Study on the Water Environment in the Mogami River Basin(1)

<p>I INTRODUCTION</p><p>The Mogami River is a first-class river flowing through Yamagata Prefecture with a 229 km length (7th in Japan) and a 7,040 km² drainage basin (9th in Japan). This paper reports the results of a survey conducted to investigate the river water quality in this basin and compares the results with open data. </p><p>II METHOD</p><p>We carried out the field survey from 7 to 10 June 2021. Air temperature, water temperature, pH, RpH, electrical conductivity (EC), and chemical oxygen demand (COD) were measured and water samples were taken at 53 sites in the mainstream and tributaries. We calculated the average of the biological oxygen demand (BOD) 75% values from 1984 to 2019 from the data by the Ministry of the Environment. </p><p>III RESULTS and DISCUSSION</p><p>The results of the survey showed the trend of water quality in the whole Mogami River basin. </p><p>The EC of the mainstream ranged from 101 to 167 µS/cm. The EC was particularly high at 167 µS/cm at the Nukanome bridge in the upper region, then decreased to 101 µS/cm at the Imogawa bridge, and then increased again to 159 µS/cm at the Murayama bridge in the middle stream. The EC of the tributaries ranged from 60 to 3075 µS/cm. Low EC values were recorded in rivers with high snowmelt inflows: 63 µS/cm in the Okitamashirakawa River and 78 µS/cm in the Tachiyazawa River. High EC values were recorded in the rivers with a high inflow of hot spring water: Sukawa River 3075 µS/cm, another Sukawa River 810 µS/cm and Zao River 634 µS/cm. </p><p>The COD values ranged from 0 to 8 mg/L. Rivers with high COD were the Yoshino River at 8 mg/L, the Inugawa River at 7 mg/L and the Sukawa River at 6 mg/L. COD in the mainstream increased from 0-1 mg/L in the upper region to 5 mg/L at the Kourai Bridge after the Yoshino and Inugawa Rivers joined. </p><p>The BOD 75% values measured by the authority ranged from 0.41 to 13.74 mg/L. Rivers with high BOD values were the Sakasa River (13.74 mg/L), the Komaki River (7.24 mg/L), and the Numagawa River (6.30 mg/L), which were found to be dispersed throughout the watershed. </p><p>Comparing the results of the survey with the spatial distribution of BOD, there were places where the high values did not match. For example, in the Yoshino River, Inugawa River and Sukawa River, which have high COD, BOD was not measured or was low. This may suggest that hidden water pollution in rivers exists and that BOD alone is limited in its assessment. It is concerned that the water quality of the mainstream will deteriorate if such rivers join the mainstream.</p><p>IV CONCLUSION</p><p>We reviewed the water quality of the rivers in the Mogami River basin in this study. In the future, it is necessary to investigate the relationship between each ionic component and other items and to clarify the factors that form the water quality and the water balance of the basin.</p>