Post-accident response of near-surface ¹²⁹I levels and ¹²⁹I/¹²⁷I ratios in areas close to the Fukushima Dai-ichi Nuclear Power Plant, Japan

The deposition density and extent of subsurface infiltration of ¹²⁹I were investigated within the area highly contaminated by the March 2011 accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in Japan. Depth profiles of ¹²⁹I activity and ¹²⁹I/¹²⁷I ratios were measured in 5-cm-long soil cores that were collected 20–27 months after the accident from three sites (S-1, S-2, and S-3) within 10 km to the west and northwest of the FDNPP. These profiles were compared with the background levels of ¹²⁹I from the same areas. The deposition densities of ¹²⁹I were 2.17 Bq m⁻² at S-1, 0.80 Bq m⁻² at S-2, and 1.41 Bq m⁻² at S-3, which are, respectively, about 160, 60, and 100 times larger than the background level (12.8–14.8 mBq m⁻²). The average ¹²⁹I/¹²⁷I ratios in each soil core after the accident (S-1: 2.1 × 10⁻⁶; S-2: 4.2 × 10⁻⁷; S-3: 2.3 × 10⁻⁶) were similar to the value typically found in contaminated surface soil within 80 km of the FDNPP. Therefore, accident-derived ¹²⁹I deposited at S-1 (4.3 km west of the FDNPP) was 2.7 times larger than at S-2 (8.2 km west of the FDNPP), and 1.5 times larger than at S-3 (7.5 km northwest of the FDNPP). Depth profiles showed that the ¹²⁹I activity and ¹²⁹I/¹²⁷I ratios decayed exponentially with depth. ∼90% of accident-derived ¹²⁹I deposited in the soil cores was concentrated in the upper 25.2–26.4 kg m⁻² of mass depth (2.1–3.1 cm depth). The relaxation mass depths (h₀) of ¹²⁹I were 9.49–11.2 kg m⁻² at the three sampling sites, and these values are comparable to those of accident-derived ¹³¹I (10.4 kg m⁻²) at a site 40 km northwest of the FDNPP.