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.