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Review

Pathways for SDG 6 in Japan: Challenges and Policy Directions for a Nature-Positive Water Future

by
Qinxue Wang
1,*,
Tomohiro Okadera
1,
Satoshi Kameyama
1 and
Xinyi Huang
1,2
1
National Institute for Environmental Studies (NIES), Tsukuba 305-8506, Japan
2
Graduate School of Life and Earth Sciences, University of Tsukuba, Tsukuba 305-8577, Japan
*
Author to whom correspondence should be addressed.
Sustainability 2026, 18(2), 994; https://doi.org/10.3390/su18020994
Submission received: 4 December 2025 / Revised: 1 January 2026 / Accepted: 12 January 2026 / Published: 19 January 2026
(This article belongs to the Special Issue Sustainable Water Management: Advanced Treatment and Nature-Based Solutions)
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Abstract

Japan has largely achieved the “first half” of SDG 6—universal access to safe drinking water and sanitation—through decades of intensive investment in water supply and sewerage systems, implementation of the Total Pollutant Load Control System, and stringent regulation of industrial effluents. National indicators show that coverage of safely managed drinking water and sanitation services is nearly 99%, and domestic statistics report high compliance rates for BOD/COD-based environmental standards in rivers, lakes, and coastal waters. Conversely, the “second half” of SDG 6 reveals persistent gaps: ambient water quality (6.3.2) remains at 57% (2023 data), while water stress (6.4.2) is at approximately 21.6%. Furthermore, SDG 6.6.1 shows that 3% of water basins are experiencing rapid changes in surface water area (2020 data), with ecosystems increasingly threatened by hypoxia in enclosed bays and climate-induced vulnerabilities. Drawing on global comparisons, this review synthesizes Japan’s progress toward SDG 6, elucidates the structural drivers for remaining gaps, and proposes policy pathways for a nature-positive water future. Using national statistics (1970–2023) and the DPSIR framework, our analysis confirms that improvements in BOD/COD compliance plateaued around 2002, reinforcing concerns that point-source measures alone are insufficient to address diffuse pollution, groundwater nitrate contamination, and emerging contaminants like PFAS. We propose six strategic directions: (1) climate-resilient water systems leveraging groundwater; (2) smart infrastructure renewal; (3) advanced treatment for emerging contaminants; (4) basin-scale IWRM enhancing transboundary cooperation; (5) data transparency and citizen engagement; and (6) scaled nature-based solutions (NbS) integrated with green–gray infrastructure. The paper concludes by outlining priorities to close the gaps in SDG 6.3 and 6.6, advancing Japan toward a sustainable, nature-positive water cycle.

1. Introduction

Over the past half-century, Japan has transformed from a state of severe industrial and domestic water pollution to one of near-universal access to safe drinking water and sanitation. During the period of rapid economic growth from the 1950s to the 1970s, untreated industrial effluents and municipal wastewater caused extremely high BOD levels and widespread contamination in rivers, lakes, and enclosed coastal waters, contributing to well-known pollution diseases such as Minamata disease, Niigata Minamata disease, Itai-itai disease, and Yokkaichi asthma [1,2,3]. In response, the national government enacted and strengthened the Water Pollution Control Law, expanded environmental quality standards, introduced total pollutant load control in major enclosed coastal waters, and rapidly invested in sewerage infrastructure [2,3,4,5,6]. These measures led to dramatic improvements in organic pollution control and human health protection.
In the context of the 2030 Agenda for Sustainable Development, these achievements correspond closely to the “first half” of SDG 6: safely managed drinking water (6.1), sanitation and hygiene (6.2), and wastewater treatment (6.3.1) [7,8,9]. Japan’s coverage of safely managed drinking water and sanitation services is now 99%, and domestic statistics report BOD/COD environmental standard achievement rates around 90% for human health and living environment items, although with differences among rivers, lakes, and coastal waters [7,8,9,10]. From a global perspective, Japan is often regarded as a front-runner in SDG 6.1–6.3.1. This trajectory offers critical insights for other nations currently navigating the transition from basic access to comprehensive water security, such as Nigeria [11], Bangladesh [12], and Bahrain [13].
However, the “second half” of SDG 6—covering ambient water quality (6.3.2), water-use efficiency and water stress (6.4), integrated water resources management (6.5), and water-related ecosystems (6.6)—presents a more complex picture. The latest UN-Water SDG 6 Data Portal reports 57% for indicator 6.3.2 (proportion of water bodies with good ambient water quality) and a −2.8% change in permanent surface water area for 6.6.1, relative to the 2000–2019 baseline [7,14,15]. These values suggest that, while basic access and point-source control are mature, Japan faces persistent and, in some cases, worsening challenges. These include diffuse nutrient pollution and groundwater nitrate contamination [16,17,18], emerging contaminants like microplastics [19], PFAS and antibiotics [20,21], hypoxia in enclosed bays [22,23,24], and ecosystem degradation driven by land-use changes and climate variability [25,26,27,28].
The aim of this review is to synthesize Japan’s progress towards SDG 6, focusing on how past water environment policies have shaped current indicator values and what policy pathways are most promising for addressing the remaining gaps. Specifically, the paper:
(1)
Maps domestic statistics and policy indicators onto the SDG 6 framework, highlighting where Japan has already reached high or near-universal coverage and where gaps remain [7,8,9,10,15,29,30,31].
(2)
Uses long-term time-series data to examine trends and structural changes in BOD/COD environmental standard achievement rates, quantifying widely held policy concerns regarding “stagnation” through rigorous statistical analysis [10,32].
(3)
Organizes the main drivers, pressures, state changes, impacts, and responses using the DPSIR framework and identifies the structural reasons why SDG 6.3.2 and 6.6.1 lag other SDG 6 targets, drawing on recent findings regarding climate drivers [26,33] and groundwater dynamics [34,35,36].
(4)
Synthesizes existing evidence on six strategic directions for completing the “second half” of SDG 6, including the potential and constraints of nature-based solutions (NbS) such as grassland restoration [37], artificial recharge [35], and eelgrass bed conservation [38,39], alongside green–gray hybrid portfolios [40].
(5)
Discusses how indicator-based dashboards and, in the longer term, basin-scale simulation platforms (“digital twins”) can support transparent scenario analysis and stakeholder dialogue [7,10,40,41].

