In this study, we examined the confirmation and characteristics of fecal contamination by measuring the coliform and Escherichia coli (E. coli) counts in the Kinki region’s water bodies. In Lake Biwa, categorized as type AA, many points exceeded environmental standards under the old standard using coliform groups, but the counts were below the standard values under the new standard using E. coli. In rivers, a positive correlation was observed between the old and new standards for each type, but the E. coli counts in type A rivers were widely distributed compared with those in type B and C rivers. Furthermore, a detailed investigation at the location of Miyamaebashi Bridge in the Katsura River suggested that E. coli from sewage treatment plant effluent impacted water quality. E. coli survival rates significantly decreased after 2-3 days at water temperatures of 20 ℃ and 30 ℃, but 73-85% survival was observed at 5 ℃ even on the third day, confirming that upstream E. coli loads can affect downstream E. coli concentrations at low water temperatures. Predictions of E. coli concentrations from pollution sources to downstream locations closely matched observed values. Thus, it was revealed that under normal river flow and weather conditions, areas far from pollution sources could still be affected.

Combined sewer overflow (CSO) causes water pollution in the Hirano River in Osaka Prefecture. In this study, we surveyed the microbial flora of the river water surface during CSO discharge. Prokaryote 16S rRNA genes in water samples collected around the Hirano Ichimachi Drainage Station in August and September 2023 were analyzed by next-generation sequencing and correlated with water quality by canonical correlation analysis. Species of Flavobacterium, Acinetobacter, Limnohabitans, Flectobacillus, and Cloacibacterium as well as strictly aerobic Leadbetterella, Rheinheimera, and Vogesella, were observed to be predominant in the samples with less CSO influence. However, in the samples with high suspended solids and chemical oxygen demand concentrations during CSO discharge, anaerobic species of Arcobacter, Bacteroides, Tolumonas, Acidovorax, and Prevotella were observed to be predominant and an increase in the ratio of bacterial genera belonging to the phylum Firmicutes (e.g., Streptococcus, Faecalibacterium, and Ruminococcus) was also observed.

The annual average air temperature in Japan had been gradually declining from 2000 to the early 2010s, but since the cool summer of 2014, it has risen rapidly with some fluctuations. We investigated the impact of this rise in air temperature on the water temperature in Osaka Bay from April 2010 to March 2024. In winter (January to March average) , the water temperature has risen in tandem with the air temperature, and the annual minimum water temperature in Akashi Strait has risen by 1.59 ℃ per decade. As a result, the number of days with a daily average water temperature below 10 ℃ has decreased from 84 days in 2011 to 0 days from 2023 onwards. On the other hand, in summer (July to September average) , there was no significant increase in either the air temperature or the surface water temperature. The surface water temperature fluctuated owing to the effects of cold water flowing into the bottom layer of Osaka Bay from the bottom water of Kii Channel and the air temperature. Therefore, even in a hot summer, if cold water flows into the bottom layer, the surface water temperature did not rise. The seasonal variation in lower water temperature was markedly affected by the presence or absence of cold-water inflow.

We examined the operation and management of the batch-type activated-sludge process widely adopted in rural sewerage treatment plants, which are necessary to achieve the required water quality levels in the International Standard ISO 16075 guidelines for treated wastewater irrigation. The study revealed that the oxidative decomposition of organic matter, promotion of nitrification in the batch tank, and sedimentation property of activated sludge were related to the BOD concentration of treated water. We found that BOD and SS concentrations of treated wastewater were likely to meet the water quality level of category A in the ISO guidelines if the DO in the batch tank was managed to be 2–3 mg L^-1 at the end of the aeration period by maintaining the SVI below 100 mL g^-1. The residual chlorine, hydraulic retention time, SS concentration, and water temperature of the treated water affected the number of coliform groups in the treated wastewater, thus, the number of coliform groups was likely to meet the category A quality level if the CT product was more than 2.8 mg min L^-1. Reducing the SS concentration of the treated water was important to suppress the reduction of the chlorine disinfection effect.

