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

Abstract

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.