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Spatial and temporal variation in vertical migration of dissolved 137Cs passed through the litter layer in Fukushima forests

Kurihara, Momo, Onda, Yuichi, Suzuki, Hiroyuki, Iwasaki, Yuichi, Yasutaka, Tetsuo
Journal of environmental radioactivity 2018 v.192 pp. 1-9
Cryptomeria japonica, autumn, canopy, cesium radioisotopes, forests, lysimeters, nuclear power, power plants, summer, surveys, temporal variation, winter, Japan
We examined spatial variation in vertical ¹³⁷Cs flux from the litter layer using lysimeters combined with copper-substituted Prussian blue in two forests (deciduous broad-leaved and Japanese cedar (Cryptomeria japonica)), approximately 40 km northwest of the Fukushima Daiichi Nuclear power plant. The study ran from August 2016 to February 2017 in three periods; summer (10 Aug–4 Oct), autumn (5 Oct–30 Nov) and winter (1 Dec–27 Feb). Twenty-five and 15 lysimeters were installed in the deciduous broad-leaved and the Japanese cedar sites within 400 and 300 m² areas with 3–5 m intervals, respectively. The geometric means of the flux in the deciduous broad-leaved site were 0.51, 0.085 and 0.060 kBq/m²/month in summer, autumn and winter periods, respectively. In the Japanese cedar site, the mean fluxes were 0.45, 0.036 and 0.023 kBq/m²/month. The ratio of ¹³⁷Cs flux during the survey period to litter ¹³⁷Cs inventory was 6% and 1% on average in the deciduous broad-leaved and Japanese cedar sites, respectively. The ¹³⁷Cs flux in the summer period was much larger than those in other periods, resulting from higher precipitation in the summer. Our fine scale observation with 5 m interval showed very large spatial variation in the ¹³⁷Cs flux and the differences between maximum and minimum range from 8 to 104 times, but were mostly 20–25 times. The spatial variations in the ¹³⁷Cs flux were affected positively by those in the litter ¹³⁷Cs inventory and negatively by canopy openness.