Export from rivers to oceans
Rivers are inherently a natural pathway, carrying nutrients and sediments from the land to the ocean. In this light, it is likely that rivers are also a major transport pathway for plastics from land. However, increased observational evidence suggests that most plastics may accumulate in and around rivers rather than exported to sea. Several studies have estimated the amount of plastic - either microplastics or macroplastics or both - in rivers worldwide and have come to different conclusions. The midpoint estimates for microplastics range from 31 kilo tons to 2.31 million tons per year. The annual delivery of macroplastics ranges from 0.15 tons to 1 million tons. And the midpoint estimates for total exported plastics range between 0.13 and 2.46 million tons. These large discrepancies illustrate the need for more data and fresh concepts to constrain the magnitudes of riverine plastic transport.
In our research we seek to find concepts to describe how diffuse plastic sources on land such as mismanaged plastic waste, reach the river networks and how they are transported within river networks. Our objective is to provide global estimates which require parsimonious approaches that only use data that is easily available. In a first step, we quantified the connectivity of each point on land by combining the downward distance to the nearest river network with the upstream accumulated area. We also aim on quantifying urban point sources such as waste water treatment plant effluents. As a first step in the direction we have estimated microplastic particle emissions from wastewater treatment plants into river networks in Germany which is currently being expanded to Europe and other regions of the world.
 Schmidt, C., Krauth, T., & Wagner, S. (2017). Export of Plastic Debris by Rivers into the Sea. Environmental Science & Technology. https://doi.org/10.1021/acs.est.7b02368
 Schmidt, C., Kumar, R., Yang, S., & Büttner, O. (2020). Microplastic particle emission from wastewater treatment plant effluents into river networks in Germany: Loads, spatial patterns of concentrations and potential toxicity. Science of The Total Environment, 737, 139544. https://doi.org/10.1016/j.scitotenv.2020.139544