Investigations in Namie Town, Fukushima Prefecture, have revealed that the rate of ¹³⁷Cs outflow (※1) in 27 months (August 29, 2013-December 1, 2015) is about 0.1% in a year, which is a bit higher than those in other investigation plots, but less than 1% in a year.(Lower-left table)

The rate of soil and ¹³⁷Cs outflow increase by heavy rain about once every ten years(Investigation period: Sep 2-Dec1, 2015), but usual rate of ¹³⁷Cs outflows (Investigation period: Aug 29, 2013-Sep 2, 2015) is estimated 0.05-0.24 % per a year(Lower-left table).

Surface soil deposited in soil saving dam, Namie Town was taken as samples (5 cm), and the concentration of cesium was analyzed. It was observed that the concentration of radioactive cesium is decreasing with time (Lower-right figure).

Table 1　Amount and rate of radioactive cesium and soil outflow from the soil saving dam, Kawafusa, Ogaki, Namie Town, Fukushima Prefecture

Period	Amount of outflowing soil	Amount of outflowing 137Cs	Rate of outflowing radioactive cesium
Aug 29,2013 ～Dec 3,2014	0.5 m3	150 MBq	0.24 ％ (0.19％/year)
Dec 3,2014 ～Sep 2,2015	0.1 m3	30 MBq	0.05 ％ (0.06％/year)
Sep 2,2015 ～Dec 1※	1.8 m3	540 MBq	0.86 ％ (3.4％/year)
Total of investigation period	2.4 m3	720 MBq	1.14 ％ (0.51％/year)

※ Rain fall in Sep 9 and 10, 2015 is 114 mm and 171 mm per a day, 20% of total rain fall of the year was fall in 2 days.

Fig.1　Concentration of ¹³⁷Cs deposited on soil surface at soil saving dam, Namie Town
Watanabe et al, KEK Proceedings 2017-6, p.122-126, （2017）

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