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

The (129)I derived from the FDNPP accident were clearly identified near the surface and showed a trend of rapid decrease with depth. The FDNPP (129)I and (137)Cs 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 (129)I existed within the top 5 g cm(-2) and 98% within the top 10 g cm(-2) and average of 100% of the FDNPP (137)Cs 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 (129)I and 0.19 ± 0.17 g cm(-2) yr(-1) for the FDNPP (137)Cs were estimated. A simple diffusion model was introduced to evaluate the downward mobility of the FDNPP-derived (129)I and (137)Cs. The apparent diffusion coefficients D of 0.0086 ± 0.0034 and 0.0011 ± 0.0010 g(2) cm(-)(4) d(-)(1) were obtained for (129)I and (137)Cs, respectively. These values might be representative for Haplic Gray lowland soils in near the steady state under humid temperate climate.