Iodine-129 concentration in seawater near Fukushima before and after the accident at the Fukushima Daiichi Nuclear Power Plant

Anthropogenic radionuclides were released into the environment in large quantities by the Fukushima Daiichi Nuclear Power Plant (1FNPP) accident. To evaluate accident-derived 129I, the 129I concentrations in seawater before and after the accident were compared. Before the accident (2008–2009), the 129I concentrations in the western margin of the North Pacific between 32° N and 44° N showed a latitudinal gradient that was expressed as a linear function of latitude. The highest and average 129I concentrations after the accident were 73 times and approximately 8 times, respectively, higher than those before the accident in this study area. Considering the distribution of 129I in surface seawater, the accident-derived 129I in the southern and northern stations of the 1FNPP was predominantly supplied by seawater advection and atmospheric deposition (including microbial volatilization), respectively. As of October 2011, depth profiles of 129I revealed that 129I originating from the 1FNPP existed mainly in the upper 100 m depth. From the depth profiles, the cumulative inventories of accident-derived 129I were estimated to be (1.6–9.6) × 1012 atoms m−2 in this study area. On the basis of the 129I data in the seawater near Fukushima, the effective dose of 129I from seafood ingestion was much smaller than the annual dose limit.