Microplastic pollution in Pearl River networks: Characteristic, potential sources, and migration pathways

Microplastic (MP) pollution has become a global environmental problem with profound impacts on aquatic ecosystems. Although the topic of MPs has attracted high attention, the sources, transport pathway, and removal of MPs in river networks is still unclear. Here, we conducted a field survey across the Pearl River Basin (PRB) (> 4.5 × 10 km) and collected the water samples to characterize the spatial distribution of MPs using a Laser Direct Infrared (LDIR) chemical imaging system. The MPs were detected in all samples with an average abundance of 1092.86 items/L, in which polyamide (PA), polyurethane (PU), and polyvinyl chloride (PVC) are the main polymer types. Population and surface runoff were identified as major factors influencing the concentrations of MPs. The Partial Least Squares Structural Equation Modeling (PLS-PM) analysis revealed that precipitation-induced surface runoff is a major pathway for MPs transferring from terrestrial environment to river networks. River hydraulic dynamics were found to have considerable influence on the selective removal of MPs from water column in the river channel. The smooth state (Froude number, Fr <0.23) promotes while the rough state (Fr > 0.23) inhibits the deposition of MPs from water column to sediments. In particular, the smooth state facilitates the deposition of large-sized and high-density MPs from the water column to sediments. The deposition processes in river channel cause considerable fractionation of polymer types and size of riverine MPs. This study provides the first-hand MP pollution status in the networks of the PRB and provide insights into sources, spatial distribution characteristics, and transmission mechanism of MPs in river networks, which would provide theoretical bases and experimental reference for river water quality management and risk control of MPs for governor, stakeholders, and policy makers.