Regional distribution and isotope ratios of radiocesium from the Fukushima Daiichi nuclear power station and global fallout in Tokai-mura

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Due to the accident at the Fukushima Daiichi Nuclear Power Station (FDNPS), radiocesium such as134Cs, 135Cs, and137Cs was dispersed over a wide area of eastern Japan and mixed with radiocesium from global fallout. The depth profiles of137Cs for samples taken in 2003 before the FDNPS accident and in 2017 after the FDNPS accident in Tokai-mura (about 115 km NE of Tokyo) were both described by exponential equations from the surface up to a depth of 15 cm. Systematic grid sampling of surface soil at a depth of 5 cm was conducted at 3 sites in Tokai-mura in 2019, and distributions of the137Cs concentration, 134Cs/137Cs radioactivity ratio, and135Cs/137Cs isotope ratio were measured. It was found that the137Cs concentration varied among sites and within individual sites, while the134Cs/137Cs radioactivity ratio was constant for all samples collected at 3 sites, 1.01 ± 0.04 (2σ). The135Cs/137Cs isotope ratio for the two sites was constant and comparable to that obtained for soil sampled near FDNPS. On the other hand, the135Cs/137Cs isotope ratio for the other site varied and showed higher values (0.355–0.446), suggesting the influence of global fallout. Based on the results, the mixture percentages of radiocesium originating from global fallout and the FDNPS accident were estimated.Supplementary InformationThe online version contains supplementary material available at 10.1038/s41598-025-25911-7.