Temporal variation of iodine-129 concentrations in kelps (Saccharina) from coastal waters off northern Japan.

Concentrations of I and I in kelps (Saccharina) collected from coastal waters off northern Japan were monitored from 2007 to 2019. During the 2007-2008 test operation of the Rokkasho nuclear fuel reprocessing plant, I discharge from the plant increased, and the I concentration and I/I atom ratio in the kelps reached maxima of 42 μBq/g-dry and 264 × 10, respectively. By 2009, both had decreased by one order of magnitude. After the Fukushima Dai-ichi Nuclear Power Plant accident in 2011, the I concentration and I/I atom ratio in the kelps increased to 2.24 μBq/g-dry and 11.6 × 10, respectively. After 2012, the ratio in kelps decreased to (2.1-8.9) × 10, which is almost the same as the seawater value off Aomori Prefecture before the test operation. The I/I atom ratio in kelps thus represents the ambient seawater ratio during the growth period of the kelps.