Assessing the state of plastic pollution in Dutch river systems

Rivers play an important role in the global distribution of plastic pollution. Plastics are transported and retained by rivers, and may be exported to sea. Large-scale, long-term and harmonized plastic monitoring data are crucial to better quantify, understand, and reduce plastic pollution in the environment. Global data availability strongly depends on the river compartment (surface, water column, riverbank, sediment) and size range (micro or macro). Despite the surge in data collection efforts, a comprehensive framework to combine those data into actionable plastic pollution indicators is still lacking. Here, we present a methodology to holistically assess the state of plastic pollution for river systems. We defined eight plastic pollution indicators representing different river compartment and size ranges. All indicators can be quantified using commonly used monitoring methods. Indicator values are coupled to effect thresholds of microplastic and macroplastic, and combined to quantify the overall state of plastic pollution. Our method can be applied at multiple spatiotemporal scales. We applied our assessment method to the Netherlands, and included four rivers, two estuaries and five urban water systems. We show that the state of plastic pollution at the annual scale varies strongly between systems, changes over time, and is driven by different indicators (e.g. suspended macroplastic, floating macroplastic or riverbank macroplastic). With our work we aim to contribute to the development of comprehensive, globally applicable tools to assess plastic pollution in rivers.