17:15 〜 19:15
[SVC31-P11] 霧島硫黄山の2023年以降の火山活動と河川汚濁
キーワード：霧島火山、えびの高原硫黄山、河川汚濁
Iwo-yama volcano in the Kirishima volcanic complex erupted in 2018 (phreatic eruptions) for the first time in 250 years. In the 2018 eruption, a large amount of highly concentrated hot water (geothermal fluids) discharged from newly opened vents (craters) flowed into the river, causing severe river pollution. After that, the craters formed hot water pools, and because the overflowing hot water from the pool continued to flow into the river, some areas of the river basin were forced to give up rice cultivation for the time being. In 2019, the river water neutralization facility started its operation, and the overflow from the hot water pools stopped at that time, and the river water quality improved significantly. However, from the middle of 2023, the hydrothermal activity of Iwo-yama volcano increased. From the end of 2023, overflow hot water from the craters often flowed into the river, and severe river pollution occurred again. In this study, We examined how the different forms of hydrothermal activity at Iwo-yama volcano affect river pollution by comparing the state of hydrothermal activity and river pollution at Mt. Iwo-yama in 2018 with the state of hydrothermal activity and river pollution after the end of 2023. Here, we referred to the data on the water chemistry of the hot water pools, hot springs, and rivers obtained by our field works and that of river water released from the Miyazaki Prefecture.
When comparing the water chemistry of the hot water pools in 2018 and 2024, the 2024 water had higher concentrations of most components. In addition, the Cl- concentration in 2024 was significantly higher than in 2018, and there was a significant difference in the Cl-/SO42- concentration ratio. The fluctuation in Cl- concentration was in harmony with the fluctuation in pH. After the end of 2023, the increase in the supply of HCl to the craters led to an increase in the acidity of the hot water pools. As a result, the inflow of overflow water from the hot water pools into the river has caused a significant drop in the pH of the river water, and it was thought that the current river neutralization facilities were unable to carry out the neutralization treatment adequately in such a situation. There was the suggestion that the deterioration in the water quality of the downstream river at the end of 2023 and in 2024 was caused by a large amount of suspended volcanic ejecta in water from the crater entering the neutralization facility and accumulating in the neutralization basin, causing a malfunction of the facilities. However, the current neutralization facility is likely incapable of dealing with the exceptionally highly acidic river water that could be produced if the volcanic activity at Iwo-yama volcano were to increase. The capacity of the neutralization facility may need to be improved.
When comparing the water chemistry of the hot water pools in 2018 and 2024, the 2024 water had higher concentrations of most components. In addition, the Cl- concentration in 2024 was significantly higher than in 2018, and there was a significant difference in the Cl-/SO42- concentration ratio. The fluctuation in Cl- concentration was in harmony with the fluctuation in pH. After the end of 2023, the increase in the supply of HCl to the craters led to an increase in the acidity of the hot water pools. As a result, the inflow of overflow water from the hot water pools into the river has caused a significant drop in the pH of the river water, and it was thought that the current river neutralization facilities were unable to carry out the neutralization treatment adequately in such a situation. There was the suggestion that the deterioration in the water quality of the downstream river at the end of 2023 and in 2024 was caused by a large amount of suspended volcanic ejecta in water from the crater entering the neutralization facility and accumulating in the neutralization basin, causing a malfunction of the facilities. However, the current neutralization facility is likely incapable of dealing with the exceptionally highly acidic river water that could be produced if the volcanic activity at Iwo-yama volcano were to increase. The capacity of the neutralization facility may need to be improved.