2. Materials and Methods

This review employs a mixed-methods approach, combining quantitative analysis of national statistics with a qualitative synthesis of academic and grey literature to assess Japan’s progress toward SDG 6.

2.1. Materials

Primary data for global SDG 6 indicators were obtained from the UN-Water SDG 6 Data Portal [7] to ensure international comparability. To provide granular, country-specific context, these were supplemented with long-term domestic statistical datasets (data as of 2023–2024 reporting cycles) [7]. Domestic datasets extended to FY2024, where available [10], from key Japanese government agencies:
  • Ministry of the Environment (MOE): Annual Public Water Body Water Quality Surveys [10], Environmental White Papers, and data on groundwater quality monitoring [2,4,15,42,43].
  • Ministry of Land, Infrastructure, Transport and Tourism (MLIT): Sewerage Statistics, Water Resources White Papers, and data on river flow and dam management [5,6,9].
  • Ministry of Agriculture, Forestry and Fisheries (MAFF): Surveys on groundwater quality with a focus on agricultural impacts [17].

2.2. Literature Search Strategy

A comprehensive literature search was conducted to identify scientific evidence regarding specific water challenges (e.g., groundwater dynamics, hypoxia, ecosystem degradation) and policy responses in Japan. We utilized EndNote™ 21 (Clarivate Analytics, Philadelphia, PA, USA) to retrieve and organize academic literature from the Web of Science Core Collection.
  • Search Criteria: The search strategy employed combinations of “Japan” with specific keywords such as “SDG 6”, “water quality”, “eutrophication”, “groundwater”, “nitrate”, “hypoxia”, “ecosystem”, “sewerage”, “johkasou”, and “integrated water resources management (IWRM)”.
  • Grey Literature: In addition to peer-reviewed articles, relevant grey literature—including MOE and MLIT white papers, basic plans for water cycle policy, and statistical yearbooks—was screened directly from the official websites of the relevant ministries and agencies to capture the latest policy directives and local case studies not yet covered in academic journals.

2.3. Analytical Framework

The collected data and literature were synthesized using the following three analytical steps:
  • Indicator Mapping: Domestic statistics (e.g., BOD/COD compliance rates) were mapped onto the SDG 6 global monitoring framework to assess alignment and identify gaps between national achievements and global targets.
  • Trend Analysis: Long-term time-series data were analyzed to identify trends and structural changes in water quality, specifically calculating simple structural breaks to evaluate the effectiveness of historical pollution control measures.
  • DPSIR Assessment: The Drivers–Pressures–State–Impacts–Responses (DPSIR) framework was applied to structure the review, linking socio-economic drivers (e.g., urbanization, climate change) to environmental states (e.g., groundwater contamination, ecosystem loss) and evaluating the sufficiency of current policy responses.

3. Progress on SDG 6 Indicators

Japan’s progress toward SDG 6 is characterized by a marked dichotomy between high achievement in basic services and persistent challenges in water quality management and ecosystem conservation. Drawing on data from UN-Water [7] and national surveys [8,9,10], this section assesses the status of each target (see Table 1).

3.1. SDG 6.1 and 6.2: Universal Access and Emerging Risks

Japan has achieved near-universal coverage for safely managed drinking water (SDG 6.1.1) and sanitation services (SDG 6.2.1), with national statistics indicating coverage rates of approximately 98–99% [8,9]. The widespread implementation of advanced water purification and sewage treatment systems has virtually eliminated waterborne diseases. However, the resilience of this infrastructure is increasingly threatened by aging facilities and seismic risks. For instance, the vulnerability of water systems to contamination was starkly highlighted by radioactive leaks following the 2011 disaster [44,45], underscoring the need for robust risk management and renewal strategies.

