Deposition of 129I in the forests of Koriyama and 129I/137Cs ratio originated from the Fukushima Daichi Nuclear Power Plant

The radionuclides ¹²⁹I and ¹³⁷Cs released during the 2011 Fukushima nuclear accident led to contamination of forested environments. The concentrations of these nuclides in precipitation, as well as their subsequent environmental behavior, are critical for assessing internal radiation exposure. In this study, deposition records of atmospheric ¹²⁹I and ¹³⁷Cs following the accident were reconstructed using a borehole drilled between 2012 and 2014 at Koriyama, located approximately 60 km from the accident site. After subtraction of contributions from global fallout and nuclear reprocessing facilities, the inventories of ¹²⁹I and ¹³⁷Cs in forest soil at Koriyama, integrated to a depth of 50 cm, were estimated to be 4.80 × 10⁵ and 81.7 mBq m⁻², respectively. The ¹²⁹I/¹³⁷Cs radioactivity ratio derived from Fukushima-derived deposition in litter and soil (0–50-cm depth) was 1.71 × 10⁻⁷, which is consistent with the ratio observed in atmospheric aerosols at the time of the accident. The ¹²⁹I/¹³⁷Cs radioactivity ratio in the litter layer was marginally lower than that in the underlying topsoil. This difference is attributed to the higher solubility and mobility of ¹²⁹I relative to ¹³⁷Cs in litter, resulting in preferential washout from the surface layer. It is therefore inferred that a fraction of ¹²⁹I originally retained in the litter layer has migrated from the forest surface toward riverine systems.