Acid river systems and the actual situation of river neutralization at the Kusatsu-Shirane volcano, Gunma Prefecture, and Ebino highlands of the Kirishima volcanoes, Miyazaki Prefecture, are reviewed. In the case of the acid river system at the Kusatsu-Shirane volcano, sources of acid water are volcanic acidic hot springs and acid mine drainage (AMD) from closed sulfur mines. Volcanic hot springs and AMD have different effects on the river environment. Arsenic is predominantly supplied from hot springs, and heavy metals such as lead are predominantly supplied from AMD. The river neutralization system at the Kusatsu-Shirane volcano targets water from Kusatsu hot springs, the region's largest source of acidic components, resulting in a significant water quality improvement in the downstream river. The system at the Kusatsu hot springs directly injects a neutralizer prepared from powdered limestone into the river. The neutralization products formed in river water are finally accumulated in a dam constructed to function as a sedimentation basin. Then, the dredged sediment from the dam is dumped at dedicated disposal sites. In the case of the Ebino Highlands (Iwoyama volcano), the Kirishima volcano complex, an eruption in 2018 caused river water pollution. Phreatic eruptions at the Iwoyama volcano triggered discharges of acidic hydrothermal fluids containing large amounts of arsenic and heavy metals from vents newly opened with the eruptive events, resulting in a deterioration of river water quality and significant damage to local agriculture. Because the discharge of acidic hydrothermal fluids from the vents continued even after eruptive activity ended, Miyazaki Prefecture decided to neutralize the river in the Ebino Highlands. After starting river neutralization, the water quality of downstream rivers improved significantly. Because the amount of treatment required for river neutralization at the Ebino Highlands is not very large, a passive treatment method was adopted to enable low-cost operation.
