The 100 Plastic Rivers Programme – A Participatory Approach to Analysing Global River Microplastic Pollution

Increasing levels of mismanagement of plastic waste are causing concern about plastic pollution of freshwater ecosystems globally. In particular, the magnitude, sources, spatial distributions, and time scales of microplastic (particles <5mm in size) and how they are transported, transformed and potentially accumulated in river catchments are still poorly understood. Emerging field-based evidence is often hard or impossible to compare due to a wide range of different field sampling, particle extraction and lab analytical techniques for analysis being used. Similarly, mathematical models that start predicting microplastic fate and transport from small (plot) to large (river basin) and even global scales are based on different source term assumptions and representations of particle transport mechanisms, and therefore frequently don't agree with each other or field observations. We here present the first results of a participatory approach adopted in the global 100 Plastic Rivers Programme that has over the last 3 years attempted to develop a first global baseline of microplastic pollution in rivers and their catchments across the world, using standardised sampling, extraction, and analysis protocols. Findings of global riverine microplastic contamination levels with identified spatial patterns and hotspots of microplastic pollution are complemented with detailed observations of the longitudinal evolution of microplastic concentrations along selected large river systems. The findings of our global and local sampling campaigns are compared with process-based mechanistic local and global microplastic fate and transport models. Our results reveal the existence of distinct spatial patterns of microplastic pollution in streambed sediments that in many cases can not only be related to suspected pollution sources and their time-variant contributions but are even more so affected by hydrodynamic controls on microplastic transport, deposition, and resuspension as well as the advective forcing of in particular smallest microplastic fractions into streambed sediments by hyporheic exchange flow processes. Our analyses also reveal that streambed sediments represent hotspots of microplastic accumulation where large quantities of these emerging pollutants can be stored for a long time and create a pollution legacy for centuries to come. Our results demonstrate the potential of participatory approaches to overcome challenges in global sampling and comparative analyses.