Abstract: In Türkiye, where agriculture consumes 75% of available water and national irrigation efficiency is only 51.3%, irrigation modernization—the conversion of classical open-channel irrigation systems to pressurized pipe systems—presents a primary strategy to achieve significant water savings. This study provides a comprehensive economic assessment of the potential of this strategy. A twofold methodology was employed: first, a cost-benefit analysis (CBA) of the 36,108 ha Ivriz irrigation project, and second, a national model to simulate the economic impact of modernizing nation’s 4.9 million hectares currently irrigated by such classical systems. The Ivriz case study reveals that project viability is entirely contingent on the on-farm efficiency achieved post-modernization. At 60% efficiency, water savings are insufficient to make the project economically feasible, whereas at 90% efficiency, substantial water savings render the project highly profitable. At the national level, the analysis indicates that the conserved water could be used to expand Türkiye’s irrigated area by 1.77-2.98 million hectares, generating an additional $3.47-$5.84 billion in annual agricultural income. The findings conclude that while modernization represents a powerful investment, its success requires a comprehensive policy framework that not only funds infrastructure conversion but also mandates integrated support programs to ensure farmers adopt the high-efficiency technologies needed to achieve these savings.

Abstract: Water shortage is a major problem that affects sustainability of agricultural sector in northeastern Thailand especially areas remote from a main river. This study aims at developing a system for water shortage risk assessment at sub-district level in Maha Sarakham Province. QSWAT model for sub-watershed streamflow simulation was integrated with WEAP model to analyze water balance and assess water shortage based on water demand from five sectors: non-irrigated agriculture, irrigated agriculture, consumption, services, and industry. The findings revealed that both models provided the results of calibration, and the validated results of streamflow analysis were at satisfactory to good level. From spatial analysis, distribution of provincial water resource was significantly different. Sub-districts located along the Chi River had high streamflow but low water shortage while those from central to southern part had limited streamflow although water demand was high especially for agricultural sector. According to temporal analysis, critical period of water shortage was found in the dry season and seasonal transition after post-harvest period. Integration of data on streamflow and water demand can be useful to divide subdistricts into three group: positive water balance, balance, and negative water balance. Most sub-districts were classified as negative water balance that needed urgent measures for water resource development. These findings have provided important data for planning water resource management locally and supporting development of drought mitigation measures for vulnerable areas in Maha Sarakham Province.

Abstract: Climate change leads to reservoir management challenges especially in areas with high risk of drought and flood. Traditional reservoir rules curves are inappropriate for addressing variation of reservoir inflow. This study presents an integration framework between GCMs from CMIP6 (ACCESS-CM2, MIROC6, and MPI-ESM1-2-LR) under SSP245 and SSP585 scenarios and WEAP, which is validated in accuracy for reservoir inflow and storage capacity. This integration contributes to Hippopotamus Optimization (HO), a technique used to develop Resilience Reservoir Rule Curve (RRRC) for Ubonrat reservoir during 2024-2055 employing dual-objective function that emphasizes the reduction of water shortage and water excess. The results indicate that RRRC developed by HO is more efficient and suitable than Honey Bee Mating Optimization (HBMO) and existing rule curve. After testing the RRRC with historical inflow and future inflow from three GCMs under SSP245 and SSP585, it can reduce average water shortage and demonstrate outstanding efficiency in water excess management. This potential reflects its adaptability under future variation of hydrological condition. This crucial finding illustrates that the integration framework can develop resilient rule curves under uncertainty. HO integrated with various models can be implemented as an optimal framework and have high potential for reservoir operation planning under climate change. The developed methodology can be implemented in other reservoirs to gather additional factors for sustainable promotion of water resource resilience.

