Depth profile and mobility of 129I and 137Cs in soil originating from the Fukushima Dai-ichi Nuclear Power Plant accident

The 129I derived from the FDNPP accident were clearly identified near the surface and showed a trend of rapid decrease with depth. The FDNPP 129I and 137Cs was 51.6 ± 1.7 mBq cm−2 and 88.2 ± 27.1 kBq cm−2 (average of four cores inventory) respectively. On average, 91% of the FDNPP 129I existed within the top 5 g cm−2 and 98% within the top 10 g cm−2 and average of 100% of the FDNPP 137Cs existed within the top 5 g cm−2. From the observation of the temporal variation of depth profiles from the same upland field (Kawauchi village, 20 km away from the FDNPP to the southwest direction), downward migration rates of 0.81 ± 0.32 g cm−2 yr−1 for the FDNPP 129I and 0.19 ± 0.17 g cm−2 yr−1 for the FDNPP 137Cs were estimated. A simple diffusion model was introduced to evaluate the downward mobility of the FDNPP-derived 129I and 137Cs. The apparent diffusion coefficients D of 0.0086 ± 0.0034 and 0.0011 ± 0.0010 g2 cm−4 d−1 were obtained for 129I and 137Cs, respectively. These values might be representative for Haplic Gray lowland soils in near the steady state under humid temperate climate.