Soil ¹³⁷Cs Activity Concentrations in Upland fields after Decontamination and Its Transfer to Soybeans

<p>Soil dressing after topsoil removal is a decontamination method for radioactive materials, which was widely applied in Fukushima Prefecture after the Fukushima Daiichi Nuclear Power Plant accident. However, information on the actual conditions of decontaminated farmland, especially the soil conditions immediately after decontamination, is limited. In this report, we describe the distribution of ¹³⁷Cs in the dressed and bulk soil layers in the upland fields in the O district of Iitate village, Fukushima Prefecture, which were decontaminated by the Ministry of the Environment, Japan, in 2014, and the results of experimental cultivation of soybean in the decontaminated fields. In 2015, immediately after decontamination was completed, a dressed soil layer at least 5 cm thick was observed in most of the fields. The average thickness was 9.9 cm. The values were almost equivalent to those of the results in the demonstration project for farmland decontamination operated by the Ministry of Agriculture, Forestry, and Fisheries, Japan. The average ¹³⁷Cs concentration in the dressed soil layer was 666 Bq kg-dry soil (ds)⁻¹, which was significantly lower than 1708 Bq kgds⁻¹ in the bulk soil layer just below it. Variations within and among fields were quite large. In 2016, no dressed soil layer was found, indicating that tillage was carried out in all fields. The ¹³⁷Cs concentration in the plow layer (0-15 cm) averaged 1864 Bq kg-ds⁻¹ (489-4533 Bq kg-ds⁻¹), consistent with previous reports. In 2015, soybean, which is known to accumulate radioactive Cs in edible parts more easily than other crops, was grown experimentally in two decontaminated fields in the same area. As a result, the average ¹³⁷Cs concentrations ( SD) of soybean grains were 7.5 (2.3) Bq kg⁻¹ (¹³⁴Cs was below the detection limit), which was much lower than 100 Bq kg⁻¹, the standard for radiocesium contamination in food in Japan, indicating the possibility of resuming farming in the decontaminated farmland.</p>