Since the eruption of Mt.Ioyama, Miyazaki Pref., Japan on 19th April 2018, arsenic was detected in the river exceeding the regulatory limit, which affected the planting of rice in 2018. In this study, we collected water and sediments along the Akago-Nagae River from the Ebino Bridge to the Hishikari canoe Stadium in order to clarify the dynamics of the drained arsenic in the river. Arsenic concentration in the river water decreased toward the downstream, however, from the Ebino Bridge to the Nagae Bridge, it was diluted by the increase of the river water. On the other hand, the adsorption of oxoanion by ferrihydride from the Nagae Bridge to the Yuda Bridge removed arsenic from river water.
