Behavior of 129I in the Abukuma River water during two high-flow events in 2018

Radioiodine is one of the most important radionuclides released by the accidents of Fukushima Dai-ichi nuclear power plant (FDNPP). Iodine-131 elevated ambient radiation dose rate immediately after the accident, but it is extinguished due to its short half-life. The long-lived ¹²⁹I can be used as a tracer to retrospectively infer the level of ¹³¹I. Understanding of environmental behavior of ¹²⁹I is important for preparedness against nuclear disaster. This study presents sampling campaigns on the Abukuma River during two high flow events, and discusses riverine ¹²⁹I behavior based on comparisons with that of ¹³⁷Cs. The study site was the Kuroiwa site locating at midstream of the Abukuma river. Its catchment area was 2886 km² and mean ¹²⁹I inventory in the catchment was 0.041 Bq m^-2. Five and seven river water samples were taken during high-flow events in July 2018 (JUL18) and October 2018 (OCT18), respectively. Suspended sediment and filtrate samples were obtained by decantation and subsequent filtration with 0.45 μm-mash membrane filter, respectively. The suspended sediment and filtrate samples were measured for ¹³⁷Cs activity concentrations with HPGe detector and then measured for ¹²⁹I/¹²⁷I ratio with accelerator mass spectrometer and for ¹²⁷I concentration with ICP-QQQ-MS. Mean ¹²⁹I activity concentration in suspended sediment during JUL18 and OCT18 were 1.0 and 0.43 mBq kg^-1, respectively. In terms of temporal trends, ¹²⁹I activity concentration in suspended sediment lowered in the peak water discharge phase, contrastive to those of ¹³⁷Cs which were the highest at peak water discharge phase. Mean dissolved ¹²⁹I activity concentrations during JUL18 and OCT18 were 0.18 and 0.067 μBq L^-1, respectively. Both ¹²⁹I and ¹³⁷Cs activity concentrations in dissolved form tended to decrease with time during two high-flow events. Mean apparent distribution coefficient (Kd) during JUL18 and OCT18 were 4.3 ×10⁴ and 6.1 ×10³ L kg^-1, respectively. The Kd values of ¹²⁹I were lower than those of ¹³⁷Cs and it reveal relatively-high solubility of ¹²⁹I. Total exportation of ¹²⁹I from the catchment during JUL18 and OCT18 were estimated as 1.0 × 10⁴ and 2.3 × 10⁴ Bq, respectively. Exported ¹²⁹I in dissolved form accounted for 80 and 27% of total exportations, respectively. By contrast, more than 95% of ¹³⁷Cs was exported in particulate form in the events. These results indicated an importance of dissolved form for understanding environmental behavior of radioiodine.