This study presents a long-term analysis of water quality using the National Sanitation Foundation Water Quality Index (NSFWQI) based on the monitoring data from 1966 to 2023 for the Sumida River in Tokyo. Although the NSFWQI has been widely used, it is not often applied to large, long-term datasets. This study illustrates how the NSFWQI can be effectively used for multidecadal analysis, allowing researchers to identify temporal dynamics and progressive changes in water quality. In this study, NSFWQI was developed from 27 sampling points using four to eight key parameters: transparency, temperature, pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), nitrate, phosphate, and fecal coliform of the monthly dataset from 1966 to 2023. The results show that the overall water quality improved from poor in 1966 to good by 2012, driven mainly by reduced BOD and enhanced DO. Moreover, the analysis identified eight significant change points, indicating gradual improvements in water quality that correspond to major policy and infrastructure advancements. However, among eight water quality parameters, fecal coliform levels remained persistently high, indicating continuing microbial risks linked to diffuse sources and aging sewer infrastructure. The findings highlight how sustained governance and infrastructure investment can reverse urban river degradation, while emphasizing that chemical recovery alone is insufficient without microbial management. Incorporating WQI-based assessments into Japan's monitoring frameworks could enhance adaptive water governance and public communication of river health.
Setiawati et al. (2026) studied this question.