3.2. SDG 6.3: Water Quality and Wastewater Management

While Japan maintains a high connection rate to wastewater treatment systems (92%), including sewerage and johkasou (on-site treatment) [6,9], the improvement in ambient water quality (SDG 6.3.2) has plateaued. Compliance with environmental standards for organic pollution (BOD/COD) has stabilized at around 90–97% since the early 2000s, with recent river surveys showing rates up to 97% in FY2024, though overall improvements have slowed post-2002 ([10], latest MOE data) (see Figure 1).
Enclosed Coastal Seas: Nutrient pollution remains a critical issue in semi-enclosed water bodies like Tokyo Bay and the Seto Inland Sea, where eutrophication persists despite total pollutant load controls [50,51,52].
  • Groundwater Contamination: Nitrate pollution from agricultural runoff continues to be a widespread problem, exceeding environmental standards in many monitoring wells [16,17,18].
  • Emerging Contaminants: New threats such as microplastics [19] and PFAS (per- and polyfluoroalkyl substances) in urban runoff [20] pose additional challenges to water safety and require updated regulatory frameworks (Figure 2).

3.3. SDG 6.4: Water Use Efficiency and Scarcity

Japan’s water use efficiency (SDG 6.4.1) is relatively high, with an estimated value of 56.2 USD m−3 reported by the FAO AQUASTAT database [31], reflecting long-standing efforts in industrial water recycling and leakage control. In contrast, water stress (SDG 6.4.2) reaches approximately 21.6%, with substantial spatial and seasonal variability across the country [7,53]. Climate change is expected to exacerbate these pressures; for example, projected declines in snow water equivalent in regions such as the Tedori River basin threaten the reliability of water resources that depend on snowmelt [46]. In response, adaptive measures, including grassland restoration to enhance groundwater recharge, are being explored as potential mitigation strategies [37].

3.4. SDG 6.5: Integrated Water Resources Management (IWRM)

Japan’s implementation of IWRM (SDG 6.5.1) scores approximately 91/100, reflecting well-established river basin planning frameworks while still revealing gaps in cross-sectoral coordination [29,43,54]. Effective IWRM requires the integration of land-use planning with water quality conservation, as demonstrated by empirical studies linking landscape metrics to river water quality in the Chugoku district [55,56,57,58]. In addition to these technical and institutional dimensions, social factors are also critical. For example, socio-cultural analyses of dam projects, such as the Makio Dam case, highlight the importance of consensus-building between upstream and downstream communities and local perceptions of water infrastructure and “harmony” in Central Japan [47].

3.5. SDG 6.6: Protection of Water-Related Ecosystems

This indicator shows a concerning trend, with indicators showing rapid changes in 3% of water basins for sub-indicator 6.6.1 (extent of surface waters, 2020 data) [7]. Ecosystems are under stress from multiple drivers:
  • Hypoxia: Stratification and nutrient loading continue to cause hypoxia in enclosed bays such as Ise Bay, Tokyo Bay, Mikawa Bay and neighboring regional waters in the Sea of Japan, threatening benthic biodiversity and fisheries [22,59,60,61,62].
  • Submarine Groundwater Discharge (SGD): Research highlights the importance of SGD as a nutrient pathway to coastal ecosystems, such as in Toyama Bay, linking terrestrial groundwater management directly to marine ecosystem health [63,64].
  • Climate Impacts: Coastal and marine ecosystems, including oyster reefs and eelgrass beds, face dual threats from warming and acidification, necessitating adaptive conservation strategies [25,38].

3.6. SDG 6.a and 6.b: International Cooperation and Participation

Japan remains a global leader in water-related Official Development Assistance (ODA) (SDG 6.a.1) [7]. Domestically, participation (SDG 6.b.1) is evolving using digital technologies. The adoption of “digital twins” for water systems [41] offers new opportunities to enhance transparency and engage local communities in water management decisions, fostering a more participatory approach to sustainability as shown in Table 2.

4. Challenges

To systematically analyze the persistent gaps in achieving SDG 6, particularly regarding water quality (6.3) and ecosystems (6.6), this review applies the Drivers–Pressures–State–Impacts–Responses (DPSIR) framework.

4.1. Drivers: Climate Change and Demographic Shifts

Two primary drivers are altering Japan’s water cycle: climate change and demographic/land-use transitions.
  • Climate Variability: Pseudo-global warming simulations for the Kanto region predict a decrease in minimum annual precipitation and an increase in intense rainfall events, heightening the risk of both droughts and floods [26,33]. In snow-dominated basins like the Tedori River, climate change is projected to significantly reduce snow water equivalent, threatening the stability of water resources during the spring melt season [27,46].
  • Socio-economic Factors: Urbanization continues to drive the “heat island” effect, altering subsurface temperatures and groundwater flow systems in megacities like Osaka and Tokyo [32]. Additionally, foreign land acquisition in headwater regions has raised concerns, requiring transparent management plans to alleviate public anxiety [65].

