A stringent environmental quality standard for zinc (Zn) has been enacted by regulators because of its toxicity to aquatic life. This study’s objective was to evaluate the variability of Zn concentrations and fluxes in the baseflow and stormflow and to estimate the contribution of Zn from point and non-point sources. By using high-resolution temporal sampling, the suspended solids (SS), iron (Fe), and Zn concentrations were measured in a small agricultural river catchment. Fe, as the natural non-point source, and Zn were evaluated using the end member mixing analysis (EMMA) to identify the source apportionment (point and non-point). The results indicate that in the baseflow, Zn mainly originated from point sources and was possibly discharged by manufacturing industries. By contrast, the non-point sources (diffuse sources) were responsible for extremely high SS, Fe, and Zn levels in the stormflow. In addition, Zn discharge during the stormflow was 93 times higher than that during the baseflow. According to the EMMA, approximately 74% of the Zn was from point sources. River management can be improved if Zn point sources are adequately treated. During a storm event, it is also important to control the particulate Zn released into the river.

Smart meters monitor hourly water consumption patterns while reducing the cost and inconvenience of traditional meters. This study comprehensively analyzes 1871 households that previously used traditional meters from the distribution point to the distribution area. All the households were equipped with smart meters and the data collected were used for analysis. On the basis of the total estimated water consumption, 227 households were classified as single households. These households were further classified into single-worker and -nonworker households. This study analyzed smart meter data to evaluate the timings and amounts of water consumption peaks. The results indicate that worker households peaked at 8:00, with 29 L/h of consumption on weekdays, and peaked again on evenings at 20:00–21:00, averaging 32 L/h. For nonworker households, the peak occurred at 9:00, with 20 L/h, with no major changes in the afternoon, and a second peak occurred at 19:00–20:00 in evening, with an average of 19 L/h. Moreover, worker households peaked at 8:00 and 20:00 on weekdays, and at 9:00 and 19:00 on weekends. It was revealed that worker households consume 10% more on weekends than on weekdays, and 262% more from 13:00 to 16:00. These findings may assist in water supply planning by supporting distribution schedules on the basis of peak household consumption, leading to more helpful resource management.

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.

For improving the management of watershed eutrophication, methods for measuring bioavailable phosphorus (BAP) are more important than measurements of total phosphorus (TP). BAP in particulate form (P-BAP) is an important substance that promotes eutrophication, especially during rainy seasons. Only a portion of particulate phosphorus (PP) is taken up by algae that contribute to eutrophication. Erosion and runoff associated with rainfall transport PP bound to sediments and soil particles to surface waters, thus increasing PP concentration. This research evaluated an extraction method using an ultrasonic washing machine for extraction time and frequency. Extraction at a frequency of 28–45 kHz and an extraction time of 1 min resulted in extracted P concentrations almost the same as concentrations extracted using conventional methods. This new method requires less time and is more efficient than conventional methods because it extracts P from multiple samples in a single step. Results indicate that extraction using an ultrasonic washing machine is a promising method for rapidly obtaining BAP from sediments and soil particles.

This study quantified the linkage between the watershed abundance of agricultural mulch film and its accumulation along riverbanks in the Hamada River catchment, Japan, by integrating riverbank monitoring, catchment-wide surveys, hydrological connectivity assessment, and delivery ratio estimation. Mulch film covered 41,748 m² of the watershed during the 2025 growing season, with abundance varying by crop type. 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 accounted for 22.5% of cumulative surface area coverage but only 5.6% of cumulative weight density, reflecting its thin, sheet-form geometry. Its accumulation was associated with episodic hydrological connectivity, with integrated hydrological exposure significantly correlating with both surface area coverage (ρ = 0.72, p < 0.05) and weight density (ρ = 0.66, p < 0.05). In contrast, the accumulation of other macroplastics showed few significant associations with the examined hydrological variables. This study provides the first quantitative estimate of the delivery ratio of agricultural mulch films from agricultural land to river systems. The 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, exhibiting the same order of magnitude despite a threefold difference in watershed abundance. These findings highlight the need for an abundance-based plastic mulch management strategy at the catchment scale.

Artisanal and small-scale gold mining (ASGM) releases mercury into tropical freshwater systems, yet integrated assessments of Southeast Asian oxbow lakes remain scarce. We quantified total mercury (THg) in water, sediment, and fish from Lake Hanjalutung, an ASGM-affected oxbow lake in Central Kalimantan, Indonesia. Water and sediment were sampled at five river and lake stations in the dry (July 2025) and rainy (November 2025) seasons, and thirteen fish of five species were obtained from a lake net-cage unit in the rainy season. THg was measured by cold-vapor and thermal-decomposition atomic absorption spectrometry. Water THg was higher in the dry season (26.13–42.66 ng/L) than in the rainy season (8.20–24.05 ng/L). Geoaccumulation index (I_geo) values were higher in the lake interior (1.65–2.01) than in the river channels (0.53–1.46), consistent with Hg retention in the lake. Fish THg ranged from 0.16 to 1.51 mg/kg dw. Bioaccumulation factors (BAF; fish/water) ranged from 5.3 × 10³ to 39.2 × 10³ L/kg, and biota–sediment accumulation factors (BSAF; fish/sediment) exceeded unity for all species. Both indices correlated positively with body length, although this association largely reflected interspecific differences. Maximum allowable weekly intake (MAWI) estimates indicate a potential exposure risk. Intake estimated from regional data exceeded the MAWI for Wallago leeri by 9- to 12-fold for one-year-old children and approximately 8-fold for pregnant women. Preferential consumption of lower-Hg species, such as Pristolepis grootii, may increase allowable intake approximately 3.5-fold, a preliminary finding requiring confirmation.