Host: The Association of Japanese Geographers
Name : Annual Meeting of the Association of Japanese Geographers, Spring 2024
Date : March 19, 2024 - March 21, 2024
I 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 to clarify the river water quality in this catchment.
II Methods
Field surveys were conducted approximately once a month from March 2022 to July 2023. Samples taken in September 2022 were analysed for major dissolved constituents by ion chromatography.
III Results and Discussion
The survey results show that nitrate ions increase rapidly in the upper reaches of the Mogami River (Fig. 1). The nitrate concentration was 0.23 mg/L at m01, but increased significantly to 1.52 mg/L at m03. It then decreased to 0.72 mg/L at m06, with no significant change in concentration until m13, and increased again from m14, with a concentration of 1.83 mg/L at m17 at the downstream end. m01-m02 and m02-m03 showed high rates of increase (269.6% and 78.8% in that order), indicating that the nitrate influx in this area, i.e. the Yonezawa Basin The nitrate influx in this area, i.e. in the Yonezawa Basin, was observed. Nitrate ion is assumed to be of human activity origin based on its correlation with the percentage of land use in the basin.
On the other hand, sulphate ion showed a high value of 17.28 mg/L at m01 in the uppermost part of the basin, decreased with the flow down to m09 and increased again to 17.92 mg/L at m10. It then decreases again to 12.52 mg/L at the most downstream m17. Unlike nitrate, sulphate ion has a higher rate of increase between m09-m10, i.e. in the Yamagata Basin (89.6%). Dissolved constituent analysis showed that this sulphate ion is supplied from tributary t19 and that sulphate ions are supplied to t19 by t12 and t13, where sulphate ions predominate (Fig. 2). t12 and t13 sulphate ions are thought to be of sulphur mine or Zao Hot Springs origin and are inferred to be volcanic in origin.
Therefore, it is clear that in the Yonezawa Basin, there is a supply of nitrate ions of human activity origin, while in the Yamagata Basin, there is a supply of sulphate ions of volcanic origin in the mainstream.
IV Conclusion.
This study clarified river water quality in the Mogami River basin with a focus on nitrate and sulphate ions. It is hoped that further clarification of the factors that form the water quality of the river basin will be possible in the future.