4.2. Pressures: Diffuse Pollution and Changing Loads

In contrast to historical point-source industrial pollution, current pressures are dominated by diffuse sources and emerging contaminants.
  • Nutrient Loads: Intensive agriculture remains a significant pressure, delivering high loads of nitrogen to groundwater and surface waters. Studies in the Miyakonojo River Basin show that agricultural land use is strongly correlated with elevated groundwater nitrogen levels [66].
  • Emerging Contaminants: Urban runoff acts as a pressure for new chemical pollutants. Specifically, first-flush stormwater in separated sewerage systems has been identified as a significant source of perfluorinated compounds (PFAS) [20]. Furthermore, high concentrations of antibiotics have been detected in river systems, driven by human and livestock waste, exerting pressure on microbial ecosystems [21].
  • Physical Alterations: Excessive groundwater pumping for municipal or seasonal use (e.g., snow melting) creates pressure on aquifer systems, leading to subsidence in regions like the Chikugo and Saga Plains and altering regional flow dynamics [36,67].

4.3. State: Complex Groundwater Dynamics and Coastal Hypoxia

The environmental state reflects the cumulative impacts of these pressures, characterized by persistent contamination and altered hydro-geochemical cycles.
  • Groundwater Quality: Long-term monitoring reveals widespread nitrate contamination in groundwater, often exceeding environmental standards [16,17,18]. In addition to anthropogenic inputs, natural geogenic factors contribute to the state of water quality, such as high fluoride concentrations derived from water-rock interactions in granitic and volcanic aquifers (e.g., Mizunami, Aso) and arsenic in specific sedimentary basins [68,69,70,71].
  • Coastal Environments: Enclosed coastal seas exhibit a degraded state due to eutrophication. Long-term trends in Hiroshima Bay show increasing hypoxia driven by seawater stratification and organic loads, which is further complicated by climate warming [23]. Similarly, circulation patterns in Ise Bay trap nutrients, maintaining hypoxic conditions in bottom waters [59].
  • Submarine Groundwater Discharge (SGD): The state of coastal ecosystems is heavily influenced by SGD, which acts as a hidden pathway transporting significant nutrients and carbon from land to sea, as observed in the Japan Sea and Toyama Bay [63,64].

4.4. Impacts: Ecosystem Vulnerability and Socio-Economic Risks

The degraded state of water resources leads to tangible impacts on biodiversity and human society.
  • Ecological Impacts: Hypoxia and acidification pose severe risks to marine life. In Osaka Bay, environmental factors including hypoxia have been linked to fluctuations in the recruitment of the gazami crab [22]. In subarctic lagoons, warming and acidification threaten Pacific oyster production, although eelgrass beds can mitigate some of these negative impacts [25,38].
  • Disaster-Related Impacts: The Great East Japan Earthquake demonstrated the vulnerability of coastal ecosystems to tsunamis, causing soil salinization that hampers vegetation recovery [39].
  • Social Resilience: Groundwater contamination and supply disruptions impact critical infrastructure. However, during the 2016 Kumamoto earthquakes, access to well water proved vital for maintaining hospital functions when municipal supplies failed, highlighting the impact of groundwater availability on disaster resilience [72].

4.5. Responses: Toward Integrated and Nature-Based Solutions

Current policy responses are shifting from hard infrastructure toward integrated and nature-based approaches, though gaps remain.
  • Legal and Institutional: The Basic Act on the Water Cycle aims to overcome vertical administrative sectionalism [54]. However, implementation of IWRM remains uneven, often lacking effective coordination between upstream forest management and downstream water users [73].
  • Technological and Engineering: Responses include the development of underground dams to secure water resources in island regions [74] and the investigation of CO2 geological storage (CCS), which must be managed to prevent groundwater displacement or contamination [75].
  • Nature-Based Solutions (NbS): There is growing adoption of NbS, such as restoring grasslands to enhance groundwater recharge and utilizing paddy fields for artificial recharge during non-irrigation periods [35,37]. Integrated coastal management concepts like Satoumi represent a holistic response to balance human use and conservation [49].

5. Policy Directions

To bridge the remaining gaps in SDG 6 and transition toward a nature-positive water future, Japan must shift from a predominant focus on point-source control to a holistic, adaptive management strategy. Building on the challenges identified in the DPSIR analysis, we propose six strategic policy directions (summarized in Table 3).

5.1. Climate-Resilient Water Systems

Adaptation to climate variability is paramount. With projections indicating altered precipitation patterns in the Kanto region [26,33] and significant declines in snow water storage in basins like the Tedori River [27,46], water infrastructure must be redesigned for resilience.
  • Diversifying Water Sources: Groundwater should be strategically positioned as a resilient backup source for emergencies. The experience of the 2016 Kumamoto earthquakes demonstrated that well water is critical for maintaining hospital functions when centralized systems fail [72].
  • Monitoring Precursors: Innovative monitoring of shallow groundwater flow and temperature can serve dual purposes: managing water resources and acting as precursors for volcanic and seismic events, as observed in the Kirishima and Izu regions [76,77].

