Effectiveness of decontamination by litter removal in Japanese forest ecosystems affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium ((137)Cs) contamination of forest ecosystems over a wide area. The removal of the forest floor litter layer has been considered a potential method for forest decontamination; however, its effectiveness remains largely unknown. We conducted a pilot-scale decontamination study in a deciduous broadleaved forest in Fukushima. The entire forest was decontaminated by removing the litter layer in July 2014, approximately 3.3 years after the accident, with the exception of two untreated plots. For three years after decontamination, we quantified (137)Cs contamination levels in the litter and topsoil layers and in the tree leaves, in the untreated and decontaminated areas. The decreased inventories of litter materials and the litter-associated (137)Cs in the decontaminated areas were observed only in the first year after decontamination. Generally, no decontamination effects were observed on the (137)Cs transfer in tree leaves. The primary reason for this was the rapid shift in the main reservoir of (137)Cs from litter layers to the underlying mineral soil, which differs from the observations in post-Chernobyl studies of European forest ecosystems. The results suggest that litter-removal decontamination can only be successful if it is implemented more quickly (within 1–2 years after the accident) for Japanese forest ecosystems.