Tracking Plastic Debris Transport Pathways from Rivers to Sea Along the Nigerian Coastline

Understanding the pathways by which riverine plastic debris is transported to the coastal ocean is essential for designing effective upstream intervention strategies and predicting accumulation zones in the nearshore marine environment. This study tracks plastic debris transport pathways from four major Nigerian rivers (Lagos, Ogun, Niger Delta network, and Cross River) to the Gulf of Guinea using an integrated field measurement and particle-tracking modelling approach. Field measurements include quarterly surveys of floating plastic debris density and composition at 12 riverine transects from 50 kilometres inland to the river mouth, using standardised visual counting from boat transects following the JRC River Input methodology. Plastic composition, morphology, and size class are recorded. Surface current and river discharge data are assimilated into a Lagrangian particle-tracking model (OceanParcels framework) driven by a regional hydrodynamic model to simulate trajectories of plastic particles released at river mouths under seasonal flow and current conditions. Model-simulated accumulation zones are compared against independent beach and sea surface plastic survey data from quarterly monitoring at five coastal stations. Back-trajectory analysis for coastal-collected plastics identifies probable river source regions. Statistical analysis quantifies the seasonal modulation of cross-shelf export efficiency by monsoon currents. Findings are expected to demonstrate that the Niger Delta river network contributes 65 to 80 percent of coastal plastic inputs in the South South geopolitical zone, with peak export during October peak discharge events. Keywords: plastic transport pathways, riverine plastic, Lagrangian modelling, Nigerian coastline, marine plastic pollution