Dispersion and potential impacts of a white wave of pellets after an accident off NW Iberian Peninsula

A particles' dispersion and coastal impact assessment study is presented following the release of 1000 million pellets by the cargo ship Tocanao offshore North Portugal. The accident took place on the 8 th of December 2023, 82 km from the coastal city of Viana do Castelo. Due to uncertainties regarding the accident, a Lagrangian dispersion model first ran in backward mode in order to identify the point and time of origin of the pellets. The model then ran in forward mode and the dispersion and environmental impact indices calculated for the next 9 weeks after the accident. The region of first arrival of the pellets, at the Galician coast, between Sálvora Island and Pindo Beach, was also the most affected area. More than 60% of the pellets arrived at this region during the first week. During the next 4 weeks the particles continued reaching the coast, affecting almost the entire Galician coastal region, although in smaller concentration. The Predicted Environmental Concentration (PEC) was calculated for the free-floating and washed-ashore pellets and compared with the Predicted No-Effect Concentration (PNEC). The Pellet Pollution Index (PPI) was also evaluated and used to categorise the pellets pollution levels. The results indicate no significant ecological risks associated with the simulated concentrations of floating pellets and pellets stranded ashore along the Galician coast. Some important limitations were however identified, particularly regarding the calculation of PNEC and the lack of data associated with possible chemical interaction between pellets and the environment (e.g. leaching of contaminants adsorved to pellets). Nevertheless, this study clarifies methodological approaches and indicates research paths for pellet contamination impact studies, which can be applied in this or other past and future accidents.