[M-IS14] Biogeochemistry

[MIS14-P03] Spatial pattern of water chemistry in the Arakawa River, Fukushima Prefecture

*Asuka Saito¹, Keitaro Fukushima¹, Yasunori Igarashi² (1.Faculty of Food and Agricultural Science, Fukushima University , 2.Fukushima University)

Keywords:spatial pattern, river water chemistry, acidity, land use

The Arakawa River, which one of the major tributaries of the Abukuma River, flows through Fukushima City, Fukushima Prefecture, has been selected as the “river with the best water quality in Japan” by Ministry of Land, Infrastructure, Transport and Tourism (MLTI) for 13 consecutive years. However, it is unknown why the water chemistry meets the stand value (BOD) in the river. Our aim of this study is to clarity the spatial pattern and determining factors of river water chemistry in Arakawa River watershed by comparing with near Matsukawa and Yuigawa River watersheds. In June and October 2023, we investigated spatial variations of river water chemistry including pH, major cations and anions including ammonium, nitrate and phosphate, total dissolved Fe and Al, and suspended sediments. We found (1) ammonium, nitrate and phosphate concentrations increased downstream with an increase of agricultural land and residential areas in three river watersheds, (2) pH decreased considerably in the middle reaches of Arakawa River because of the mixing of strongly acidic tributary originating from the volcano, (3) precipitation of some Al and Fe on the riverbed due to neutralizing strongly acidic tributary river by neutral main river. This neutralization and precipitation processes form an environment unfavorable for aquatic livings in Arakawa River, which is one of the possible factors in meeting the water standard by MLIT.