Risk assessment of microplastic pollution in urban lakes and peripheral Rivers of Dhaka, Bangladesh

Although microplastics (MPs) are considered as a ubiquitous pollutants, most of the studies on MPs in water have paid little attention to the analysis of the ecological risk of MPs, which is decisive in understanding the effect of MPs on the aquatic ecosystem. Therefore, to evaluate possible ecological risks, this study investigated the abundance of MPs in surface water and underlying sediments of several lakes and peripheral rivers of Dhaka city, Bangladesh. 19 sites (in five lakes and five rivers) were sampled to ensure maximum city coverage. Density separation, microscopic inspection and FTIR analysis were conducted to analyze the characteristics of MPs and the type of polymers. The average MPs abundance in water and sediment samples was 36000 MPs/m3 and 13607 MPs/kg dry weights. Most of the MPs are fiber and fragments in shape in sediment samples. However, foam is also a significant fraction of water samples. The high MPs numbers have occurredespecially for the particles having diameter <0.5 mm. Using FTIR, eight polymer types were identified in the MPs samples, including polypropylene, polystyrene, nylon, low-density polyethylene, ethylene-vinyl acetate, acrylonitrile butadiene styrene, and polyethylene terephthalate. For assessing the ecological risk of MPs, the predicted no-effect concentration (PNEC) values were calculated through a species' sensitivity distribution (SSD) model. The PNEC for MPs in the surface water of those lakes and rivers are 43870 n/m3. Ecological risk assessment for MPs suggests that most of the water bodies posed low risks (Risk quotient (RQ)<1) to freshwater biota, except the five sites with high risk. The estimated RQ values based on SSD analysis in this pilot study may serve as a baseline to understand the possible ecological risks of MPs in fresh water.