Vertical distribution of ¹²⁹I and radiocesium in forest soil collected near the Fukushima Daiichi Nuclear Power Plant boundary

Three soil core samples were collected from a forest located about 1.1 km south of the Fukushima Daiichi Nuclear Power Plant (FDNPP) boundary in 2017, and the vertical profiles of ¹²⁹I from the FDNPP accident were determined by the combination of TMAH (tetramethyl ammonium hydroxide) extraction and ICP-MS/MS analysis. The humus layer above the soil layer was heavily contaminated with ¹³⁴Cs (1983-5985 Bq g^-1) and ¹³⁷Cs (1947-5902 Bq g^-1) (decay-corrected to March 11, 2011). The ¹²⁹I activity concentrations decreased sharply with the soil depth, from 1894 to 34.1, from 9384 to 78.9, and from 2536 to 51.3 mBq kg^-1, for the three sites. Downward migration of ¹²⁹I was slightly faster than the one of ¹³⁴Cs. In addition, the cumulative ¹²⁹I inventories were observed to be 43.4 ± 1.0, 71.7 ± 1.8, and 56.5 ± 1.8 Bq m^-2, respectively. Subsequently, the cumulative ¹³¹I inventories were estimated to be 1.76 ± 0.06, 2.90 ± 0.11, and 2.28 ± 0.10 GBq m^-2 (decay-corrected to March 11, 2011), respectively. Finally, the total atmospheric deposition of ¹²⁹I on the land of Japan due to the FDNPP accident was estimated to be around 1.09-1.71 kg (7.11-11.2 GBq).