Abstract: Recently, several studies from developing economies have reported the presence of per- and polyfluoroalkyl substances (PFAS) in water bodies, with a dominance of Perfluorooctanoic acid (PFOA), a potential endocrine disruptor. In this study, an engineered sugarcane bagasse biochar–chitosan composite (SBCT) was designed, synthesized, and evaluated as an adsorption medium for the removal of PFOA from aqueous systems at concentrations up to 500 ppb in water. Batch adsorption experiments were conducted to investigate the effects of initial PFOA concentration, contact time, pH, adsorbent dosage, and temperature. Scanning electron microscopy (SEM) showed that SBCT has a significant porous structure. The composite showed over 90% of PFOA removal from water. Further, the presence of peaks corresponding to C-F bonds after adsorption by Fourier transform infrared (FTIR) Spectroscopy analysis confirms the adsorption of PFOA on SBCT. The protonated amine groups (NH₃⁺) in chitosan enhanced the adsorption of anionic PFOA through electrostatic attraction with carboxyl groups (COO⁻). The Kinetic study revealed that Pseudo-first order best described the adsorption process, with equilibrium adsorption capacity (qeq) of 2.78 mg/g, suggesting that physisorption is the predominant mechanism. The Langmuir Isotherm model gave the best fit, establishing a maximum adsorption capacity (qmax) of 9.08 mg/g. Thermodynamic analysis revealed that the adsorption process was spontaneous and exothermic, consistent with physisorption. The regeneration capacity of the SBCT composite demonstrated exceptional reusability across five adsorption-desorption cycles with methanol. The adsorption kinetics, equilibrium behavior, and regeneration efficiency suggest that SBCT is a viable low-cost adsorbent for batch adsorption-based treatment systems targeting PFOA removal, particularly in decentralized and resource-constrained water treatment applications.

Abstract:

With accelerating climate change and urbanization, river catchments continue to experience structural modifications through dam construction and concrete-lining of natural channels as adaptation measures. These interventions can alter the natural hydrology. This necessitates assessment of their influence on hydrology at a catchment scale. However, such evaluations are particularly challenging in data-scarce regions such as the Chongwe River Catchment, where hydrometric records capturing conditions before and after structural modifications are limited. Therefore, we applied a 2D rain-on-grid approach in HEC-RAS to evaluate changes in high-flow characteristics in the Chongwe River Catchment in Zambia, where structural interventions have been implemented. The terrain was modified in HEC-RAS to represent 21 km of concrete drains and ten dams. Sensitivity analysis was conducted on five model parameters and showed that Manning’s roughness coefficient had by far the largest impact on peak flows. Model calibration and validation showed strong performance with R² = 0.99, NSE = 0.75 and PBIAS = – 0.68 % during calibration and R² = 0.95, NSE = 0.75, PBIAS = – 2.49 % during validation. Four scenarios were simulated to determine the hydrological effects of channel concrete-lining and dams. The results showed that concrete-lining of natural channels in the urban area increased high flows at the main outlet by approximately 4.6%, generated very high channel velocities of up to 20 m/s, increased flood depths by up to 11%, and expanded flood extents by up to 15%. The existing dams reduced peak flows by about 28%, increased lag times, reduced flood depths by about 11%, and reduced flood extents by up to 8% across the catchment. The findings demonstrate that enhancing stormwater conveyance through concrete-lining must be complemented by storage to manage high flows, while future work should explore nature-based solutions to reduce channel velocities and improve sustainable flood mitigation.

Abstract: This study investigates how a Long Short-Term Memory (LSTM) model inter-nally represents baseflow contributions in snowmelt-driven, semi-arid mountain basins with heterogeneous geologic characteristics. Five basins in the Sangre de Cristo Mountains of northern New Mexico, spanning fractured Precambrian bedrock and sedimen-tary-volcanic terrain, were used to evaluate both model performance and interpretability. Baseflow dynamics were inferred post hoc using the Baseflow Index (BFI) and a two-reservoir HEC-HMS (Hydrologic Engineering Center’s Hydrologic Modeling System) model. Although baseflow was not explicitly included in model training, internal cell state activations showed strong correlations with both shallow and deep baseflow com-ponents derived from the HEC-HMS model. To better understand how these relationships may change under climatic stress, BFI-based baseflow patterns were further analyzed un-der pre-drought and drought conditions. Results indicate that the LSTM learned to inter-nally distinguish between short- and long-residence flowpaths, encoding physically meaningful hydrologic behavior. This work demonstrates the potential for LSTM models to offer valuable insights into baseflow generation and groundwater–surface water inter-actions, particularly critical in water-scarce regions facing increasing drought frequency.