When managers of water areas plan water quality management strategies in enclosed water areas, they refer to the calculation results of predicted impacts on water quality by using ecosystem models. Several phytoplankton growth and nutrient uptake equations in ecosystem models have been developed in previous studies; however, the differences in the calculation results of models for phytoplankton and nutrients in oligotrophic environments have not been clarified. In this study, we constructed a one-box ecosystem model with a closed condition and compared the differences in calculation results of models for phytoplankton biomass and nutrients obtained using different phytoplankton growth and uptake equations for an oligotrophic environment. The phytoplankton biomass was higher when using the model with no fixed phytoplankton cellular composition than when using the model with a fixed phytoplankton cellular composition. This is because the cellular composition of the phytoplankton decreased in response to a decrease in the nutrient concentration of the surrounding water in the model with no fixed phytoplankton cellular composition, leading to a decrease in the amount of nutrients required for growth. The nutrient concentrations of the surrounding water were lower when using the model that had no fixed phytoplankton cellular composition and considered cellular nutrients explicitly than when using other models. Therefore, this model was seen as more likely to represent oligotrophic environments.

To investigate a method of evaluating the maximum risk of various metal elements eluted from illegally dumped home appliances, pellet samples of copper, lead, zinc, and antimony, and crushed waste PC parts (printed circuit boards, cathode-ray tube glass, and liquid-crystal displays panels) were immersed in elution solution for 60 days. Then the elution amounts and behaviors of various elements were determined. The elution amounts of metals increased over a long period of time in both the pellet and waste PC crushed material samples, suggesting that the Notification 13 elution test with a shaking elution time of 6 h is not applicable to the evaluation of long-term elution behavior, such as estimating the maximum risk of illegally dumped waste. Although the elution rate after 60 days of immersion was lower in some cases than that of long-term shaking using a high-speed stirrer, the maximum elution amounts were not significantly different. Such results suggested that the immersion test method without continuous shaking was effective. Moreover, we found that the relationship between time and elution rate can be classified into several patterns.

A test was conducted to cultivate Asari clams using fertilizer on an artificial tidal flat located in Urasaki-cho, Onomichi City, Hiroshima Prefecture. A numerical simulation model was constructed to calculate the material circulation including water and mud, and the data obtained in the field test were explained well. It was found that the limiting factor of benthic microalgae (BMA) , which is the main food of Asari clams, changes with the season: phosphorus in autumn, water temperature and solar radiation in winter, and nitrogen from spring to summer. It was also found that growth inhibition in terms of the food requirement of Asari clams occurs significantly in spring. For this reason, it is recommended to time fertilization before the spring growth period. It was shown that the use of a large amount of fertilizer materials results in larger individuals, higher sales prices, and better cost₋benefit performance. However, considering the risk of hydrogen sulfide generation in bottom mud, it was concluded that the appropriate amount of fertilizer applied is approximately 0.4 pieces m^-2 (distance of approximately 2 m between fertilizer pieces) .

In this study, we examined the impacts of fly-ash-shell-blocks (FSBs) on the pH and nutrient environments in seawater to determine the usefulness of utilizing fly ash and scallop shell to form a marine macroalgal reef. Ordinary portland cement paste (OPC) , FSB, and FSB blended with blast furnace slag were placed in seawater at a liquid : solid ratio of 10 : 1, and the changes in pH and nutrient (SiO₂, PO₄, NH₄, NO₂, and NO₃) concentrations for 7 days were investigated. The experiments were conducted using blocks four months and one year after production. Whereas the pH of the original seawater was about 8.0, the pH after the OPC treatment increased to about 9.9 and only SiO₂ was eluted. With the FSB and blended FSB treatments, the pH increased to 8.2–8.5 and all the nutrients were eluted. The source of the nutrients in FSB and blended FSB should be the fly ash. The eluted nutrients would positively affect the growth of the attached kelp. FSB and blended FSB may be more useful for a macroalgal reef than OPC because of their smaller pH increase and greater nutrient leaching.

