Assessment of microplastics in the Mahananda, Barnoi, and Baral rivers of Bangladesh and their ecological risks

Microplastic (MP) pollution has become an emerging threat to freshwater ecosystems, yet limited information is available for river systems in Bangladesh. This study assessed the occurrence, characteristics, and ecological risks of microplastics in the Mahananda, Barnoi, and Baral rivers. The analysis results revealed widespread contamination in both water and sediment, with clear spatial and compositional differences among the rivers. Morphological analysis showed that blue-colored microplastics were predominant (27-31%), followed by red particles (15-17%), suggesting inputs from fishing activities, textile fibers, construction materials, and plastic packaging. The majority of particles were smaller than 1 mm, indicating extensive fragmentation and enhanced ecological exposure risk. FTIR analysis identified six major polymer types, with polyethylene and polypropylene dominating surface waters, while denser polymers such as polyethylene terephthalate and polyvinyl chloride were more enriched in sediments, reflecting density-driven segregation. Quantitative analysis showed the highest MP abundance in the Barnoi River (water: 24.23 MPs L⁻¹; sediment: 2,453.85 MPs kg⁻¹), followed by the Mahananda River (water: 20.69 MPs L⁻¹, sediment: 2,177.56 MPs kg⁻¹), whereas the Baral River exhibited comparatively lower levels (water 10.82 MPs L⁻¹, sediment 1,389.23 MPs kg⁻¹). Contamination factor (CF) and pollution load index (PLI) revealed potential microplastic pollution load in the rivers. Polymer hazard index (PHI) values indicated a very high level of polymer hazard, and ecological risk index (ERI) values ranged from high in water to dangerous levels in sediments, emphasizing sediments as major sinks of microplastics. Because of the severe ecological threats and potential risks to aquatic organisms, urgent implementation of pollution control and waste management strategies are required.