Abstract:

Accurate snow monitoring is critical for understanding hydrological processes and managing water resources. However, traditional snow sensing networks in the United States, such as the United States Department of Agriculture’s (USDA) SNOwpack TELemetry (SNOTEL) system, are costly and limited in spatial coverage. This study presents the design and deployment of a lower-cost, open-source snow sensing station aimed at improving the accessibility and affordability of snow hydrology monitoring. The system integrates research-grade environmental sensors with an Arduino-based Mayfly datalogger, providing high temporal resolution measurements of snow depth, radiation fluxes, air and soil temperatures, and soil moisture. Designed for adaptability, the station supports multiple sensor types, various power configurations—including solar and battery-only setups—multiple telemetry options, and capability for diverse deployment environments, including forested and open terrain. A multi-site case study at Tony Grove Ranger Station in northern Utah, USA demonstrated the station’s performance across different physiographic conditions. Results show that the system significantly reduces costs while increasing the spatial resolution of data, offering a scalable solution for enhancing snow monitoring networks. This study contributes an open-source hardware and software design that facilitates replication and adaptation by other researchers, supporting advancements in snow hydrology research.

Abstract: Fish passage through turbines is one of the main environmental impacts of hydropower. Turbine type is a key factor influencing fish survival, and widespread Kaplan turbines are generally considered less dangerous than other turbine types. Nevertheless, while large Kaplan turbines have been extensively studied, there is limited empirical evidence about the biological impact of small, high-speed Kaplan turbines on fish survival. In this study, we conducted controlled in situ fish experiments at a small and low head hydropower plant (1 MW; head 8 m) using balloon tags and pressure sensors to quantify real mortality in two horizontal Kaplan turbines operating at full capacity: one small turbine (1.2 m Ø, 500 rpm and 5 m³/s) and one larger unit (1.55 m Ø, 300 rpm and 8 m³/s). Fish (9.5–19 cm) were released into the intake flow and monitored post-passage. Results showed higher mortality in the small turbine, with ~80% in 24 hours, many exhibiting severe mechanical injuries such as complete sectioning of the head or spinal cord, with significantly higher mortality in greater fish. In contrast, the larger turbine showed a ~60% mortality rate, and fewer traumatic injuries. Our findings highlight the underestimated impact of small, high-rpm Kaplan turbines on fish survival and underscore the need for adaptive turbine operation or structural modifications to minimize ecological damage during critical migration periods.

Abstract: Accurate snowmelt runoff prediction is critical for water resource management in mountainous regions where seasonal snowpack constitutes the dominant water supply. This study demonstrates operational application of the degree-day based Snowmelt Runoff Model (SRM) integrated with Geographic Information Systems (GIS) and multi-platform remote sensing for discharge forecasting in the Kirkgoze Basin (242.7 km², 1823-3140 m elevation), Eastern Anatolia, Turkey. Three automatic weather stations spanning 872-m elevation gradient provided meteorological forcing, while MODIS MOD10A2 8-day composite products supplied operational snow cover observations validated against Landsat-5/7 (30-m resolution, 87.3% agreement, Kappa=0.73) and synthetic aperture radar imagery (RADARSAT-1 C-band, ALOS-PALSAR L-band). Uncalibrated model performance was modest (R²=0.384, volumetric difference=29.78%), demonstrating necessity of site-specific calibration. Systematic adjustment of snowmelt and rainfall runoff coefficients yielded excellent calibrated performance for 2009 melt season: R²=0.8606, correlation coefficient R=0.927, Nash-Sutcliffe Efficiency=0.854, volumetric difference=3.35%. Enhanced temperature lapse rate (0.75°C/100m vs. standard 0.65°C/100m) reflected severe continental climate. Multiple linear regression analysis identified temperature, snow-covered area, snow water equivalent, and calibrated runoff coefficients as significant discharge predictors (R²=0.881). Results confirm SRM's operational feasibility for seasonal forecasting and flood warning in data-scarce snow-dominated basins, with modest requirements (daily temperature, precipitation, satellite snow cover) aligning with operational monitoring capabilities. The methodology provides transferable framework for regional water resource management in climatically-vulnerable mountain environments where snowmelt supports agriculture, hydropower, and municipal supply.

