On modeling the fate of microplastics along river networks

The increasing amount of microplastics (hereafter MPs) in freshwater ecosystems appears as a highly relevant environmental issue as MPs represent a group of contaminants of emerging concern responsible for water pollution worldwide. Within MPs are included particles with the potential to enter the environment, persist in it and be easily ingested by aquatic organisms with important adverse effects on both the ecosystems and human health. Research has revealed that the presence of MPs in organisms can cause negative effects and the risk associated with their long-term exposure is still under debate. Within freshwater ecosystems, streams and rivers represent one of the most important delivery vectors responsible for the transfer of MPs from terrestrial to marine environments. Therefore, understanding their transport dynamics became crucial to propose mitigation strategies and to manage and possibly reduce adverse health effects. Here, we present a simple model able to predict the fate of MPs along the different reaches that compose a river network. The model solves the general advection-dispersion-reaction equation (ADRE) along each reach of the river network considering the release of MPs from the Waste Water Treatment Plants (WWTPs). Using a combination of free access databases (MERIT, ReachHydro) and computational algorithms we estimate MPs concentrations at different locations in the river network. The model capability to capture MPs transport was tested by using available literature data where MPs samples were collected upstream and downstream of WWTPs. Our proposed method was able to satisfactorily reproduce the measured values proving a useful tool to understand the role of river networks in controlling the fate of MPs and to provide the basis for introducing suitable mitigation strategies.