In this study, we examined the characteristics of river water quality in the Abukuma River basin through the results of a survey, and regional differences in water quality were clarified. The tributaries on the left bank of the river contained more diverse ionic components than the tributaries on the right bank. There are significant topographical differences between the right and left banks, such as the gradient of the river and the presence of volcanoes. Several factors, such as topography, geology, and land use, which may have a strong influence on water quality, are thought to cause these differences.

Ⅰ In Japan, water pollution became a problem nationwide during the period of high economic miracle, but due to legislation and raising environmental awareness of society as a whole, water quality has improved rapidly. However, even now, not only is it concentrated in Tokyo, but urbanization is progressing in rural areas, and there are still areas where water pollution is severe, such as in the suburbs. The former point source pollution has spread as surface source pollution, and due to problems with wastewater treatment facilities in mountain village areas, there are many basins in large river basins where contaminated areas are more conspicuous in the upstream than in the downstream. The results of the "Water Environment Survey of Public Water Areas" that has been continued by the government since 1971, and the nationwide observation records such as the "National Simultaneous Survey of the Familiar Water Environment" that started in 2004 centered on civil society groups. We have mainly examined long-term fluctuations in river water quality in Japan, but in this paper, in addition to the 17th "Simultaneous Survey of Familiar Water Environments" in 2020, the 18th survey in 2021 was measured by Hosei University. The results will also be considered.

Ⅱ Using the database "Water quality survey results of public water areas" of the National Institute for Environmental Studies, the changes in water quality since 1971 are organized, and for the "National simultaneous survey of familiar water environment", his COD from 2004 to 2018 The results of the survey were organized and long-term changes were considered. In addition, before 1971, the data extracted from various research results were organized, and after 2018, the nationwide observation records conducted in the laboratory were used. Furthermore, for 2020, we targeted the observation results of more than 2000 points conducted by the laboratory and related parties.

Ⅲ 1. Results of water quality survey in public water areas The number of observation points, which was about 1,000 in 1971, exceeded 5,000 in 1986, 15 years later, and observations have been continued at points of less than 6,000 since then. In terms of changes over time in BOD values, 3 or more accounted for half (1971), but in 1976, 2 or less accounted for half, and recently, 2 or less accounted for about 80% (2018). The number of points 1 to 4 has not changed, the number of points 4 or more has decreased, and the number of points 1 or less has increased to about half of the total. 2. In 2004, when the survey started, there were about 2,500 points, but in 2005 it was about 5,000 points, and after that it changed to around 6,000 points, but in 2018 it was about 7,000 points. About half have a COD of 4 or less. In 2020, the number of observation points decreased due to the influence of the new virus, and the observation results of Hosei University were not included in the total, so the number was 3,802, which is the second lowest after the first time in 2004. We analyzed the survey results of Hosei University at 2,000 points.Since the results were observed mainly in the coastal area and in the terminal areas of rivers of various scales nationwide, it is suitable for examining the regional differences of rivers nationwide, and it is possible to compare with the previous observation results. Continued research is expected in the future.

3. Although there are research results by pioneering Kobayashi (1961), it is difficult to obtain systematically observed water quality data. It was not possible, and the difficulty of clarifying the past water quality was highlighted.4. From 2017 to 2020, he organized his data surveyed at more than 2000 locations nationwide every year and clarified the current state of river water quality in recent years.

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Keyword:Air passenger, OFOD statistics, World cities, Intercity flow, Urban network structure