Abstract: CeO₂–ZnO materials were synthesized via a modified sol–gel method, varying the ZnO loading to evaluate its effect on the photocatalytic degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) under UV irradiation. XRD analysis confirmed the cubic fluorite structure of CeO₂ and, at higher loadings, the coexistence with a hexagonal wurtzite phase. At low loadings, Zn²⁺ incorporation into the CeO₂ lattice was evidenced, leading to lattice parameter contraction and the generation of oxygen vacancies. BET and SEM analyses indicated that an intermediate ZnO content (CZ1.5) increased the surface area (~41%) without collapsing the mesoporosity. XPS revealed Ce³⁺ species, defects, and Zn–O–Ce bonds, consistent with the formation of an S-scheme heterojunction. UV–Vis spectra showed similar Eg values (~3.13 eV) across the series, indicating that the photocatalytic enhancement is driven by structural and interfacial effects rather than optical changes. The CZ1.5 catalyst achieved ≈80% degradation and 74% mineralization (TOC), approximately twice that of pure CeO₂, attributed to efficient charge separation and the generation of reactive species O₂•⁻ and •OH. Furthermore, a temperature treatment CZ1.5 calcined to 400°C was the most active above the material treated at 300 and 500 °C.

Abstract: Quality control of drinking water is essential for safeguarding public health, particularly in densely populated urban environments. Environmental microbiological monitoring can enhance traditional surveillance by providing deeper insights into the dissemination of pathogens and antimicrobial resistance genes in aquatic systems. In this study, we evaluated the quality of wastewater and treated water from two urban water supply systems, representing the southern and northern regions of Porto Alegre, Rio Grande do Sul, Brazil, across four climatic seasons between 2024 and 2025. Fifteen water samples, comprising raw water from Guaíba Lake and treated water from public distribution points, were analyzed. The Water Quality Index was calculated, microbiological indicators were quantified, and carbapenem resistance genes were detected using molecular assays. Most treated water samples complied with bacteriological standards; however, the blaOXA-48-like gene was recurrently identified in both wastewater and treated water. No resistance genes were detected during the summer, while the blaVIM gene appeared exclusively in spring samples. The detection of carbapenem resistance genes in the absence of cultivable coliforms suggests the persistence of extracellular DNA or viable but non-culturable bacteria, underscoring limitations of conventional microbiological monitoring. Integrating classical microbiological methods with molecular assays provides a more comprehensive evaluation of water quality and supports decision-making within a One Health framework.

Abstract: Ningbo (NGB), one of the world's most important port cities located on the East Coast of China, contains more than 100 rivers and streams across three major catchments, the Yong, Yuyao and Fenghua Rivers. During the 1970s – 2000s, extensive river engineering, including channelisation, conversion of natural rivers into artificial canals, and construction of sluice gates and embankments were undertaken to cope with rapid urbanisation and development. Since the 2010s, the Ningbo Government and Water Bureau have initiated smart river and fluvial flood management strategies to enhance digital twins and smart flood management technologies, such as 3D flood mapping and real-time water level and discharge monitoring, significantly improving precision. In this study, we demonstrate that smart technology has performed effectively in Ningbo, with applications in the recent climate extreme events such as Typhoon In-Fa and Muihua in 2021 and 2022, during which the Municipal Bureau has safeguarded public safety and welfare. This further strengthening both municipal and national commitment to enhance climate resilience. Nevertheless, further advancement of the DT platform remains necessary. Key areas for improvement include faster computational capacity, enhanced coordination across departments and open data sharing mechanisms, and integration of artificial intelligence (AI) to support more effective decision-making processes in response to the climate extremes and adverse water hazards conditions.

