Changes in water quality around Kirishima volcano after Mt. Shinmoe eruption in 2018(3)

Abstract

From the time of the eruption in April 2019 to the time of the survey in March 2019, the pH was lower than 3.5 and the EC was almost always higher than 800 μS/cm in the upper reaches of the Akago River and the Nagae River, which have headwaters around Mt. Iou. The lower reaches of the Nagae River and the downstream Sendai River were also affected by the eruption, with low pH and high EC observed at the beginning of the eruption. The levels of arsenic in the Akago River, the Nagae River and the downstream Sendai River also exceeded environmental standards from the beginning of the eruption. The water quality of the Akago River and the upper Nagae River also shows that the water is strongly influenced by volcanic activity of Mt.Iou, because the ratio of SO ₄^{2 −} and Cl ⁻ is high. Since the June 2019 survey, pH and EC values have risen and declined significantly in the upper reaches of the Akago and Nagae rivers. Arsenic levels have also fallen below environmental standards, and SO ₄^{2 -} and Cl ^- levels have also declined. This may be due to the construction of a waterway by Miyazaki Prefecture to improve water quality in May 2019 and the gradual weakening of volcanic activity since April 2019.

On the other hand, no major changes in water quality have been observed in rivers around Mt. Shinmoe due to volcanic eruptions or volcanic activity. There are several sites with low pH and high EC values, all of which are hot spring sites. The water quality composition of many rivers around Kirishima volcano was mostly SO ₄^{2 -}, HCO₃ ^-, etc., and it was proven that the origin of water quality was mostly due to the action of volcanic activity.

In the future, it is necessary to clarify the water environment of Kirishima Volcano by grasping the correlation between the hot spring existing in Kirishima Volcano and the river water, and to grasp the change of the water environment by the volcanic activity of Mt.Iou.