This article provides a detailed report on the annual water quality survey results and improvement status of Japan's Class A rivers, based on the 2024 National Water Quality Status of Class A Rivers report announced by the Ministry of Land, Infrastructure, Transport and Tourism on July 4, 2025.
Key Points
Scale of Water Quality Survey and Environmental Standard Achievement Status
- Water quality surveys conducted at 1,086 locations in 109 water systems nationwide (1,102 locations including 16 prefectural observation points)
- Large-scale survey with annual total of 262,841 samples
- River environmental standard (BOD) satisfaction rate: 97% (1 point increase from previous year)
- Lake environmental standard (COD) satisfaction rate: 42% (2 point decrease from previous year)
- Overall environmental standard satisfaction rate: 91% (same as previous year)
Significant Improvement Trend in BOD Water Quality
- Approximately 97% of all survey points have BOD 75% value of 3.0mg/ℓ or less, suitable for salmon and ayu habitats
- 62.1% of points with BOD 75% value of 1.0mg/ℓ or less, 29.5% between 1.1-2.0mg/ℓ, indicating mostly good water quality
- In 1971, 27% of all survey points had BOD average values exceeding 5.0mg/ℓ, now significantly improved
- 99.7% of points satisfied health item environmental standards (annual average)
Notable Water Quality Improvement Examples in Urban Rivers
- Tama River (Denenchofu Weir): BOD 75% value 1.5mg/ℓ (improved from polluted river with detergent foam in 1960s)
- Yamato River (Asaka New Water Intake): BOD 75% value 1.3mg/ℓ (dramatically improved from over 30mg/ℓ BOD in 1960s)
- Tsurumi River (Otsuna Bridge): BOD 75% value 3.6mg/ℓ
- Ayase River (Teshiro Bridge): BOD 75% value 3.8mg/ℓ
- Yamato River has now recovered to water quality levels suitable for salmon and ayu habitats
Health Item Standard Exceedance Status
- Survey of 27 health items conducted at 860 locations nationwide (16,632 extended points), with 29,938 total samples
- 3 points exceeded environmental standards in annual average (1 point for arsenic, 2 for boron, 1 for fluorine)
- Exceedances mainly due to natural causes (arsenic/fluorine) and seawater influence (some boron)
- 8 points (9 extended points) had at least one sample exceeding standards, satisfaction rate 99.1%
Continuous Water Quality Improvement Efforts
- River purification projects started in 1969, promoting water quality improvement through sewerage development and discharge regulations
- 1997 River Act revision promoted comprehensive river management integrating flood control, water utilization, and environment
- Some mid-downstream urban areas and tributary inflows still show high BOD values
- Lakes have low environmental standard satisfaction rates due to closed water body characteristics, requiring continuous improvement measures
The article emphasizes that over half a century of water quality improvement efforts have resulted in remarkable improvements in Japan's Class A river water quality, with good water quality secured in almost all rivers, while highlighting the continued need for water environment improvement measures in some lakes and urban rivers.