Abstract: The toxic effects of fluorides on aquatic organisms have raised widespread concerns on their pollution in water bodies. This study derived water quality criteria for fluorides by collecting acute and chronic toxicity data globally and conducted an ecological risk assessment of fluoride concentrations in China's surface water: the acute toxicity data covered 34 species across 14 families and 4 phyla, while the chronic toxicity data covered 7 species across 5 families and 3 phyla; Using the species sensitivity distribution (SSD), the water quality criteria SWQC and LWQC were determined to be 17.47 mg/L and 3.334 mg/L, respectively; An ecological risk assessment of fluoride concentrations in China's surface water was conducted using the risk quotient (RQ), identifying the Shahe River, Xihe River, Ebinur Lake, and Chagan Lake as high risk areas among 32 river and lake basins. Among 21 provincial-level administrative regions, the Guangxi Zhuang Autonomous Region was assessed as no risk, while the remaining river and lake basins and administrative regions were classified as moderate risk or low risk. The findings of this study can serve as a reference for subsequent research and detailed optimization in related fields.

Abstract: Rapid urbanization has intensified challenges in urban water management, highlighting the growing importance of sponge city development. However, at present, the unique role of groundwater aquifers in regulating the water cycle process has been ignored, and overly simplistic construction methods are relied on. To address these limitations, this study introduces an integrated spatial optimization framework for urban-scale sponge ecosystems by coupling hydrodynamic modeling, ArcGIS-based spatial analysis, and Monte Carlo simulation. The framework systematically incorporates surface water depth, formation lithology, and groundwater depth to construct a comprehensive evaluation system for sponge ecological suitability. Shenzhen serves as the empirical case to demonstrate the framework’s applicability. Surface water depth is quantified using the Finite-Volume Community Ocean Model (FVCOM), while groundwater conditions are assessed through a calibrated groundwater model, thereby improving the scientific precision of ecological suitability evaluation. Leveraging GIS and Monte Carlo simulation, the study develops a streamlined yet robust methodology for optimizing the spatial configuration of sponge ecological infrastructure. Building on ecological redlines and protected areas, a landscape ecology–oriented matrix–corridor–patch analysis is applied to delineate ecologically suitable zones. Tailored planning and management strategies are subsequently formulated for each zone based on its ecological attributes. The findings offer methodological advances for sponge ecosystem construction and contribute to enhancing urban climate resilience and adaptive capacity under accelerating environmental change.

Abstract:

Driving rain or ‘wind-driven rain’ (WDR) arrives at the ground on an oblique trajectory, and drops may strike at a speed greater than their still-air terminal velocity. Oblique rain can affect a range of geomorphic processes including the splash dislodgment and transport of soil particles, and hydrological processes including overland flow, canopy interception and the generation of stemflow. The mean rain inclination angle at which WDR strikes the ground has been estimated from the catch of paired gauges, one with a conventional horizontal orifice, and one with a vertical orifice. Such data allow the resolution of rain vectors to find the rain inclination. This can only be carried out over periods sufficiently long for a measurable rain depth to be measured, and does not permit the real-time recording of rain inclination. Here, a new acoustic method for measuring rain inclination is introduced that provides an inexpensive tool for the continuous, real-time monitoring of WDR. Furthermore, the method also permits the simultaneous recording of rainfall duration and intermittency at high temporal resolution, with no additional apparatus. Data on rain inclinations collected during showers on a tropical coast exposed to strong trade-winds are presented to illustrate the operation of the acoustic measurement system. However, the focus of this paper is the presentation of the new method itself, and not on the climatology of WDR.

