Assessment of microplastics contamination and ecological risks associated with urban canals: insights into a case of Dhaka city

Urban canals carry microplastics (MPs) into river systems. This study measured MP abundance and characteristics in five major canals of Dhaka City: Kallyanpur, Gobindapur, Norail, Digun and Abdullahpur. 30 samples were collected, 15 from water and 15 from sediment. Sampling took place in the dry season, when low discharge and minimal dilution contamination to its peak, the results give a worst-case baseline for these canals. Sediment held more MPs (162–1188 items/kg) than water (13–185 items/L). Fibers (33.93%) and fragments (22.99%) were the dominant shapes and came mostly from the breakdown of larger plastic debris. Transparent particles were most common in both matrices (28.54% in water, 28.7% in sediment), most particles fell between 0.5 and 1 mm, and FTIR identified polyethene and polypropylene as the main polymers. SEM images showed heavy weathering and surface degradation. The Nemerow Pollution Index (NPI) placed the canals between low and high contamination. Estimated flux to peripheral rivers ranges from 50 to 189 billion particles per day, which marks these canals as a route for MP transfer. This is the first comprehensive assessment of microplastic contamination in Dhaka’s urban canals, quantifying flux to peripheral riverine and providing a framework for managing microplastic contamination.