Temporal stability of macroplastic on Dutch riverbanks

Is the distribution of macroplastic along rivers stable over time? Rivers transport and accumulate macroplastic litter, and large amounts of macroplastic are deposited on riverbanks. It seems there are factors and variables linked to macroplastic deposition at a specific location, e.g. riparian vegetation or population density. However, the specific transport and deposition processes that lead to a plastic item being deposited on a specific riverbank are not well understood yet. So we took a step back and instead of analyzing local riverbank characteristics that might link to plastic deposition, we investigated the stability of plastic deposition over time. If macroplastic distribution is indeed stable over time, it further indicates that there is a link between macroplastic deposition and site specific factors. 

We adapted a method, which was originally developed to analyse the temporal stability of soil moisture. With that method, we assessed the temporal stability of macroplastic distribution on 229 riverbanks along eight major Dutch rivers. Each location was surveyed between six and eight times from 2020 to 2024. Our results demonstrate clear temporal stability in macroplastic distribution over those four years. Riverbanks were classified into four categories, hotspots, coldspots, and persistently above- or below-average sites, based on their relative plastic concentration over time. Between 10% and 42% of sites along each river were identified as coldspots with a plastic concentration always below average. At the other end, 0% to 8% were persistent hotspots, with a plastic concentration always above average. We also identified that the hotspots contained a disproportionally large amount of macroplastic, between 13% to 35% of macroplastic items for the rivers that had hotspots. 

The proposed method offers a practical and relatively easy approach to investigate the temporal stability of macroplastic distribution between locations. Assessing this temporal stability (or lack thereof) can provide a new perspective on macroplastic pollution in the investigated system. It can also contribute to optimize macroplastic monitoring and focus mitigation measures. Further, the demonstrated spatial and temporal stability implies underlying mechanisms governing macroplastic deposition. This provides a direction for future process-based investigations.