A first approach to estimate the leakage of polymer-coated fertilizer-derived microplastics from paddy fields to beaches

The fate of plastic debris that enters the ocean (whether it floats or sinks) remains largely unknown except for plastics found on coastlines. Plastic debris on beaches provide a basis for estimating the return rates of riverine debris derived from land-based sources. Polymer-coated fertilizers represent a traceable form of land-based plastics. These fertilizers use plastic microcapsule as coating agents and are commonly applied to paddy fields in Japan. This study investigates the occurrence and runoff rates of fertilizer-derived microplastics in coastal environments, focusing on their distribution along beaches near river mouths. Samples were collected from 147 plots across 17 beaches along the Japanese coastline. The identified microplastics were composed of polyethylene (PE) and polyurethane (PU). On the Sea of Japan coast, ten beaches contained an average of 18.1 kg ha (670.7 items m), whereas seven Pacific Ocean coast beaches averaged 6.3 kg ha (231.7 items m). We observed two transport pathways: (1) runoff through rivers and (2) direct drainage from paddy fields to beaches. Beached microcapsules transported via rivers represented <0.2 % of the total applied amount, while beaches directly connected to paddy field drainage retained 27.9 %. River runoff resulted in high losses of microcapsules to the open sea, whereas direct drainage led to substantial beach retention. The high areal density of microcapsules indicates that wave and tidal action promotes their accumulation on beaches. Overall, the main transport pathway involves movement from paddy fields into rivers, subsequent entry into the ocean, deposition on beaches, and eventual offshore transport by rip currents.