Macroplastic and microplastic presence in riverbank, surface water, and sediment of a Japan agricultural river

Macroplastics and microplastics are increasingly being documented in riverine systems, yet their pathways remain poorly understood, especially in agricultural catchments. In this study, macroplastics along riverbanks and microplastics in surface water and sediments of the Hamada River in Toyohashi, Japan, were examined, complemented by a survey of mulch sheet and sewage sludge usage. Sampling and analyses were conducted based on the Japanese Guidelines of Riverine Microplastic Survey and were determined using attenuated total reflectance Fourier transform infrared spectroscopy. The abundance of macroplastics was 2.06 ± 0.56 pieces/m². Almost half the macroplastics found were mulch sheets with polyethylene of varying colors, such as white, black, transparent, and gray, corresponding to an estimated leakage rate of 0.013% from tobacco mulch sheet applications. Mesoplastics averaged 0.01 ± 0.001 particles/m³ in surface water and 20 ± 15 particles/kg in sediment, while microplastics averaged 0.038 ± 0.02 particles/m³ and 75 ± 87 particles/kg, respectively. Films were the dominant type in water, and coated fertilizers were the major type in sediment. The polymer composition was dominated by polyethylene and polypropylene. The dominance of film-shaped plastics in both macroplastic and microplastic fractions suggests that agriculture is a major source of plastic pollution, which may help to inform mitigation strategies and proper waste management in agricultural catchments.