The Processes and Transport Fluxes of Land-based Macroplastics and Microplastics Entering the Ocean via Rivers

Approximately 80% of marine plastic waste originates from land-based sources and enters oceans through rivers. Hence, to create effective regulations, it is crucial to thoroughly examine the processes by which land-based plastic waste flows into marine environments. To this end, this review covers the complete journey of macro- and microplastics from their initial input into rivers to their ultimate release into oceans. Here, we also discuss the primary influencing factors and current popular research topics. Additionally, the principles, applicability, accuracy, uncertainty, and potential improvement of the standard methods used for flux estimation at each stage are outlined. Emission estimates of land-based macro- and microplastics are typically assessed using the emission factor approach, coefficient accounting approach, or material flow analysis. Accurately estimating mismanaged plastic waste is crucial for reducing uncertainty in the macroplastic emission inventory. In our review of the processes by which land-originating plastics enter rivers, we categorized them into two major types: point-source and diffuse-source pollution. Land surface hydrological models simulate transport from diffuse sources to rivers, necessitating further research. Riverine (micro)plastic flux to the ocean is often estimated using monitoring statistics and watershed hydrological models at the watershed scale; however, standardized monitoring methods have not yet been established. At the global scale, algorithms based on river datasets are often used, which require further improvements in river data selection and microplastic number-mass conversion factors. Furthermore, the article summarizes the accuracy and sources of uncertainty of various methods. Future research efforts should focus on quantifying and mitigating uncertainties in resultant projections. Overall, this review deepens our understanding of the processes by which land-based plastic waste enters the ocean and helps scholars efficiently select or improve relevant methods when studying land-ocean transport fluxes.

Environmental Implication

This review elucidates the processes and methods for quantifying the transport of land-based macro- and microplastics into oceans via rivers. It highlights plastic pollution poses ecological and health risks. Accurately estimating emissions and transport fluxes is critical for science-based plastic waste management to mitigate risks. However, considerable uncertainty remains regarding emissions and riverine transport. Advancements in monitoring, process understanding, and integrated modeling across scales are imperative to constrain plastic inputs into oceans. Interdisciplinary efforts to refine quantification methodologies for emissions, overland transport, and riverine fluxes will inform evidence-based policies to curb plastic discharge and contamination.