The particle behavior during stormwater runoff depends on particle characteristics, such as size and density, and for analyzing the behavior of particles, including microplastics, it is necessary to obtain basic data on the characteristics of various particles in environmental samples. In this study, we obtained number-based particle characterization data for all particles in road dust and sediment in a stormwater inlet by estimating particle density using solutions of different densities, calculating shape property values by image analysis, and identifying the material composition by ATR-FTIR. More than 90% of the particles in all samples were distributed in the density fraction higher than 1.6 g cm^-3. For morphological characteristics, the sediment in the stormwater inlet contained more small particles than road dust, and there might also be many aggregated particles, where fine particles had adhered to each other. About 40-50% of the particles were identified as synthetic polymer, and many synthetic polymer particles were distributed in the density fraction higher than 1.6 g cm^-3 and the Feret diameter fraction of 500-800 μm.

In the Dainineya River, located in the central area of Osaka City, microplastics were collected and analyzed regarding their density, polymer types, shapes, and colors. The investigation followed the “Guidelines for Microplastic Investigation in Rivers and Lakes (Ministry of the Environment, Government of Japan) .” Microplastic collection was conducted by the natural water flow method, and under low flow conditions, the net towing method (an original method) was employed. Sample analysis was performed according to the guidelines. Upon comparing the results of the natural water flow method and the net towing method for microplastics sized 1–5 mm, the densities were found to be 41.0 particles m^-3 and 9.68 particles m^-3, respectively. While questions were raised regarding the quantitative comparison between the net towing method and the natural water flow method, the former was considered to capture the characteristics of microplastics in the target river equally well from a qualitative perspective.

In Lake Inbanuma, located in the northern part of Chiba Prefecture, the ratios of nonpoint sources to the total runoff load are significant, being approximately 80% of COD, 70% of T-N, and 40% of T-P in the total runoff load originating from nonpoint sources. The Lake Water Quality Conservation Plan estimates the nonpoint source load through unit loading, but the load may vary significantly depending on precipitation volume and patterns. In this study, we focused on road runoff, a major component of urban areas, which contributes significantly to the nonpoint source load, and investigated the relationships between precipitation volume and the loads of COD_Mn, nitrogen, and phosphorus in Chiba Prefecture. On the basis of 22 rainfall surveys using an automatic water sampler and the results of 10 surveys where runoff from road drainage was sampled from the start to the end of the runoff, relationships were obtained between total precipitation and the one-rainfall loads of COD_Mn, T-N, NO₃-N, T-P, and PO₄-P. These relationships enable load estimation based on precipitation volume, suggesting that COD_Mn in the unit values for urban areas in Chiba Prefecture may be underestimated.

We investigated the optimization of the automated analysis of 12 metals (B, Al, Cr, Fe, Mn, Cu, Zn, As, Se, Cd, Hg, and Pb) and the effect of additives using an automated acid decomposition system based on the continuous flow analysis (CFA) method. We investigated the effect of additives, including HCl, Na-containing reagents, and various oxidizing reagents, on the recovery of Hg in HNO₃. Adding HCl and NaClO had a remarkable effect and improved the recovery from 70% to 100%. On the basis of the conditions obtained in this study, it was possible to automate the measurement process through pretreatment and the simultaneous inductively coupled plasma mass spectrometry (ICP-MS) analysis of metal elements containing Hg in various samples even if the samples contain interference substances.

The inflow amounts and transport processes of microplastics from every source should be further understood to determine the risk management of microplastics in terrestrial water bodies. Fertilizers containing coated microcapsules (diameter of 2–5 mm) applied to paddy fields can be a source of microplastics. In this study, the amount and characteristics of microcapsules discharged from the water drain of paddy fields were investigated in six paddy fields for two years. Year-round monitoring in two fields showed that more than 90% of the annual amount was discharged between starting irrigation and transplanting. The amounts of microcapsule discharges before transplanting from four fields were 70–520 × 10³ particles ha^-1 (ave. 243 × 10³ particles ha^-1) , 0.17–1.29 kg ha^-1 by weight (ave. 0.61 kg ha^-1) , and the discharge rate relative to the amount applied in the previous year was 0.9–6.7%. The microcapsule discharge was considered to be caused by standing water drainage, as well as overflow due to rainfall events. From one field out of six, the amount of discharge after transplanting may be equivalent to that before transplanting owing to insufficient water management.

