Monthly variation and transport of microplastics from the Soan River into the Indus River

The presence of plastic and microplastic pollution in freshwater systems receives extensive concerns for its accumulative trend and potential ecological impacts. This is the first annual study that investigated the monthly profile of plastic pollution in the mouth of the Soan River. Plastic pollutants comprising microplastic content up to 91.7 % were abundantly found during different seasons around the year, ranging from 132.7 items/m3 to 641.3 items/m3. The average abundance of plastics was significantly higher in August (641.3 +/- 23.7 items/m3) than in other months. Overall, fibers, large microplastics (L-MPs), and transparent items were dominant by shape (57.7 %), size (61.9 %), and color (24.6 %), respectively. The highest average number of fibers (374.3/m3) and L MPs (396 items/m3) were recorded during May and remained higher in the surface water from December to May. Fragments (432.3/m3) and S-MPs were observed higher (362.3 items/m3) during the peak rainy month of the summer monsoon season (August). Variations in the abundance and morphotypes were seemingly not only influenced by the seasonal change but also might be due to hydromorphological characteristics of the river, especially riverbed morphology, and the flow of the water. Only 5.2 % of the total items found were identified using mu-FTIR (micro-Fourier Transformed Infrared Spectroscope) which consisted of 70.7 % plastic items. Spectroscopy revealed that polyethylene terephthalate was an abundantly found polymer that largely prevailed in the form of fibers, followed by polypropylene and polyethylene. Most of the fragments, foams, and films were composed of polypropylene, polystyrene, and rayon respectively. Being an urban river, the polymeric profiledemonstrated that anthropogenic activities had a significant impact on polluting the river. These findings are a very important source to understand the profile of plastic pollution in the Soan River and also a significant reference for policy-making in controlling plastic pollution among the riverine networks.

Automated summary

This study investigates the monthly profile of plastic pollution in the Soan River mouth and finds that plastic pollutants, particularly microplastics, are abundantly present throughout the year with variations in abundance and morphotypes influenced by seasonal changes and river characteristics. The study also reveals the significant impact of anthropogenic activities on polluting the river, providing important insights for understanding plastic pollution and developing policies to control it in riverine networks.