Measurement, quantification, and potential risk of microplastics in the mainstream of the Pearl River (Xijiang River) and its estuary, Southern China

The goals of this study were to investigate the distribution profiles, sources, and inventory of microplastics in the surface water of the mainstream of the Pearl River (Xijiang River) and its estuary, China, and to assess the potential ecological risks of the microplastics in this subtropical riverine habitat. The results showed that the microplastic abundances of the Humen (HUM, 16.33 ± 0.88 items/L), Zhaoqing (ZQ, 15.33 ± 0.67 items/L), and Hutiaomen (HTM, 14.67 ± 1.33 items/L) sites were significantly higher than those of the other sampling sites, indicating that the microplastics in Xijiang River and its estuary exhibited an unbalanced spatial distribution. The most common microplastic color was transparent and the major shape of the microplastics was fragments. Microplastics with a size of 0.01-0.1 mm were dominant throughout the 16 sampling sites. Polyolefin elastomer (POE, 33.33%), polyethylene terephthalate (PET, 23.81%), and polyurethane acrylate (PUA, 14.29%) were the dominant microplastic polymers. The microplastic abundance co-varied with most of the selected socio-economic indicators, including the population density, urban land area, gross domestic product (GDP), freshwater aquatic products, and freshwater cultivated area, but without significant differences, indicating that there are additional factors affecting the microplastic abundance. The potential risk (i.e., the potential ecological risk, RI; polymer risk index, H; and pollution load index, PLI) posed by the microplastics was higher in HUM, followed by ZQ and HTM, and it was lower for the other sites, suggesting that the main outlets and the biggest city on the Xijiang River contributed more to the high risks of microplastic pollution. Consistent with microplastic abundance distribution profiles, the average potential risk index values of the western river outlets were higher than those of the eastern river outlets and the Xijiang River, implying that the western river outlets had non-negligible potential ecological risks. Our findings deepen the understanding of the risks posed by microplastics and further contribute to microplastic risk management of riverine ecosystems.