Quantifying the Linkage Between Agricultural Mulch Film Watershed Abundance and Riverbank Accumulation

This study quantified the linkage between the watershed abundance of agricultural mulch film and its accumulation along riverbanks in Hamada River catchment, Japan, by integrating riverbank monitoring, catchment-wide surveys, hydrological connectivity assessment, and delivery ratio estimation. Black film was predominantly associated with red perilla, while silver film was almost exclusively used for tobacco. Riverbank monitoring from 2023 to 2025 showed that mulch film contributed disproportionately more to surface area coverage than to weight density, reflecting its thin, sheet-form geometry. Its accumulation was significantly associated with episodic hydrological connectivity, whereas other macroplastics showed few significant hydrological associations. This study provides quantitative estimate of the delivery ratio of agricultural mulch films from agricultural land to river systems, a linkage rarely quantified in prior studies. Annual delivery ratio of silver film ranged from 0.17% to 0.31%, with the highest value in the year for the lowest watershed abundance, indicating that delivery was not determined solely by abundance. In 2025, the delivery ratios were 0.18% for silver and 0.06% for black film, both below 1% but differing by a factor of three despite black film’s greater watershed abundance. These findings highlight the need for an abundance-based plastic mulch management strategy at the catchment scale.