Study on long-term variation of river water quality in Japan

１．introduction

During Japan's period of rapid economic growth, water pollution became a problem throughout the country, but water quality has improved rapidly thanks to the enactment of laws and the heightened environmental awareness of society as a whole. However, even now, not only is the concentration concentrated in Tokyo, but urbanization is also progressing in rural areas, and there are still areas with severe water pollution in the suburbs. What used to be point source pollution has spread to non-point source pollution. Due to problems with wastewater treatment facilities in mountainous areas, many large river basins are more polluted upstream than downstream. The results of the ``Survey of the Water Environment in Public Water Areas,'' which has been continued by the government since 1971, and the ``National Simultaneous Survey of Familiar Water Environments,'' which was started in 2004 by citizens' groups, are available on a nationwide scale. I have mainly studied long-term changes in river water quality in Japan. We will also consider the results of measurements by Hosei University in 2020 and 2021.

２．Results and considerations

１）Water quality survey results for public water areas

There were about 1,000 observation points in 1971, but 15 years later, in 1986, the number exceeded 5,000, and since then observations have continued at just under 6,000 points. In 1971, half of the BOD values were 3 or more, but in 1976 half were 2 or less, and recently, 2 or less accounted for about 80% (2018). The number of points with 1 to 4 remained unchanged, but 4 or more decreased, and 1 or less increased to about half of the total.

２）Nationwide Simultaneous Survey of Familiar Water Environment

In 2004, when the survey began, there were about 2,500 sites, but in 2005 there were about 5,000 sites, and after that, although it remained around 6,000 sites, it reached about 7,000 sites in 2018. COD4 or less is about half.

３．conclusion

In addition to the nationwide long-term observation results, data from before 1971 were collected and organized in an attempt to reconstruct past water quality. With regard to recent water quality, we independently conduct observations at more than 1,500 points nationwide every year to clarify the current situation. In addition to the coastal area in 2020, we were able to investigate the inland area in 2021 and the municipality unit in 2022. We would like to improve the accuracy while continuing the wide-area survey.