National First-Class River Water Quality Status for Fiscal Year 2024

Overview

This report details the annual water quality survey results and improvement status of Japan's first-class rivers, based on the Ministry of Land, Infrastructure, Transport and Tourism's announcement on July 4, 2025, regarding the national first-class river water quality status for fiscal year 2024. ## Key Points ### Scale of Water Quality Survey and Environmental Standards Achievement - Water quality surveys conducted at 1,086 points across 109 river systems nationwide (1,102 points including 16 prefecture observation points) - Large-scale survey with total annual samples reaching 262,841 - River environmental standards (BOD) satisfaction rate: 97% (1 percentage point increase from previous year) - Lake environmental standards (COD) satisfaction rate: 42% (2 percentage point decrease from previous year) - Overall environmental standards satisfaction rate: 91% (same as previous year) ### Significant Improvement in BOD Water Quality - Approximately 97% of all survey points had BOD 75% values of 3.0 mg/ℓ or below, indicating environments suitable for salmon and sweetfish habitation - 62.1% of points had BOD 75% values of 1.0 mg/ℓ or below, 29.5% had 1.1-2.0 mg/ℓ, showing good water quality in most areas - Dramatic improvement from 1971 when 27% of all survey points exceeded BOD average values of 5.0 mg/ℓ - Health-related environmental standards satisfaction rate: 99.7% (annual average values) ### Notable Urban River Water Quality Improvement Examples - Tamagawa River (upstream of Denenchofu Weir): BOD 75% value 1.5 mg/ℓ (improved from polluted river with detergent foam in the 1960s-70s) - Yamato River (Asaka New Intake): BOD 75% value 1.3 mg/ℓ (significant improvement from BOD over 30 mg/ℓ in the 1960s-70s) - Tsurumi River (Otsuna Bridge): BOD 75% value 3.6 mg/ℓ - Ayase River (Teshiro Bridge): BOD 75% value 3.8 mg/ℓ - Yamato River has now recovered to water quality levels suitable for salmon and sweetfish habitation ### Health-Related Standards Exceedance Status - Health-related surveys conducted for 27 items at 860 points nationwide (16,632 extended points), total survey samples: 29,938 - 3 points exceeded environmental standards in annual average values (1 point for arsenic, 2 points for boron, 1 point for fluorine) - Exceedance causes mainly natural origin (arsenic, fluorine) and seawater influence (partial 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 began in 1969, promoting water quality improvement through sewerage development and discharge regulations - 1997 River Act revision promoted comprehensive river improvement incorporating flood control, water utilization, and environmental considerations - Some points in urban middle/lower reaches and tributary streams still show high BOD values - Lakes require continuous improvement measures due to closed water body characteristics and low environmental standards satisfaction rates The report emphasizes that over half a century of water quality improvement efforts have significantly improved Japan's first-class river water quality, ensuring good water quality in almost all rivers, while noting that continued water environment improvement measures are needed for some lakes and urban river sections.