Effects of local-scale decontamination in a secondary forest contaminated after the Fukushima nuclear power plant accident

We investigated whether local-scale decontamination (removal of the litter layer, superficial soil layer, and understory) in a secondary forest contaminated by the Fukushima nuclear power plant accident reduced Cs-137 contamination of the soil and litter. We also measured Cs-137 concentrations in plants and in the web-building spider Nephila clavata (Nephilidae: Arachnida), as an indicator species, to examine Cs-137 contamination in arthropods. One month after decontamination, the total Cs-137 contamination (soil + litter) was reduced by 20% (100 kBq center dot m(-2)) relative to that in an adjacent untreated (i.e., contaminated) area, which was however not statistically significant. Four months after decontamination, Cs-137 in the decontaminated area had increased to a level similar to those in the untreated area, and the air radiation dose in the decontaminated area was about 2.1 mu Sv center dot h(-1), significantly higher than that in the untreated area (1.9 mu Sv center dot h(-1)). This may have been attributed to a torrential rain event. Although no statistically significant reduction was observed, most spiders had a lower Cs-137 contamination than that before the decontamination. This implied that the decontamination may have reduced Cs-137 transfer from soil via litter to N. clavata through the detrital food chains, but may not have reduced the amount of Cs-137 transfer through grazing food chains because the concentration of Cs-137 in living tree leaves was not reduced by the decontamination. In autumn, about 2 kBq center dot m(-2) of Cs-137 was supplied from foliage to the ground by litterfall. The results suggested that removal of the litter and superficial soil layers in a contaminated forest may be ineffective. The present study suggests that the local-scale decontamination in a secondary forest had no effect on the reduction of Cs-137 contamination in the treated area. (C) 2017 Elsevier Ltd. All rights reserved.