5.2. Infrastructure Renewal and Smart Asset Management

As Japan faces a population decline, maintaining aging infrastructure poses a significant fiscal challenge.
  • Digital Transformation: The adoption of “digital twins” and smart sensors is essential to optimize the operation of water supply and sewerage systems, reducing leakage and maintenance costs [41].
  • Water Reuse and Recycling: Promoting water reuse technologies is vital for closing the loop in the urban water cycle [78].
  • Marine Safety: Recent engineering investigations on ballast tank flow phenomena and drainage system design provide foundational understanding of advanced ballast technologies. For example, analytical and experimental studies have detailed fluid flow behaviors within ballast tanks and performance implications for drainage systems [79].
However, the implementation of these digital technologies faces severe fiscal constraints, particularly in small-scale municipalities suffering from depopulation [6,9]. Technology alone is not a panacea. To make “Smart Asset Management” feasible, structural reforms are essential. Specifically, the regional consolidation of water utilities (Koiki-ka) across municipal boundaries must be accelerated to secure the financial base for investing in digital twins and sensor networks [41,78]. Furthermore, adopting Public–Private Partnerships (PPP) can bridge the technical gap in shrinking local governments. Without these governance reforms, advanced technologies will remain accessible only to major metropolitan areas, widening the regional gap in water security.

5.3. Advanced Pollution Control and Emerging Contaminants

Achieving good ambient water quality (SDG 6.3.2) requires tackling diffuse pollution and new chemical threats.
  • Emerging Contaminants: Urgent regulatory frameworks and advanced treatment technologies are needed to address PFAS in urban stormwater [20] and antibiotics in river systems [21], which pose long-term health and ecological risks.
  • Agricultural Runoff: Addressing nitrate pollution from agriculture is particularly challenging in Japan. Unlike the EU’s Nitrates Directive, which mandates strict controls, Japan lacks legally binding regulations on agricultural nutrient inputs (e.g., fertilizer application rates). Current measures primarily rely on voluntary guidelines, which have proven insufficient given the widespread detection of nitrates in groundwater [16,17,18]. Therefore, policy must shift from relying on voluntary cooperation to creating structural incentives. This includes linking agricultural subsidies to environmental compliance (cross-compliance) and promoting slow-release fertilizers [81]. Additionally, utilizing underground dams in island regions like Okinawa offers a technical solution to secure water resources while managing nitrate flux [74].

5.4. Deepening IWRM and Groundwater Governance

Integrated Water Resources Management (IWRM) must evolve to encompass the full hydrological cycle, including invisible groundwater flows.
  • Land-Water Linkages: Policies should explicitly recognize the link between land use and water quality. Studies in the Chugoku district and Yamaguchi Prefecture have shown that integrating landscape metrics into water management plans significantly improves water quality prediction and management [55,56,57].
  • Managing SGD: Recognizing Submarine Groundwater Discharge (SGD) as a critical nutrient pathway [63,64] necessitates “Ridge-to-Reef” management strategies that coordinate upstream terrestrial activities with downstream coastal conservation.
  • Social Coordination: Successful IWRM requires managing social dynamics. Case studies like the Makio Dam highlight the importance of fostering “harmony” and consensus between upstream and downstream communities to resolve conflicts [47,48]. Additionally, understanding Japan’s virtual water trade structure is crucial for global responsibility [82].
A critical barrier to effective IWRM in Japan is the civil law framework, where groundwater rights are traditionally tied to land ownership. While this makes direct national control difficult, progressive municipalities have bypassed these limitations through local ordinances. For example, Kumamoto City established a “collaborative governance” model that effectively treats groundwater as a common pool resource despite the constraints of national property law [69,83]. While the Basic Act on the Water Cycle (2014) provides a national philosophy aimed at overcoming vertical administrative sectionalism [54], it lacks strong regulatory power over private pumping. To overcome this, local ordinances (e.g., in Kumamoto and Ono) have pioneered “collaborative governance” that bypasses the rigid national legal limitations [67,72]. The future of Japanese IWRM lies in empowering these local “groundwater councils” to negotiate agreements between private land rights and public water security, effectively operationalizing the concept of water as a “public good” from the bottom up [47,48].

5.5. Enhancing Data Transparency and Citizen Engagement

  • Community Involvement: Local stakeholders should be active participants in water management. The communal use of groundwater for snow melting in Obama City illustrates how local practices can balance economic benefits with resource sustainability [67].
  • Satoumi Initiatives: The concept of Satoumi (coastal seas managed by local communities) in the Seto Inland Sea provides a model for participatory management that enhances both biodiversity and biological productivity [49].

