Host: The Association of Japanese Geographers
Name : Annual Meeting of the Association of Japanese Geographers, Spring 2023
Date : March 25, 2023 - March 27, 2023
Introduction
The Mogami River is a first-class river flowing through Yamagata Prefecture, with about 229 km length (7th in Japan) and about 7,040 km2 drainage basin (9th in Japan). This paper reports the results of a survey conducted to clarify the river water quality in this catchment.
II Methods
We carried out field surveys monthly from March to November 2022. At a total of 53 sites in the main stream (m01-06, 08-17) and tributaries (t01-37), air temperature, water temperature, pH, RpH and electrical conductivity (EC) were measured and water samples were taken in the field. In June, chemical oxygen demand (COD) and total organic carbon (TOC) were also measured.
III Results and discussion
From the survey results, trends in water quality were found for the entire Mogami River basin. The EC in the main stream ranged between 95 and 134 µS/cm (Fig. 1). Particularly high values of 132 and 134 µS/cm were observed at m03 and m04 in the upper reaches of the river, before dropping to 95 µS/cm at m09 and rising again to 120 µS/cm at m10 in the middle reaches. This is influenced by the water quality of the joining tributaries: the inflow of tributaries with high EC, the Haguro, Tenno, Omono and Yoshino Rivers (t01-04), increases the EC of the main stream in the upper reaches, and temporarily the EC decreases due to the joining of tributaries with low EC, but the EC increases again in the middle reaches when the Sukawa (t19) merges.
Furthermore, COD exceeded 4 mg/L in the Sakasa River (t17), Yoshino River (t04) and Inu River (t05), while TOC was higher than 3 mg/L in the Sakasa River (t17), Mogami River (m09), Sukawa River (t11) and Su River (t13) (Fig. 2), suggesting a pollution load at these sites. However, EC in the Inu River(t05) and Mogami River(m09) was below 100 µS/cm. This indicates that the quality of contamination may differ from other sites.
The Zao River (t12), which has high EC but low COD and TOC, has a pH of around 3.5 and is not polluted, suggesting that the EC is high due to contamination by hot springs. The Su River (t13) also has a pH of around 2.5 and was thought to be mainly composed of hot spring water, but the high TOC suggests that some organic matter is mixed in.
IV Conclusion.
In this study, the river water quality in the Mogami River basin was overviewed from the viewpoint of pollution load based on the survey results. In the future, we plan to analyse each ionic component, investigate the relationships between other items, and clarify the factors that shape water quality and the water balance in the basin.
