Microplastic pollution is an emerging threat to marine environments with potential environmental, social, economic, and health consequences and has become a major global environmental issue. The objective of the present study was to evaluate microplastic contamination in the ecosystem of the Qara Su River in Ardabil, Iran. Fifteen sampling sites along the Qara Su River in the Ardabil urban area were selected to collect sediment samples. The abundance and morphological characteristics of microplastics were determined by counting using a digital stereomicroscope. Micro-Fourier transform-infrared spectroscopy (μ-FTIR) analysis was used to identify polymer type of the microplastics. In this study, the frequency and distribution of microplastics (< 5 mm) in the sediment of Qara Su River were investigated as one of the sources containing microplastics. Sediment samples were collected from five different sites (Karkarq, Sar band, Anzab Sulfa, Dolat abad, and Samian) from September to March 2020. For microplastic examination in sediment, the amount (approximately 1000 g of fresh sediment from each site) was sampled from 15 sites. Fiber microplastics (53%) and fragments (33%) microplastics were predominant. It has shown the abundance and heterogeneity of space. In this study, the highest amount of microplastics detected in sediment samples was related to the size of less than 5000 µm (92%). The frequency of microplastics below 5000 µm in sediment was attributed to the low capacity of existing processes in Ardabil wastewater treatment plant. The abundance of microplastics shows significant differences between sampling sites. The Qara Su River along the city of Ardabil, like many other rivers, is contaminated with microplastics. This study was the first study conducted for evaluation of the sediment environment in terms of the microplastic pollution of Qara Su River in Ardabil. Considering the amount of microplastics in sediments, further research is required to estimate the amount of microplastics released by Ardabil wastewater treatment plants and industrial town wastewater, and other possible sources of emission and to evaluate their contribution to microplastic pollution in water, sediment, and soil. This study provided a framework for future studies of microplastics pollution in the sediment of urban areas around the Qara Su River in Iran.

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