Atmospheric Fallout of 129I in Japan before the Fukushima Accident: Regional and Global Contributions (1963-2005)

Atmospheric I-129 deposition was studied in different locations of Japan (Akita, Tsukuba, Tokyo, and Ishigald Island) with samples collected between 1963 and 2005 in order to understand the distribution and sources of this nuclide and provide a reference deposition level prior to the Fukushima accident. Over this time period, the deposition pattern of I-129 in Tsukuba and Tokyo (on the Pacific side) differed from that of Akita (on the Japan Sea side). The primary source of deposition in Tsukuba and Tokyo is related to the I-129 discharge from domestic reprocessing in Tokai-mura. In contrast, the time-series pattern of deposition in Akita seems to have been influenced by I-129 discharges from reprocessing facilities in Europe and the transport of this radionuclide by westerly winds to coastlines of the Japan Sea. The I-129 deposition in Ishigaki (one of the southernmost islands in Japan) is influenced primarily by oceanic air masses (easterly winds), and deposition was 1 order of magnitude lower than that observed in Tsukuba and Tokyo. Cumulative I-129 deposition in Tokyo before the Fukushima accident was estimated at 13 mBq/m(2). The results of this study on deposition contribute to understanding the deposition levels of I-129 prior to the accident.