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

Due to the accident at the Fukushima Daiichi Nuclear Power Station (FDNPS), radiocesium such as ¹³⁴Cs, ¹³⁵Cs, and ¹³⁷Cs was dispersed over a wide area of eastern Japan and mixed with radiocesium from global fallout. The depth profiles of ¹³⁷Cs 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 the ¹³⁷Cs concentration, ¹³⁴Cs/¹³⁷Cs radioactivity ratio, and ¹³⁵Cs/¹³⁷Cs isotope ratio were measured. It was found that the ¹³⁷Cs concentration varied among sites and within individual sites, while the ¹³⁴Cs/¹³⁷Cs radioactivity ratio was constant for all samples collected at 3 sites, 1.01 ± 0.04 (2σ). The ¹³⁵Cs/¹³⁷Cs isotope ratio for the two sites was constant and comparable to that obtained for soil sampled near FDNPS. On the other hand, the ¹³⁵Cs/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.