5.6. Mainstreaming Nature-Based Solutions (NbS)

Nature-based solutions offer sustainable, cost-effective alternatives to gray infrastructure.
  • Ecosystem Restoration: Restoring grasslands in headwaters like Aso has been proven to significantly enhance groundwater recharge compared to neglected plantations [33,71].
  • Coastal Protection: Conserving and restoring eelgrass beds is a dual-benefit strategy that mitigates local ocean acidification while reducing the risk of malformation in oyster aquaculture [49].
  • Artificial Recharge: Utilizing agricultural lands, specifically paddy fields, for artificial groundwater recharge during non-irrigation periods is a proven method to replenish aquifers and improve water quality.
  • Forest Management: Proper thinning and management of forests are essential to maintain low-flow water resources, although the trade-offs with dam development must be carefully balanced [74].

6. Discussion

6.1. Bridging the Gap Between Infrastructure and Ecosystems

This review has highlighted a distinct “SDG 6 paradox” in Japan: While the divergence between domestic achievement (~90%) and SDG 6.3.2 (~57%) can be technically attributed to stricter aggregation rules [7,10,15], it fundamentally signals a “policy mismatch.” The high compliance with organic pollution standards (BOD/COD) has historically created a sense of security. However, under the strictly aggregated “One-Out, All-Out” methodology of SDG 6.3.2, water bodies frequently fail due to nutrient exceedances (nitrogen or phosphorus) even when organic pollution levels are low. This discrepancy masks the urgency of addressing “invisible” threats such as nutrient imbalance and hypoxia [22,59,60,61,62]. Therefore, Japan should not merely aim to maintain its current standards but must recalibrate its national targets to align more closely with the “One-Out, All-Out” principle of SDG 6.3.2. This shift is necessary to transition from “sanitation success” to “ecosystem integrity,” ensuring that water bodies are not just clean enough for human use but healthy enough to sustain biodiversity [25,38].
  • Limits of Gray Infrastructure: The stagnation in BOD/COD compliance rates since the early 2000s suggests that the marginal utility of traditional point-source control measures (e.g., expanding sewerage systems) is diminishing [10,32]. The persistent issues of hypoxia in enclosed bays [22,67,84] and nutrient loads from agriculture [81] indicate that “gray” infrastructure alone cannot solve diffuse pollution or restore complex ecological functions.
  • Trade-offs and Synergy: Managing water resources often involves trade-offs. For instance, while dams provide stable water supply and flood control, they can alter river flow regimes and sediment transport, impacting downstream ecosystems [47,72]. Conversely, Nature-based Solutions (NbS), such as the restoration of grasslands in Aso [37] or the conservation of eelgrass beds [38], offer synergistic benefits by enhancing groundwater recharge, sequestering carbon, and providing habitats, aligning water security with biodiversity goals.

6.2. The Invisible Resource: Mainstreaming Groundwater Governance

Groundwater has historically been managed primarily as a resource to be exploited or a hazard causing subsidence [69,83]. However, this review underscores its critical role in resilience and ecosystem support.
  • Resilience: The usage of well water during the Kumamoto earthquakes highlighted groundwater as a vital lifeline during disasters [23].
  • Connectivity: Recognizing Submarine Groundwater Discharge (SGD) as a major nutrient pathway [63,70] fundamentally shifts coastal management from a terrestrial focus to a comprehensive “Ridge-to-Reef” approach.
  • Quality Management: Addressing nitrate contamination requires a shift from end-of-pipe treatment to source control in agriculture, potentially utilizing denitrification functions in aquifers and artificial recharge zones [64].

6.3. Global Implications

Japan’s trajectory offers valuable lessons for other nations.
  • For Developing Economies: Countries like Nigeria and Bangladesh, currently focusing on basic access (SDG 6.1/6.2) [11,12], can anticipate future challenges. Japan’s experience suggests that integrating wastewater treatment with ecosystem conservation early in the development phase prevents long-term environmental debts.
  • For Water-Scarce Regions: Japan’s advances in water reuse [78] and leakage control offer technical solutions for arid regions like Bahrain [13]. Conversely, Japan must learn from global best practices in transboundary aquifer management and water diplomacy to secure its virtual water interests [27].

7. Conclusions

Japan stands at a crossroads in its water policy. To achieve the “second half” of SDG 6 and realize a Nature-Positive Water Future by 2030, the following shifts are essential:
  • From Sectoral to Integrated Management: Breaking down silos between river, forest, agricultural, and coastal management to implement true basin-scale IWRM (SDG 6.5) that accounts for groundwater and SGD.
  • From Gray to Green-Gray Hybrid: Systematically incorporating NbS (grasslands, wetlands, tidal flats) into infrastructure planning to enhance climate resilience and biodiversity.
  • From Static to Adaptive Governance: Leveraging digital twins and real-time monitoring to adaptively manage water resources amidst rapid climate change and demographic shifts.
By addressing these challenges, Japan can not only secure its own water future but also contribute a leading model of sustainability to the global community.

