Rahel Hauk

Plastic is an essential part of everyday human life, and even though plastic mass production started less than 100 years ago, plastic pollution is now a global problem. In this thesis the focus is on one small facet of macroplastic (> 2.5 cm) pollution, macroplastic on riverbanks in the Netherlands. Macroplastic can enter the riverine environment via multiple pathways such as surface run-off, sewer overflow, or littering. Rivers transport and store a variety of natural and anthropogenic materials, nowadays including (macro-)plastic. Macroplastic in rivers can be transported and emitted to the sea, or (intermittently) stored in the river bed, in riparian vegetation, or in and on riverbanks and floodplains.

In this thesis I investigated different aspects of riverbank macroplastic pollution along Dutch rivers. In particular the research in this thesis followed four questions:
1. How stable is macroplastic deposition on riverbanks over time and space?
2. How does macroplastic concentration and composition on riverbanks and floodplains differ following a flood event vs. non-flood conditions?
3. How far are macroplastic items transported during a flood event and over the following years, and how does their condition develop in terms of mass and fragmentation?
4. Following flood events, how do macroplastic concentrations and characteristics differ between landcover elements on riverbanks and floodplains, and how is plastic distributed in the lateral direction?

Chapter 1 is the introduction to the thesis topic. I introduce the history of plastic production and plastic pollution as well as its consequences. I then introduce the role of rivers in plastic pollution (transport), and plastic transport and deposition during flood and non-flood conditions. And finally, macroplastic deposition on Dutch riverbanks which is the main focus of this thesis.

Chapter 2 assesses and quantifies the temporal stability of macroplastic concentrations across riverbanks and rivers over time. The results show that macroplastic pollution across riverbanks is not evenly distributed and does not appear random over time. Hotspot and coldspot locations which are consistently above/below the macroplastic concentration average are identified.

Chapter 3 compares plastic deposition along the Meuse river following a flood event to plastic deposition along the Meuse during non-flood conditions. This is done by fieldwork following the July 2021 flood event and comparing this fieldwork to multi-year monitoring data on the same riverbanks.

Chapter 4 focuses on a case study of plastic spilled into the Meuse river during the July 2021 flood. The study followed the transport and fate of spilled buttertubs over the two years after the flood event. Their transport path and velocity was tracked, as well as their fragmentation and mass development.

Chapter 5 investigates flood induced deposition of macroplastic on riverbanks and floodplains. Where exactly plastic is deposited, spatially on riverbanks, and in relation to riverbank and floodplain elements such as different vegetation and debris piles. In this chapter also the characteristics of macroplastic deposited off the ground, in trees and other riparian vegetation, vs. macroplastic deposited on the ground and in groundcovering vegetation, are investigated.

Chapter 6 is the synthesis of this thesis, I discuss the main findings and how the results from the individual chapters connect and fit into a broader context. I also discuss what these results reveal about plastic pollution and what we can learn from this for plastic pollution research. Especially implications of the thesis results in view of modeling plastic transport and deposition along rivers, as well as sampling and monitoring strategies. I will discuss emerging and open questions in the research field, and potential future research directions.