Cyanobacterial blooms caused by the genus Microcystis have been confirmed to occur in Lake Shinji. In this study, we used preserved specimens of the genus Microcystis to measure cell diameter and intercellular distance. Differences in cell diameter were observed, with M. ichthyoblabe being 3.3 to 4.0 μm and M. aeruginosa being 4.5 to 4.9 μm. The distance between M. aeruginosa cells was relatively small with an average of 1.1 to 2.8 μm, whereas that between M. ichthyoblabe cells was from 0.9 to 5.7 μm. It was found that there are cases where they are densely packed and where they are relatively dispersed. Regarding the water quality in the upper layer of Lake Shinji when cyanobacterial blooms occurs, the average Cl⁻ concentration for two months before the occurrence is often less than 2,000 mg L^-1. It became clear that the PO₄-P concentration tended to be higher when cyanobacterial blooms occurred.

To increase the atmospheric CO₂ absorption amount achieved through conservation and restoration activities for seaweed beds, it is important to select a place suitable for the growth of seaweed on the basis of information on the growth conditions of seaweed in the past and present. In this study, to contribute to the selection of activity location, and the setting of target values for CO₂ absorption, we selected sea areas and locations on the basis of previous surveys of Japanese seaweed beds carried out before the 1990s, when the decline and disappearance of seaweed beds began to become notable, and estimated the CO₂ absorption potential (tons CO₂ ha^-1 yr^-1) for 68 locations nationwide. The CO₂ absorption potential ranged from 0.02 to 18.0. When comparing the absorption potential by sea area and seaweed bed type, we found that the absorption potential was particularly high in the northern Sea of Japan coastal area and the Seto Inland Sea for Sargassum beds, in the East China Sea coastal area and the Pacific coast of Shikoku for Eisenia and Ecklonia beds, and in the Tohoku Pacific coastal area for Laminaria beds.

The pond in a park serves as a recreational waterside area; however, problems such as foul odors and landscape deterioration due to eutrophication persist. Despite these problems, quantitative evaluations of the impact of fallen leaves on water quality remain limited. Such evaluations are essential for developing effective tree planting and leaf litter management strategies. This study focused on Lake Nanko, a park pond in Shirakawa City, Fukushima Prefecture, in 2023. Fallen-leaf-derived nutrient loads were estimated through field surveys and laboratory experiments. The total nutrient load in Lake Nanko was determined by summing the fallen-leaf-derived nutrient load, river inflow, and nutrient elution from sediments. Annual leaf-derived nutrient loads entering the lake from the water surface were estimated at 9.51 kg-N (total nitrogen, T-N) and 0.480 kg-P (total phosphorus, T-P) , while riverine loads were 0.35 kg-N and 0.022 kg-P. The proportions of leaf-derived loads relative to the total nutrient load were 0.1% for T-N and 0.7% for T-P annually. Even during the leaf-fall season, the contributions were 0.1% for T-N and 2.6% for T-P. These findings indicate that nutrient loads from fallen leaves have only a limited impact on the overall water quality of Lake Nanko.

In this study, we evaluated the characteristics of the days of the week and parts of the day that sampled river water for which environmental quality standards are available for the conservation of the living environment based on the analysis of regular water quality monitoring data collected from FY2013 to FY2022 at environmental reference points in Japanese rivers. We also evaluated days-of-the-week- and parts-of-the-day-specific variations of each water quality parameter. The days-of-the-week sampling of river water concentrated in the middle of the week, particularly on Wednesdays. In addition, most river water samplings were conducted in the morning. There were no apparent days-of-the-week-specific variations in water quality parameters. However, the values for pH, biochemical oxygen demand, suspended solids, dissolved oxygen, and total zinc tended to be on average slightly higher in the afternoon than in the morning on many days of the week. The potential biases resulting from the timing of samplings and parts-of-the-day-specific variations in water quality parameters should be considered when discussing the environmental quality and effluent standards for these parameters.