Author Contributions

Conceptualization, Q.W.; Methodology, Q.W.; Investigation (literature search and synthesis), Q.W. and X.H.; Formal analysis, Q.W., T.O. and S.K.; Visualization, X.H.; Writing—original draft, Q.W.; Writing—review & editing, T.O., S.K. and X.H.; Supervision, Q.W. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Institute for Environmental Studies (NIES), Japan, through the projects “Proposal and Evaluation of Environmentally Efficient Technologies and Systems in Collaboration with Local Communities (No. 2125AA117)” and “Regional Environment Conservation Domain: Foresight and Advanced Basic Research (No. 2125AV007)”.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data are publicly available from the Ministry of the Environment, Japan (Public Water Body Water Quality Survey Results) and the UN-Water SDG 6 Data Portal.

Conflicts of Interest

Authors are affiliated with NIES; no external funding influenced the work.

Abbreviations

The following abbreviations are used in this manuscript:
BODBiochemical Oxygen Demand
CCSCarbon Capture and Storage
CODChemical Oxygen Demand
DODissolved Oxygen
DPSIRDrivers–Pressures–State–Impacts–Responses
ECElectrical Conductivity
IWRMIntegrated Water Resources Management
MAFFMinistry of Agriculture, Forestry and Fisheries
MLITMinistry of Land, Infrastructure, Transport and Tourism
MOEMinistry of the Environment
NbSNature-based Solution
ODAOfficial Development Assistance
PFASPer- and Polyfluoroalkyl Substance
PFOAPerfluorooctanoic Acid
PFOSPerfluorooctanesulfonic Acid
SDGSustainable Development Goal
SGDSubmarine Groundwater Discharge

