Iodine Isotopes in Precipitation: Temporal Responses to ¹²⁹I Emissions from the Fukushima Nuclear Accident

The Fukushima Dai-ichi Nuclear Power Plant accident in 2011 has released a large amount of radionuclides to the atmosphere, and the radioactive plume has been dispersed to a large area in Europe and returned to Asia. To explore long-term trend of the Fukushima-derived radioactive plume and the behavior of harmful radioiodine in the atmosphere, long-term precipitation samples have been collected over 2010–2012 at Fukushima, Japan for determination of long-lived 129I. It was observed that 129I concentrations of 1.2 × 108 atom/L in 2010 before the accident dramatically increased by ∼4 orders of magnitude to 7.6 × 1011 atom/L in March 2011 immediately after the accident, with a 129I/127I ratio up to 6.9 × 10–5. Afterward, the 129I concentrations in precipitation decreased exponentially to ∼3 × 109 atom/L by October 2011 with a half-life of about 29 days. This declining trend of 129I concentrations in precipitation was interrupted around October 2011 by a new input of 129I to the atmosphere following a second exponential decrease. Such a cycle has occurred three times until the present. This temporal variation can be attributed to alternating 129I dispersion and resuspension from the contaminated local environment. A 129I/131I atomic ratio of 16 ± 1 obtained from rainwater samples is comparable with a value estimated for surface soil samples. 129I results from Denmark suggest an insignificant effect of 129I released from Fukushima to the 129I levels in Europe.