Abstract: Leachate in landfills becomes difficult to treat due to its complex and widely variable composition, containing a large amount of organic, inorganic substances and heavy metals. When it seeps into the ground, leachate pollutes groundwater, and if discharged into surface water, it will harm the aquatic environment in the corresponding area. Therefore, it is extremely necessary to treat leachate before discharging it into the environment to prevent this negative impact. In this study, a lab-scale A2O (Anaerobic–Anoxic–Oxic) system integrated with a Moving Bed Biological Reactor (MBBR) was established. We evaluated key water quality indicators of wastewater pretreated by internal electrolysis, the effluent from the A2O–MBBR system, and the combined treatment process. The wastewater was taken from Nam Son landfill, Soc Son, Hanoi, in Viet Nam. COD, BOD5, NH4+-N, and pH of the input leachate wastewater were 2140 mg/L, 250 mg/L, 895 mg/L, and 8 ± 0.5, respectively. The conditions of internal electrolysis were as follows: 120 minutes of reaction time, pH =4, 4.0 g/L Fe/Cu dosage and 100 mg/L PAM dosage. Following the internal electrolysis pretreatment, the removal efficiencies of COD, BOD₅, and NH₄⁺–N reached 49.0%, 4.8%, and 11.2%, respectively. After 24 hours of operation, the integrated treatment process exhibited markedly enhanced performance, achieving removal rates of 85.0% for COD, 85.2% for BOD₅, 94.1% for total nitrogen, 98.0% for total phosphorus, and 96.7% for NH₄⁺–N. These results demonstrate the high synergistic efficiency of the combined internal electrolysis–A₂O–MBBR system. Furthermore, all post-treatment parameters complied with the Vietnamese standard QCVN 40:2011/BTNMT (Column B2) for leachate wastewater, confirming its effectiveness and environmental suitability.

Abstract: Flood susceptibility mapping is crucial for understanding flood-prone areas and mitigating the associated risks, particularly in vulnerable regions like the Sebeya Catchment. This study has adopted a GIS-AHP approach integrated with local community knowledge over flood susceptibility factors such as Topographic Wetness Index (WTI), Digital Elevation Model (DEM), Precipitation, Slope, Land Use/ Land Cover (LULC), Normalized Vegetative Index (NDVI), Distance to Roads, Distance to River, and Drainage Density. The pairwise comparison matrix was used to determine each factor's weight according to its influence in inducing flood. The findings revealed that 33.1% of the total area has a very and high susceptibility to floods, whereas the rest part of the catchment is moderately susceptible to floods. Most social economic activities in this study are located in high-risk zones, which significantly to appearance of flooding impacts. Current study indicates that, damage to infrastructure, loss of livelihoods, displacement of communities, and increased costs of disaster response are key consequences observed in affected regions. A confusion matrix approach was employed to validate the flood susceptibility map, and the results indicate an overall accuracy of 0.92, confirming strong model performance and reliability. The study further proposes adaptive strategies and provides recommendations for enhancing flood resilience, including improvement in land-use planning, use of early warning systems, and sustainable catchment management. Further studies should develop an economic-loss prediction model based on flood-susceptibility mapping.

Abstract: Minerals extraction from brine solutions is a vital issue for resource recovery in many fields of industry, especially in desalination processes. Usually, the solubility limit is viewed as a key factor that plays a determinant role in the efficiency of a prescribed process. This paper suggests the investigation of the influence of ionic strength, which is a measure of the total concentration of all dissolved ions, on the solubility limits in brines that are extracted from desalination facilities in Kuwait before discharging them into the Arabian Gulf. This information may contribute to the optimisation of the extraction process, especially when treating complex brines with high salinity, and in the case of interest, the mineral extraction from the brine stream may result in the reduction of environmental risk. For this purpose, the solubility of two main minerals (CaSO4 and Mg(OH)2) was measured for several values of ionic strength achieved by adjusting the concentration of the brine solutions. Results show a non-linear relationship between ionic strength and the solubility limit of the target minerals, with behavior similar to that that could be found in the literature. In the case of CaSO4, it was found that the behavior of the solubility in the case of a diluted solution or low ionic strength is not the same as in the case of a brine solution; the latter has a reverse pattern. On the other hand, the solubility of Mg(OH)2 in Kuwait brine water was shown to decrease as the ionic strength of the brine solution decreased. Other minerals obtained by the extraction process are under analysis. The findings of this work provide crucial insights for process design, enabling more precise control over precipitation steps and enhancing the overall yield and economic viability of mineral extraction from complex brine resources.

