Radiocesium fallout in the grasslands on Sakhalin, Kunashir and Shikotan Islands due to Fukushima accident: the radioactive contamination of soil and plants in 2011

The accident at the Fukushima Dai-ichi Nuclear Power Plant has resulted in radioactive contamination of environmental media and food in the Far East of Russia, particularly in the Sakhalin Region. To obtain the knowledge about the 134Cs and 137Cs spatial distribution in the Sakhalin Region, soil samples were collected at 31 representative grassland sites on Sakhalin, Kunashir and Shikotan islands (43.80°–46.40° N and 142.73°–146.84° E) in the middle of May and around the end of September to early October 2011. In the autumn, vegetation samples (mixed grass/forb crop and bamboo, Sasa sp.) were collected together with soil samples. Maximum measured activity concentrations (on dry weight) of 134Cs and 137Cs in soil were 30 Bq kg−1 and 210 Bq kg−1, respectively. Within soil profile, 134Cs activity concentrations declined rapidly with depth. Although for both sampling occasions (in the spring and autumn) the radionuclide was completely retained in the upper 3–4 cm of soil, a deeper penetration of the contaminant into the ground was observed in the autumn. In contrast with 134Cs, activity concentrations of 137Cs demonstrated a broad range of the vertical distribution in soil; at most sites, the radionuclide was found down to a depth of 20 cm. This resulted from interfering the aged pre-accidental 137Cs and the new Fukushima-borne 137Cs. To calculate contribution of these sources to the inventory of 137Cs, the 134Cs:137Cs activity ratio of 1:1 in Fukushima fallout (the reference date 15 March 2011) was used. The maximum deposition density of Fukushima-derived 137Cs was found on Shikotan and Kunashir Islands with average density of 0.124 ± 0.018 kBq m−2 and 0.086 ± 0.026 kBq m−2, respectively. Sakhalin Island was less contaminated by Fukushima-derived 137Cs of 0.021 ± 0.018 kBq m−2. For the south of Sakhalin Island, the reference inventory of pre-Fukushima 137Cs was calculated as 1.93 ± 0.25 kBq m−2 (reference date 15 March 2011). For Shikotan and Kunashir Islands, the pre-Fukushima reference levels of 137Cs ground contamination appeared to be higher: on average, 2.81 ± 0.35 kBq m−2. Maximum measured activity concentrations (on wet weight) of 134Cs and 137Cs in the vegetation were 5 Bq kg−1 and 18 Bq kg−1, respectively. Soil-to-plant aggregated transfer factors, Tags, for 134Cs were more than an order of magnitude higher than those for 137Cs. For the above-ground biomass density of 1 kg per m2 (wet weight), plant contamination may contribute approximately 2% and 0.1% to the ground deposition of Fukushima-derived and pre-accidental 137Cs, respectively. -- Highlights: ► Soil and grass samples from Sakhalin and Kuril islands were analyzed. ► 134Cs of Fukushima origin and 137Cs have been quantified in the samples. ► In 2011, 134Cs ground deposition for the 31 sampling plots ranged from 8 Bq m−2 to 155 Bq m−2. ► 137Cs ground deposition ranged from 83 Bq m−2 to 3200 Bq m−2. ► Maximum activity concentration of 134Cs in the vegetation was 5 Bq kg−1