most MP particles consisted of polyethylene terephthalate, cellophane, polyethylene, polyvinyl chloride type material. Annual flux discharge calculation showed that Saptagandaki River discharges 0.7×10^8 MP/s. The findings of this study provide baseline data for MPs contamination in one of the major Himalayan River water systems of Nepal and the data could be useful to identify potential control measures.