Iodine isotopes in precipitation: Four-year time series variations before and after 2011 Fukushima nuclear accident

Rainwater samples were collected monthly from Fukushima, Japan, in 2012-2014 and analysed for ¹²⁷I and ¹²⁹I. These are combined with previously reported data to investigate atmospheric levels and behaviour of Fukushima-derived ¹²⁹I before and after the 2011 nuclear accident. In the new datasets, ¹²⁷I and ¹²⁹I concentrations between October 2012 and October 2014 varied from 0.5 to 10 μg/L and from 1.2 × 10⁸ to 6.9 × 10⁹ atoms/L respectively, resulting in ¹²⁹I/¹²⁷I atomic ratio ranges from 3 × 10-8 to 2 × 10^-7. The ¹²⁷I concentrations were in good agreement with those in the previous period from March 2011 to September 2012, whereas the ¹²⁹I concentrations and ¹²⁹I/¹²⁷I ratios followed declining trends since the accident. Although ¹²⁹I concentrations in five samples during the period of 2013-2014 have approached the pre-accident levels, ¹²⁹I concentrations in most samples remained higher values in winter and spring-summer. The high ¹²⁹I levels in winter and spring-summer are most likely attributed to local resuspension of the Fukushima-derived radionuclide-bearing fine soil particles deposited on land surfaces, and re-emission through vegetation taking up ¹²⁹I from contaminated soil and water, respectively. Long-term declining rate suggests that contribution of the Fukushima-derived ¹²⁹I to the atmosphere would become less since 2014.