Distribution of Radiocesium in Gunma Prefecture Released by the Fukushima Daiichi Nuclear Power Plant Accident

We investigated the distribution of radiocesium in soils of Gunma Prefecture released by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The arithmetic mean of the Cs-134/Cs-137 radioactivity ratio of soil samples (n = 253) from Gunma Pref. is 0.996±0.0014 (decay corrected to March 11, 2011), which may indicate that the radiocesium precipitated in Gunma Pref. is mainly derived from the Reactors 2 and 3 in the FDNPP. The geometric mean of the concentrations (decay corrected to March 25, 2011) of radiocesium (Cs-134 + Cs-137) in soil samples (n = 267) collected from Gunma Pref. is about 1 kBq kg−1. However, the geometric mean (1.4 kBq kg−1, n = 171) of mountainous soil samples of Gunma Pref. is two or three-times higher than that (0.54 kBq kg−1, n = 96) of plain soil ones of the same pref. The profiles of vertical distribution of radiocesium in soil samples collected from 3 different environmental situations are quite similar despite the difference in the kinds and amount ratios of clay minerals included. Moreover, more than 95 % of radiocesium is retained within the upper part (0-4 cm), even though 5 years after elapsed. The relationship between the spatial radiation dose rate of sampling points (1 m height) and the concentrations of radiocesium in soil samples is positive but weak (r = 0.65), probably because the concentration of radiocesium in soil samples changes widely even in a small area. The distribution of radiocesium deposition obtained in this work is substantially consistent with that by the MEXT (Ministry of Education, Culture, Sports, Science and Technology) airborne radiation monitoring, except the area (≤10 kBq m−2) of the plain region of Gunma Pref.