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

Abstract Concentrations of 129I and 127I 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, 129I discharge from the plant increased, and the 129I concentration and 129I/127I atom ratio in the kelps reached maxima of 42 μBq/g-dry and 264 × 10−11, respectively. By 2009, both had decreased by one order of magnitude. After the Fukushima Dai-ichi Nuclear Power Plant accident in 2011, the 129I concentration and 129I/127I atom ratio in the kelps increased to 2.24 μBq/g-dry and 11.6 × 10−11, respectively. After 2012, the ratio in kelps decreased to (2.1–8.9) × 10−11, which is almost the same as the seawater value off Aomori Prefecture before the test operation. The 129I/127I atom ratio in kelps thus represents the ambient seawater ratio during the growth period of the kelps.