Heavy metal contamination in the urban river has become the most severe problem in the urban environment, especially the zinc (Zn) compound is harmful to the environment at current levels of exposure. This study aims to assess the impact of urbanization and industrialization on Zn pollution in surface sediment at the Umeda River. Triplicate surface sediment samples were collected at nine sample points. An atomic absorption spectrophotometer was used for analyzing the Zn and Fe concentrations. The surface sediments in the Umeda River contained 48–159 mg/kg of Zn. The results indicated that Zn concentrations were abundant in the ascending order of 600–1,000 μm, 300–600 μm, and smaller than 300 μm. The higher the grain size, the higher the particulate organic carbon which might contribute to the higher Zn and Fe concentrations. In addition, the sediment quality guidelines indicated that the Zn levels in the study area did not constitute a major threat to biota. Normalization with Fe showed that the Zn pollution in surface sediment had been influenced by anthropogenic activities such as industrialization and urbanization.

Agricultural rivers are often silent receivers of microplastics (MPs) from diffuse, non-point sources; however, their pollution dynamics during rainfall events remain poorly understood. In this study, MP transport was investigated at three sampling points in an agricultural river catchment, where mulching films are used, and sewage sludge is not applied. Sampling was conducted in the Umeda River and its tributaries during six sampling events. MP flux exhibited a strong positive correlation with river discharge (L–Q relationship; n = 1.49–1.61, R² = 0.67–0.87). The L–Q model indicates that a tenfold increase in discharge results in approximately a 600-fold increase in MP flux and a 1000-fold increase in total suspended solid flux. MP abundance during rainfall was up to four times higher than that during baseflow, ranging from 73 ± 64 to 200 ± 111 particles/m³, while peak flux reached 6736 particles/s, with an MP mass of 811 mg/s. Regarding particle characteristics, rainfall enhanced the heterogeneity of MPs, although fragments and polyethylene/polypropylene polymers remained consistently dominant across all hydrological stages. First-flush behavior was observed at HU, with more than half of the total MP mass exported within the initial 50% of the event flow volume. These findings help to inform mitigation strategies that should prioritize a reduction in upstream plastic inputs in order to effectively manage MP transport in agricultural rivers.

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.

Conceptual diagram of agriculture as cryptic plastic pollution hotspot, linking mulch sheets, macroplastics on river banks, runoff pathways, microplastics in water and sediment, and leakage bar charts. 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.

Microplastics, plastic particles smaller than 5 mm, are significant pollutants of growing global concern due to their persistence, widespread distribution, and potential ecological and human health risk. Numerous studies have proposed different methods for sampling microplastics in river sediments and water. However, comparative studies evaluating how different sampling methods affect the quantification of microplastic abundance in surface water remain limited. This study focused specifically on surface water, as it represents the primary pathway of microplastic transport, and the yield is most directly influenced by sampling method selection. Therefore, this study aimed to compare the abundance and characteristics of microplastics collected using three sampling methods: plankton nets, buckets, and pumps, which were selected to represent the full spectrum of commonly used riverine surface water collection approaches. Each method collected an equivalent volume of water, 1000 L. Statistical analysis showed no significant differences among the methods in terms of microplastic abundance and characteristics. These results suggest that the selection of a sampling method should be primarily based on river characteristics such as flow velocity, depth, and channel width rather than methodological differences. The findings provide practical guidance for selecting appropriate sampling approaches in low-flow river environments.