Iodine isotopes in precipitation: temporal responses to (129)i 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 (129)I. It was observed that (129)I concentrations of 1.2 × 10(8) atom/L in 2010 before the accident dramatically increased by ∼4 orders of magnitude to 7.6 × 10(11) atom/L in March 2011 immediately after the accident, with a (129)I/(127)I ratio up to 6.9 × 10(-5). Afterward, the (129)I concentrations in precipitation decreased exponentially to ∼3 × 10(9) atom/L by October 2011 with a half-life of about 29 days. This declining trend of (129)I concentrations in precipitation was interrupted around October 2011 by a new input of (129)I 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 (129)I dispersion and resuspension from the contaminated local environment. A (129)I/(131)I atomic ratio of 16 ± 1 obtained from rainwater samples is comparable with a value estimated for surface soil samples. (129)I results from Denmark suggest an insignificant effect of (129)I released from Fukushima to the (129)I levels in Europe.