Anthropogenic influences on seasonal microplastic dispersion in the Pasur River of Bangladesh based on field observations and numerical simulation

Microplastics are a growing concern in the contemporary world. Many regions of the world are on the way to combat it. Unfortunately, research in Bangladesh has yet to advance. This study assessed the spatial distribution of microplastics and used the MIKE 21 Particle Tracking Module to depict its seasonal variations within the Pasur River, whose flow presents a marked influence from both the tidal cycle and river discharge. In-situ measurements revealed the highest concentration of microplastics at 11944.44 ± 294.07 MP/m3 near the Mongla port, implying the impact of anthropogenic activities. Six morphotypes of microplastics were identified, with fiber (52%) being the most dominant. Almost three-fourths of the particles were (> 1 mm) in size, and colored particles were more prevalent (54%). A total of seven types of polymers were identified through ATR-FTIR spectroscopy, with polypropylene (PP) (40%) and polyethylene (PE) (26.67%) being the most abundant. The ecological risk assessment (ERA) revealed moderate to high pollution levels with PLIzone = 1.43, NPI ≈ 2.00, and PHI = 819.15. Furthermore, the MIKE 21 particle tracking module portrayed that due to high river discharge in monsoon (September), all the particles were washed out within 10 days from the Pasur River to the ocean. In contrast, during the dry season (January), which has a low discharge rate, most of the particles were retained in the river. The persistent presence of microplastics during the dry season is a major finding that will provide policymakers with a blueprint for initiating waste management programs.