Closing the Plastic Budget Equation in Fluvial Systems: A Review of Monitoring Methods

As plastic waste increasingly accumulates in aquatic environments, recent efforts have focused on quantifying the contribution of riverine sources to the ocean's plastic pollution. Accurate monitoring of fluvial plastics is challenging because plastics are transported and stored in different layers and compartments. Some processes, such as surface transport, can be measured relatively easily, while others, including bed load transport or storage in biota, remain difficult to assess. We start with a generalized formulation of the plastic budget equation, where we identify several transport and storage terms. Subsequently, we review common monitoring methods for plastics transported as surface load, suspended load, and bed load, as well as for plastics stored in sediments, floodplains, and fluvial biota. Overall, we hope to contribute to a better understanding and monitoring of fluvial plastic dynamics, with the aim to support the development of viable strategies regarding plastic pollution management and remediation.