Plastics in estuaries: estimating an emission budget, transport and fate of plastics through high resolution model simulations

Estuaries account for 88% of the global coastline and act as filters for water, sediments and particles (Dürr et al. 2011) and may act as important pathways of plastic pollution. Recent studies suggested that rivers can contribute between 1.15 – 2.41 million tons of plastic per year globally (Lebreton et al. 2017) or 307 and 925 million items per year from Europe alone (Gonzalez-Fernández et al. 2021). However, the processes of transport and retention at rivers, estuaries and beaches are still poorly understood. Recent results from Schernewski et al. (2024) showed that floating plastics released at a city harbor at the beginning of the Warnow estuary (Germany) were trapped in reeds or beaches within 6 days, with only 0.4% transported to the Baltic Sea (11 km) during storms, while sinking plastics accumulated near the source during calm winds (7 m/s) but were resuspended and transported up to 4 km away during storms (< 20 m/s). However, this study considered only one emission location and there is still a knowledge gap regarding the role of item size and density, as well as the emission locations from multiple litter sources, and thereby the retention potential at coastlines and seafloor.

In this study, we investigate the emission, transport and retention of plastics in the Warnow estuary, Germany, an exemplary estuary exposed to urban areas, harbor activities, tourism and other land-uses, using a highly resolved 3D hydrodynamic model of 20 m and a Lagrangian particle tracking approach using the Ocean Parcels framework. Emission source locations were defined with GIS. First simulations considered the emission of particles from the tourism and recreation sector over 10-days, with 1000 particles released per point. The results showed for emission sources closer to the estuary opening and the beach, 68% of particles were retained, whereas for particles released at the harbor and urban area (beginning of the estuary), 92% were trapped on beaches or reed belts, with the majority beaching within the first 2 days only 3.5 km away from the source. These results already indicate that the emission of plastics may be overestimated since few studies take into account emission location and retention dynamics.

Building upon these learnings we aim to i) provide a detailed emission budget for different sources of plastics at the Warnow estuary and ii) assess the influence of item size and density in the transport and retention of particles. This high resolution model together with the emission approach shall provide a more comprehensive assessment of emission, transport and accumulation of plastics and contribute to the understanding of the plastic budget at other estuaries as well as for the elaboration of effective mitigation strategies based on specific accumulation hotspots and emission sources.