Modelling transboundary plastic pathways to Norwegian coastal and inland waters

Marine plastic debris (MPD) is a form of transboundary pollution, with many common types having far-field transport potential. Such movement can undermine efforts to tackle marine plastic pollution and disincentivize taking action. Norway's extended coastline, which borders a major transport pathway out of the North Sea - the Norwegian Coastal Current (NCC) - makes it susceptible to transboundary fluxes of MPD from other countries. The complex network of fjords along the coast can act as natural traps for marine litter transported via the NCC. In this study, we investigate the origins of transboundary plastic reaching Western Norway using a nested modelling approach. Buoyant plastic emissions from major European rivers serve as input for a Lagrangian particle tracking model. Background currents are provided by three nested ocean models with increasing resolution: a 4 km model of the North-East Atlantic, an 800 m model of the Norwegian coast, and a 160 m hydrodynamic model for western Norway. As particles move through these grids, we track their pathways to the Norwegian Exclusive Economic Zone (EEZ) and into western Norwegian fjords. Between 15%-88% of released plastic mass enters the Norwegian EEZ from the countries bordering North Sea, of which 1%-35% enters fjords along the west coast. Five major rivers are identified as significant contributors to the simulated plastic load (both in terms of mass and number of particles) in these fjords, where concentrations can be an order of magnitude higher than in surrounding waters. The study highlights key processes in both near- and far-field plastic transport and offers insight for agencies working on international pollution policy and cleanup efforts.