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Fog containing radiocesium covered mountain forests immediately after the Fukushima Daiichi Nuclear Power Plant accident

Article title

Fogwater deposition of radiocesium in the forested mountains of East Japan during the Fukushima Daiichi Nuclear Power Plant accident: A key process in regional radioactive contamination

Author (affiliation)

Naohiro Imamura (a), Genki Katata (b), Mizuo Kajino (c)(d), Masahiro Kobayashi (a), Yuko Ito (a), Akio Akama (e)

(a) Department of Forest Soils, FFPRI, Tsukuba, Ibaraki, Japan.

(b) Ibaraki University, Mito, Ibaraki, Japan.

(c) University of Tsukuba, Tsukuba, Ibaraki, Japan.

(d) Meteorological Research Institute,Japan Meteorological Agency (JMA), Tsukuba, Ibaraki, Japan.

(e) Center of Forest Restoration and Radioecology, FFPRI, Tsukuba, Ibaraki, Japan.

Publication Journal

Atmospheric Environment, 224: 117339, Elsevier, 1 March 2020, DOI:10.1016/j.atmosenv.2020.117339( External link )

Content introduction

After the Fukushima Daiichi Nuclear Power Plant accident, large amounts of radioactive materials were deposited over wide areas of mountain forests. In order to clarify the distribution and movement of radiocesium in forest ecosystems and make predictions for the future, it is necessary to understand patterns with which radiocesium entered forests immediately after the accident.

Immediately after the nuclear accident, the authors measured fogwater (photo), throughfall and rainwater in mountain forests where radioactive contaminants had been dispersed. As a result, it was found that the radiocesium concentrations in fogwater and throughfall were higher than in rainfall. Normally, when rainwater passes through a forest canopy, the radiocesium concentration in throughfall decreases compared to rainfall because radiocesium is removed by the canopy. However, in the present study, when highly contaminated fogwater adhered to a canopy, the concentration in throughfall increased compared to rainfall (Fig. 1). Therefore, the radiocesium concentrations in rainfall and throughfall that had been observed at 21 locations in mountain forests in East Japan in March 2011 were additionally analyzed. As a result, it could be seen that forests that existed in high elevation areas tended to have relatively high concentrations in throughfall (Fig. 2). Such locations were corresponded to the fog deposition areas by atmospheric model calculations and cloud base height analyses, indicating that radiocesium had been deposited by clouds and fog in those mountain forests.

Compared with rainwater, fogwater with high radiocesium concentrations continued to adhere to leaf surfaces and tree bark for long periods. Therefore, it appears that radiocesium could be easily absorbed from tree surfaces. This study will be useful for explaining the radiocesium cycling in relatively high elevation mountain forests that have been contaminated by radioactive fogwater.


Photo: A fogwater sampler that was set up in the Chichibu Mt. 

Photo: A fogwater sampler that was set up in the Chichibu Mountains.


Figure 1: Relationship between concentrations of radiocesium

Figure 1: Relationship between concentrations of radiocesium in rainfall and throughfall for cases of rainfall and fog.

The thickness of the arrows indicates the concentration of radiocesium.


Figure 2:Relationship between elevation.

Figure 2:Relationship between elevation and the concentration of radiocesium in throughfall/concentration in rainfall at 21 forest locations in East Japan.