Macrolitter in Groyne Fields

Abstract

Plastic pollution and accumulation in the riverine environment is of increasing concern. While most research focuses on microplastic contamination, the dynamics of macrolitter remain largely unknown. Large scale riverbank monitoring initiatives in the Netherlands reveal that macrolitter hotspots occur at several locations. Unfortunately, current knowledge on how these hotspots emerge and how this is influenced by hydrology and meteorology remains limited. As most studies are based on data from seasonal monitoring activities, short term variability remains unknown. This study is the first attempt to monitor and analyse the variability of riverbank macrolitter within a single location for over a period of three months. Behaviour of individual items is tracked and macrolitter exchange between water and riverbank is studied with regards to hydrology and wind. Finally, a conceptual model on riverbank macrolitter dynamics is presented in favour of supporting future research design.
A remote groyne field in the Waal has been monitored 21 times within the period of November 2021 until January 2022. The location of macrolitter items was recorded using Real Time Kinematic positioning. This allowed for analysing spatial patterns throughout time. Additionally, photographs of items were made in order to categorise the items without removing them from the riverbank. The river OSPAR protocol was used for item categorisation.The data shows that macrolitter primarily accumulates in the floodmarks. Rising water pushed items higher on the riverbank. Wind had a limited effect on item mobilisation as most items are wet and sandy. Analysis of item exchange between riverbank and water revealed that macrolitter deposition was observed at a relatively constant rate with minor deviations. Item uptake was heavily dependant on changes in water level. Uptake was initiated when the water level rises (dH > 0). The rate of uptake was higher with a larger water level increase. However, correlation was not statistically significant as riverbank morphology, substrate and vegetation may also influence uptake. After three months and two moderate discharge peaks, almost all items found on day 1 (estimated 99.6%) had been taken up. This indicates that under normal hydrologic conditions, the retention time of items within groyne fields is defined by the timing and magnitude of moderate water level fluctuations (assuming no accumulation under water within the groyne fields).
A conceptual model of riverbank macrolitter dynamics under natural processes is presented. Macrolitter can be stored in three domains: water, sediment and riverbank surface. Exchange within these domains occurs in four directions: in/out of sediment (storage/mobilisation) and in/out of water (uptake/depositions). Exchange is promoted by an interplay between item attributes, environmental processes and riverbank morphology. Future research on the interaction between these variables is needed in order to fully understand macrolitter dynamics.