Study on the migration behavior of Fukushima accident-derived iodine-129 from land area to the marine environment

The March 2011 accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in Japan results in the massive release of high volatility fission products, including ¹²⁹I (8.1 GBq) and ¹³¹I (120 - 160 PBq). The long-lived ¹²⁹I (T_1/2: 15.7 million years) is one of the important radionuclide that we need to evaluate its migration behavior from Japan's land area to the marine environment because of its relatively high chemical reactivity, biological concentration in the marine ecosystem, and affinity for the thyroid gland although it is less radiologically harmful than the short-lived ¹³¹I (T_1/2: 8.02 days). The present study aimed to evaluate the following three points: 1) the source and the discharge of particulate ¹²⁹I in the Niida River, and 2) ¹²⁹I distribution in the sea water and the marine organism in area close to the FDNPP. In the Niida River system where the upstream are in the relatively high-contaminated area located 30 - 40 km northwest from the FDNPP, total suspended solid (SS) each month from December 2012 to January 2014 were continuously collected at Haramachi site (5.5 km upstream from the river mouth) of the downstream river by installing the time-integrative SS sampler. On the other hand, sea water samples of 2 L in water depths of 0 m, 5 m, 10 m, 15 m, 20 m, and rock fish were collected at a site 6 km south-southeast of the FDNPP in July 2014. The iodine in SS and rock fish of 0.2 - 0.5 g was volatilized and trapped in an organic alkaline solution (∼ 10 ml) by pyrohydrolysis method. After adding 2 mg iodine carrier to the trap solution and the filtered water sample of 1 L, the iodine was isolated and precipitated as AgI. The ¹²⁹I/¹²⁷I ratio of AgI targets were measured using an AMS system at the Micro Analysis Laboratory Tandem Accelerator (MALT), The University of Tokyo. ¹²⁷I in the trap solution and the water sample were measured by an ICP-MS at the University of Tsukuba. The original ¹²⁹I activities and ¹²⁹I/¹²⁷I ratios in the samples were calculated using ¹²⁷I concentration obtained from ICP-MS and ¹²⁹I/¹²⁷I ratio obtained from AMS. ¹²⁹I activities and ¹²⁹I/¹²⁷I ratios in SS of the Niida River were 0.9 - 4.1 mBq kg^-1 and (2.5 - 4.4)x10^-8, which were strongly correlated with the total dry weight of monthly SS samples with correlation coefficient (R²) of over 0.79. ¹²⁹I activity and ¹²⁹I/¹²⁷I ratio in SS were considered to infer the source of SS (relatively high-level contaminated upstream area or low-level contaminated downstream area). Meanwhile, the discharged particulate ¹²⁹I at Haramachi site were estimated to be 7.6 - 9.0 kBq month^-1 from September to October 2013. Therefore, relatively massive amount of particulate ¹²⁹I from the upstream were transported to the downstream of the Niida River from September to October 2013. The ¹²⁹I activity in surface sea water 40 months after the accident was 2.2 μBq L^-1, 2 times larger than the previous ¹²⁹I data in surface sea water 3 months after the accident. ¹²⁹I activities in the rock fish were 42 - 48 μBq kg^-1, approximately 20 - 400 times larger than that of sea water of same site. Further investigations are needed to evaluate the mechanism. (author)