Exploring visual counting and visual classification as monitoring tools to quantify macroplastic emissions: findings from 50 campaigns across the globe

Quantifying plastic pollution is a key activity to unlock: understanding of plastic transport processes; verification of modelling efforts; baseline estimations at river basin level; performance measurement of cleanup efforts; and others. Visual counting and visual classification are cornerstone methodologies to quantify macroplastic fluxes in rivers, providing comparable datasets and replicable methodologies. This study compares monitoring conducted in 50 locations over the past 10 years; it includes some datasets already published while others are novel. This is a growing dataset, part of ongoing monitoring efforts. Most data collection was done so far in Southeast Asia (>50% of surveys), while efforts in Central America (appr. 16%) were done mostly within the same river basin in Guatemala. The dataset covers 37 rivers and also include a few surveys in North America, Europe and Africa (appr. 7%). Most of the surveys were conducted in natural waterways, with widths varying between 6 and 550 meters, while at least 40% were up to 100 meters in width. In this study, we compare these datasets in terms of fluxes and composition and assess what they can tell about plastic pollution and its correlation with the environment (e.g. precipitation, flow regime, tides). We also discuss opportunities and shortcomings in the methodology and its applicability in such diverse contexts. The main outlook is that these findings reflect the diversity of fluxes and composition across different river systems. These methodologies can be a cost-effective tool to bridge the gap in quantifying plastic pollution across the globe, whilst other techniques (e.g. camera-driven, GPS drifters), can cover its limitations or complement the efforts.