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Sustainability 18 00994 g001
Figure 1. Long-term trends in wastewater treatment coverage versus compliance rates with environmental standards (BOD/COD) in Japan (1970–2024, with recent data indicating slight improvements in rivers). The graph shows treatment coverage rising sharply from 1970s to 90% by 2000, with BOD/COD compliance plateauing at ~90% post-2002.
Figure 1. Long-term trends in wastewater treatment coverage versus compliance rates with environmental standards (BOD/COD) in Japan (1970–2024, with recent data indicating slight improvements in rivers). The graph shows treatment coverage rising sharply from 1970s to 90% by 2000, with BOD/COD compliance plateauing at ~90% post-2002.
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Figure 2. Spatial distribution of PFOS and PFOA concentrations in Japan, based on MOE surveys conducted between FY 2020 and FY 2023.
Figure 2. Spatial distribution of PFOS and PFOA concentrations in Japan, based on MOE surveys conducted between FY 2020 and FY 2023.
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Table 1. Current Status of SDG 6 Indicators in Japan.
Table 1. Current Status of SDG 6 Indicators in Japan.
IndicatorDescriptionCurrent Value (Approx.)TrendKey Challenges & DriversReferences
6.1.1Safely managed drinking water99%Stable (High)Aging infrastructure, seismic resilience, radioactive contamination risks.[8,9,44,45]
6.2.1Safely managed sanitation99%Stable (High)Population decline affecting maintenance costs, rural-urban disparities.[6,9,11]
6.3.1Wastewater treatment92%ImprovingOptimization of johkasou (on-site systems), advanced treatment for nutrients.[6,9]
6.3.2Ambient water quality57% (UN) ~90% (Domestic)StagnatingDiffuse pollution (nitrate), emerging contaminants (PFAS, antibiotics), hypoxia.[7,10,16,20,21]
6.4.1Water-use efficiency$56.2/m3 (FAO AQUASTAT, 2024)ImprovingEconomic growth driving efficiency, but low in agriculture; need for sector-specific strategies.[7,31]
6.4.2Water stress21.6%FluctuatingClimate variability, declining snow water storage, agricultural demand; high stress in certain regions.[7,27,33,46]
6.5.1IWRM implementation91/100ModerateCross-sectoral coordination, groundwater governance, stakeholder consensus; insufficient finance and gender mainstreaming.[29,43,47,48]
6.6.1Water-related ecosystemsRapid changes in 3% of basins (2020 data)DegradingHypoxia in enclosed bays, loss of wetlands/tidal flats, climate impacts on biodiversity; regional degradation.[7,15,22,25,49]
Table 2. Schematic Comparison between SDG 6.3.2 and Domestic Environmental Standard Achievement Metrics in Japan.
Table 2. Schematic Comparison between SDG 6.3.2 and Domestic Environmental Standard Achievement Metrics in Japan.
FeatureSDG 6.3.2 (Good Ambient Water Quality)Domestic Environmental Standard Achievement (BOD/COD)
Primary FocusComprehensive water quality status relative to natural background or target conditions.Organic pollution control for protecting human health and living environments.
Core ParametersFive Core Parameter Groups:
1. Dissolved Oxygen (DO)
2. Electrical Conductivity (EC)
3. Nitrogen (Total Nitrogen or Nitrate/Ammonia)
4. Phosphorus (Total Phosphorus or Orthophosphate)
5. pH		Organic Pollution Indicators:
Biochemical Oxygen Demand (BOD) for rivers.
Chemical Oxygen Demand (COD) for lakes and coastal seas.
Aggregation Rule“One-Out, All-Out” Principle:
A water body is classified as “good” only if at least 80% of monitoring values for all core parameters meet their targets. Failure in a single parameter (e.g., nitrogen) results in a failure classification.		Parameter-Specific Compliance:
Typically reported as the percentage of measurement points achieving the standard for a specific parameter (e.g., “BOD compliance rate”). It does not usually fail a site based on a single exceedance in the headline statistic.
Sensitivity to NutrientsHigh: Explicitly includes Nitrogen and Phosphorus as core parameters for all water body types. Many water bodies fail due to nutrient levels despite good BOD.Variable: Nitrogen and Phosphorus standards exist but are applied primarily to lakes and coastal seas to prevent eutrophication, not universally to all river sections in the headline BOD/COD statistic.
Resulting Value (approx.)~57% (2020 reporting)
Reflects strict aggregation where nutrient or DO failures in specific samples downgrade the entire assessment.		~90% (Recent years)
Reflects high success in controlling organic pollution (BOD/COD) from wastewater, potentially masking issues with nutrients or other specific parameters.
ImplicationHighlights persistent issues in nutrient management, diffuse pollution, and hydro-morphological alterations that don’t necessarily spike BOD.Demonstrates success in sanitation and point-source control (sewerage expansion) but may overestimate overall ecosystem health.
Table 3. Overview of Six Strategic Directions for Completing the “Second Half” of SDG 6 in Japan.
Table 3. Overview of Six Strategic Directions for Completing the “Second Half” of SDG 6 in Japan.
Strategic DirectionKey Focus AreasProposed Actions & TechnologiesPrimary SDG TargetsReferences
1. Climate-Resilient Water SystemsAdaptation to altered precipitation, snowmelt decline, and disaster resilience.• Positioning groundwater as a strategic emergency backup (e.g., hospital resilience).
• Real-time monitoring of groundwater for volcanic/seismic precursors.
• Adaptive reservoir operations for flood/drought extremes.		6.4, 6.1[27,33,46,72,76,77]
2. Infrastructure Renewal & Smart Asset ManagementAging infrastructure, population decline, circular economy.• Digital Twins: Virtual simulation for leak detection and optimization.
• Water Reuse: Closing the loop in industrial/urban cycles.
• Marine Tech: “No-water-ballast tankers” to stop invasive species/pollution.		6.1, 6.3, 6.4[41,78,79]
3. Advanced Pollution Control & Emerging ContaminantsDiffuse pollution, new chemical threats beyond organic load.• Advanced treatment for PFAS and antibiotics in urban runoff/rivers.
• Underground dams for securing resources and managing nitrate flux.
• Precision fertilizer management.		6.3[20,21,71,74]
4. Deepening IWRM & Groundwater GovernanceVertical/horizontal coordination, land-water linkage, transboundary issues.• Landscape-based Management: Integrating land use metrics into water planning.
• Ridge-to-Reef: Managing Submarine Groundwater Discharge (SGD) zones.
• Social Harmony: Consensus building between upstream/downstream users.		6.5, 6.6[47,48,55,63,64]
5. Enhancing Data Transparency & Citizen EngagementStakeholder participation, local knowledge integration.• Satoumi: Community-based coastal management.
• Public dashboards for real-time water quality/quantity data.
• Local resource sharing models (e.g., groundwater for snow melting).		6.b[7,49,67]
6. Mainstreaming Nature-Based Solutions (NbS)Ecosystem services, biodiversity, green-gray hybrid infrastructure.• Grassland Restoration: Enhancing groundwater recharge in headwaters.
• Artificial Recharge: Using paddy fields during non-irrigation periods.
• Blue Carbon: Conserving eelgrass beds to mitigate acidification and hypoxia.		6.6, 6.4[35,37,38,39,80]
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Wang, Q.; Okadera, T.; Kameyama, S.; Huang, X. Pathways for SDG 6 in Japan: Challenges and Policy Directions for a Nature-Positive Water Future. Sustainability 2026, 18, 994. https://doi.org/10.3390/su18020994

AMA Style

Wang Q, Okadera T, Kameyama S, Huang X. Pathways for SDG 6 in Japan: Challenges and Policy Directions for a Nature-Positive Water Future. Sustainability. 2026; 18(2):994. https://doi.org/10.3390/su18020994

Chicago/Turabian Style

Wang, Qinxue, Tomohiro Okadera, Satoshi Kameyama, and Xinyi Huang. 2026. "Pathways for SDG 6 in Japan: Challenges and Policy Directions for a Nature-Positive Water Future" Sustainability 18, no. 2: 994. https://doi.org/10.3390/su18020994

APA Style

Wang, Q., Okadera, T., Kameyama, S., & Huang, X. (2026). Pathways for SDG 6 in Japan: Challenges and Policy Directions for a Nature-Positive Water Future. Sustainability, 18(2), 994. https://doi.org/10.3390/su18020994

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