Tracing marine litter sources along the Barcelona coastline: Insights from observations and numerical modelling

Plastic debris concentrations in Barcelona coastal waters are among the highest in the Mediterranean Sea, comparable to those observed in subtropical gyres. This study quantifies the marine litter budget and fluxes along the Barcelona coastline using Lagrangian numerical backtracking simulations informed by one year of sampling data (2020-2021) from five urban and two metropolitan area beaches. Sensitivity analyses were conducted to determine optimal simulation parameters, identifying a simulation duration of -28 days and a median time of particles to come from a known source of -26.35 h. These parameters were used to develop a probabilistic module accounting for unresolved nearshore processes. Additional behavioural conditions for this module were derived from the sensitivity assessments. Analysis of beach water marine litter as simulation input revealed multiple pollution hot-spots, linking marine litter sources to known emission points such as combined sewer overflows, marinas, and coastal structures such as breakwaters. Marine litter amounts in urban beach waters reached 1.8 × 10⁹ items km^-2 y^-1, with simulations indicating that >55% originated from outside the study domain. These findings highlight the Barcelona coastal zone as both a significant emitter and recipient of plastic debris due to regional hydrodynamic connectivity and transport. High levels of plastic pollution highlight the urgent need for upstream waste reduction and targeted mitigation measures to prevent litter from reaching the sea.