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

We examined spatial variation in vertical ¹³⁷ Cs 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 m ² 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/m ² /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/m ² /month. The ratio of ¹³⁷ Cs flux during the survey period to litter ¹³⁷ Cs inventory was 6% and 1% on average in the deciduous broad-leaved and Japanese cedar sites, respectively. The ¹³⁷ Cs 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 ¹³⁷ Cs 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 ¹³⁷ Cs flux were affected positively by those in the litter ¹³⁷ Cs inventory and negatively by canopy openness.
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