[M-AG34] Radioisotope migration: New Development of Environmental Dynamics Research on the 1FNPS Accident

[MAG34-P04] Effect of water quality on the dynamics of dissolved radioactive Cs in river water from forested headwaters to downstream

*Tomura Kosuke¹, Yuichi Onda¹, Junko Takahashi¹, Taichi Kawano¹, Naoyuki Wada¹, Yutaro Nagata¹ (1.University of Tsukuba)

Keywords:dissolved Cs-137, ion competitiveness, decomposition of organic matter, leaching

Radioactive cesium Cs-137 deposited in forests after being dispersed into the atmosphere due to the Fukushima Daiichi Nuclear Power Plant accident is thought to be transferred to rivers in forested watersheds and transported to upstream, midstream, and downstream areas, and to be leached into the river water as dissolved Cs-137 mainly from organic matter through microbial decomposition or from soil particles through ion competition with other dissolved cations. In actual rivers, the contribution of these two leaching pathways is considered to be determined largely by the water quality of the river water, but there have been no studies to clarify the contribution of both leaching pathways from the headwaters to the downstream areas of rivers focusing on water quality. In this study, we analyzed stream and river water to examine the dissolved Cs-137 leaching pathways from the forest headwaters in the Yamakiya district, Fukushima, to the lower reaches of the Kuchibuto River. As a result, the concentrations of potassium ion (K⁺), which is involved in the leaching due to the ion-competitiveness , and dissolved organic carbon (DOC), which is leached by organic matter decomposition, both showed significant correlation with the concentration of dissolved Cs-137, indicating the existence of both leaching pathways for dissolved Cs-137 in the headwaters area. Here, overall, the correlation was lower with DOC concentration than with K⁺ concentration. However, considering that significant differences in K⁺ concentrations were observed between sites in correlation with distance from the spring, the contribution of organic matter decomposition to the leaching of dissolved Cs-137 was considered to be significant enough in headwater area which was especially close to the spring and K⁺ concentrations were especially low and close to the spring. On the other hand, in the upper, middle, and lower reaches of the river, where K⁺ concentrations are significantly higher than in the headwaters, the contribution of the leaching due to ion competition was suggested to be significant. In this way, we clarified the changes in the leaching dynamics of dissolved Cs-137 from the forested headwaters to the downstream area of the river, which is significantly affected by the river water quality represented by K⁺ concentration.