Abstract: Contaminated water detoxification remains difficult due to the presence of persistent halo-organic contaminants, such as perfluorooctanoic acid (PFOA) and chlorophenols, which are chemically stable and resist conventional purification methods. Functionalized membrane-based separation and decontamination have garnered immense attention in recent years. Commercially available microfiltration membrane (PVDF) and polymeric non-woven fiber filters (glass and composite) are functionalized with Poly (methacrylic acid) (PMAA) that shows outstanding pH responsive performance and tunable water permeability under ambient conditions perfect for environmental applications. Polymer loading based on weight gain measurements on PMAA-Microglass composite fibers (137%) and Microglass fibers (116%) confirmed their extent of functionalization, which was significantly greater than that of PVDF membrane (25%) due to its wide effective pore diameter. Presence of chemically active hydrogel within PVDF matrix was validated by FTIR (hydroxyl/carbonyl) stretch peak, substantial decrease in contact angle (68.8° ± 0.5° to 30.8°± 1.9°), and decrease in pure water flux from 509 to 148 LMH/bar. Nanoparticles are generated both in solution and within PVDF membranes using simple redox reactions. This strategy is extended to PVDF-PMAA membranes, which are loaded with Fe/Pd nanoparticles for catalytic conversion of 4-chlorophenol and PFOA, forming Fe/Pd-PVDF-PMAA systems. 0.25 mg/L Fe/Pd nanoparticles synthesized in solution displayed alloy-type structures and demonstrated a strong catalytic performance, achieving complete hydrogenation of 4-chlorophenol to phenol and 67% hydrogenation of PFOA to its reduced form at 22-23 °C with ultrapure hydrogen gas supply at pH 5.7. These results underscore the potential of hybrid polymer–nanoparticle systems as a novel remediation strategy, integrating tunable separation with catalytic degradation to overcome the limitations of conventional water treatment methods.

Abstract: Japan has largely completed the “first half” of SDG 6—universal access to safe drinking water and sanitation—through decades of investment in water supply and sewerage systems, total pollutant load control and stringent regulation of industrial effluents. National indicators show that coverage of safely managed drinking water and sanitation services is close to 99%, and domestic statistics report high achievement rates of BOD/COD-based environmental standards in rivers, lakes and coastal waters [6–11]. At the same time, SDG 6.3.2 (good ambient water quality) remains at 57% and SDG 6.6.1 (extent of water-related ecosystems) shows a −2.8% change in permanent surface water area compared with the 2000–2019 baseline [9,12,13]. The present review synthesizes how far Japan has progressed towards SDG 6, why gaps remain in the “second half” of the goal and which policy pathways are most promising for a nature-positive water future. Using national statistics from the Ministry of the Environment and the Ministry of Land, Infrastructure, Transport and Tourism (1970–2023), UN-Water SDG 6 indicators and recent literature, we map domestic indicators onto the SDG 6 framework, analyse long-term trends in BOD/COD achievement rates and organise remaining challenges using the DPSIR (Drivers–Pressures–State–Impacts–Responses) perspective [2–4,8,9,17,29]. A simple structural break analysis confirms that improvement in BOD/COD achievement essentially stalled around 2002, reinforcing policy concerns that point-source measures alone can no longer deliver additional gains [8,22]. A comparison of SDG 6.3.2 with domestic environmental standard metrics explains why the former is systematically lower: multi-parameter assessment, stricter aggregation rules and greater sensitivity to nutrients and ecological conditions mean that water bodies passing BOD/COD standards may still fail under SDG 6.3.2 [8,9,13]. Building on this diagnosis, the review proposes six strategic directions—climate-resilient water systems, infrastructure renewal and smart asset management, advanced treatment and pollution prevention, integrated water resources management and governance, citizen participation and nature-based solutions (NbS) and green–gray hybrids—and summarises their expected contributions and constraints in a concise overview table [12–14,16,17,24,26]. Finally, we outline how integrated indicator dashboards and, in the longer term, basin-scale simulation platforms (“digital twins”) could help link SDG 6 indicators, domestic statistics and scenario analysis in a transparent, reproducible way [8,9,25,26]. The paper concludes with research and policy priorities for closing the remaining gaps in SDG 6.3 and 6.6 and for advancing towards a nature-positive water future.