The influence of velocity on plastic transport in rivers – Improving a global river model to map riverine plastic emissions into the marine environment

Pollution of the marine environment by plastic is an ever-growing problem. One mayor source of coceanic plastic are rivers and other waterways, and their pollution has an influence on riverine and marine environments. 

The Ocean Cleanup aims to reduce this harm by, among other things, closing the tap. To be able to tackle this task, it is important to know when and where the plastic is entering the oceans, so that targeted and efficient action can be taken. A crucial step therefore is understanding the drivers of riverine plastic transport on a global scale. We approach this problem with a global probabilistic river model.  

Previous iterations relied on discharge to scale the in-river transport. Studies have since indicated that discharge might not be a good predictor of plastic transport for a wide range of rivers. Thus, the latest version now includes the influence of velocity by using terrain slope as a proxy, which allows us to investigate its influence  on a global and local scale. 

First results show that including slope in the formulation impacts the seasonal variation of the calculated plastic transport. The most interesting difference, however, is how different types of rivers contribute to the total emissions. So is the relative influence of large and shallow catchments decreased compared to results of the previous model, while the relative contribution of steep catchments increased.  

This tool will later be finetuned by comparing it to the analysis of a large-scale drifter field study along the Amazon, Mekong and Motagua Rivers. With this we strive to gain more detailed insight into this complex topic.