The sink is leaking! Enabling citizen science for global mapping of microplastic leakage from coastal soils

Abstract Marine plastic pollution increasingly infiltrates coastal soils, yet little is known about their role as potential sources of microplastics (MPs) leaking back into the ocean. This study documents and quantifies MP leakage from plastic-infiltrated coastal soil on Smøla island, Central Norway, and evaluates a low-cost, citizen-science-friendly methodology for future global monitoring. Nine soil cores were extracted and subjected to simulated rainfall. Leachate samples were filtered, oxidized (H₂O₂), Nile Red-stained and examined under ultraviolet-stereomicroscopy. MPs in the size range of 1 mm–100 μm were detected in all samples, from 6.2 to 33.9 MPs/L (mean±SD = 20.0±10.8 MPs/L), corresponding to an estimated annual leakage of ~27,000 MPs/m 2 /year. A significant positive correlation ( ρ Spearman = 0.72, p = 0.030) was found between macroplastic concentration and MP leakage. Coastal soils may only act as a temporary sink, facilitating breakdown into secondary MPs and redistribution to the ocean. To enable further studies, we present a pedagogical step-by-step guide for application in citizen science and educational contexts. We also emphasize its potential to empower research in developing countries. Together, these outcomes lay the foundation for accessible, globally comparable monitoring of MP leakage from coastal soil – an underexplored yet potentially significant pathway in the plastic pollution cycle.