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 (127)I and (129)I. These are combined with previously reported data to investigate atmospheric levels and behaviour of Fukushima-derived (129)I before and after the 2011 nuclear accident. In the new datasets, (127)I and (129)I concentrations between October 2012 and October 2014 varied from 0.5 to 10 μg/L and from 1.2 × 10(8) to 6.9 × 10(9) atoms/L respectively, resulting in (129)I/(127)I atomic ratio ranges from 3 × 10(-8) to 2 × 10(-7). The (127)I concentrations were in good agreement with those in the previous period from March 2011 to September 2012, whereas the (129)I concentrations and (129)I/(127)I ratios followed declining trends since the accident. Although (129)I concentrations in five samples during the period of 2013-2014 have approached the pre-accident levels, (129)I concentrations in most samples remained higher values in winter and spring-summer. The high (129)I 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 (129)I from contaminated soil and water, respectively. Long-term declining rate suggests that contribution of the Fukushima-derived (129)I to the atmosphere would become less since 2014.