Temporal stability of riverbank macroplastic

Macroplastic pollution (>0.5 cm) is prevalent on riverbanks and floodplains globally. Macroplastic can reach riverbanks via various pathways. These pathways include direct littering, overland transport by wind or surface runoff, or transport and deposition by the river. Local factors and variables such as the type of riparian vegetation can influence macroplastic deposition on riverbanks. However, the combination of physical processes and anthropogenic influences that control macroplastic transport and deposition are not fully understood yet. As a result, it is often unknown why certain macroplastics were deposited on a specific riverbank. While macroplastic transport and deposition processes are not thoroughly understood, there is growing evidence that there are preferential deposition locations for macroplastic on riverbanks. Yet, it is unclear whether the spatial distribution is consistent over time. To understand why plastics accumulate at certain locations, we first need to investigate if the spatial distribution is consistent over time. We quantified the temporal stability on riverbanks using a multi-year dataset of bi-annual macrolitter measurements on Dutch riverbanks. We applied this method to 229 riverbanks along eight Dutch rivers between 2020 and 2024. All riverbanks included in this study were measured at least six times during those four years. With this method we found that plastic distribution on Dutch riverbanks exhibits temporal stability. We classified riverbanks into hotspots, coldspots, and above and below average, based on the amount of macroplastic per 100 m riverbank for each measurement round. Per river between 10 - 42% of riverbanks were coldspots, with measurements always below average, and 0 - 8% of riverbanks were hotspots with all measurements above average. The ratios of riverbanks classified as hotspots, coldspots, and above and below average, and their temporal stability, are similar between all eight rivers we investigated. Our proposed method could be used to improve and streamline macroplastic pollution monitoring, as well as clean-up efforts. Among other uses, the method allows to identify riverbanks representative of the average plastic concentration in the monitoring area. Further, it also allows to identify hotspots or coldspots that accumulate disproportionally high or low amounts of plastic respectively. This method can serve as a base for investigation of factors that influence and determine macroplastic transport and deposition dynamics. Spatial and temporal stability in macroplastic distribution indicates underlying factors and mechanisms which can be investigated as a next step.