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

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.