Behavior of 129I in the Abukuma River water during two high-flow events in 2018
Authors: Wakiyama Yoshifumi , Matsumura Masumi , Matsunaka Tetsuya , Hirao Shigekazu , Sasa Kimikazu 
Published: March 2021

&lt;p&gt;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 &lt;sup&gt;129&lt;/sup&gt;I can be used as a tracer to retrospectively infer the level of &lt;sup&gt;131&lt;/sup&gt;I. Understanding of environmental behavior of &lt;sup&gt;129&lt;/sup&gt;I is important for preparedness against nuclear disaster. This study presents sampling campaigns on the Abukuma River during two high flow events, and discusses riverine &lt;sup&gt;129&lt;/sup&gt;I behavior based on comparisons with that of &lt;sup&gt;137&lt;/sup&gt;Cs. The study site was the Kuroiwa site locating at midstream of the Abukuma river. Its catchment area was 2886 km&lt;sup&gt;2 &lt;/sup&gt;and mean &lt;sup&gt;129&lt;/sup&gt;I inventory in the catchment was 0.041 Bq m&lt;sup&gt;-2&lt;/sup&gt;. 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 &amp;#956;m-mash membrane filter, respectively. The suspended sediment and filtrate samples were measured for &lt;sup&gt;137&lt;/sup&gt;Cs activity concentrations with HPGe detector and then measured for &lt;sup&gt;129&lt;/sup&gt;I/&lt;sup&gt;127&lt;/sup&gt;I ratio with accelerator mass spectrometer and for &lt;sup&gt;127&lt;/sup&gt;I concentration with ICP-QQQ-MS. Mean &lt;sup&gt;129&lt;/sup&gt;I activity concentration in suspended sediment during JUL18 and OCT18 were 1.0 and 0.43 mBq kg&lt;sup&gt;-1&lt;/sup&gt;, respectively. In terms of temporal trends, &lt;sup&gt;129&lt;/sup&gt;I activity concentration in suspended sediment lowered in the peak water discharge phase, contrastive to those of &lt;sup&gt;137&lt;/sup&gt;Cs which were the highest at peak water discharge phase. Mean dissolved &lt;sup&gt;129&lt;/sup&gt;I activity concentrations during JUL18 and OCT18 were 0.18 and 0.067 &amp;#956;Bq L&lt;sup&gt;-1&lt;/sup&gt;, respectively. Both &lt;sup&gt;129&lt;/sup&gt;I and &lt;sup&gt;137&lt;/sup&gt;Cs 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 &amp;#215;10&lt;sup&gt;4&lt;/sup&gt; and 6.1 &amp;#215;10&lt;sup&gt;3&lt;/sup&gt; L kg&lt;sup&gt;-1&lt;/sup&gt;, respectively. The Kd values of &lt;sup&gt;129&lt;/sup&gt;I were lower than those of &lt;sup&gt;137&lt;/sup&gt;Cs and it reveal relatively-high solubility of &lt;sup&gt;129&lt;/sup&gt;I. Total exportation of &lt;sup&gt;129&lt;/sup&gt;I from the catchment during JUL18 and OCT18 were estimated as 1.0 &amp;#215; 10&lt;sup&gt;4&lt;/sup&gt; and 2.3 &amp;#215; 10&lt;sup&gt;4&lt;/sup&gt; Bq, respectively. Exported &lt;sup&gt;129&lt;/sup&gt;I in dissolved form accounted for 80 and 27% of total exportations, respectively. By contrast, more than 95% of &lt;sup&gt;137&lt;/sup&gt;Cs was exported in particulate form in the events. These results indicated an importance of dissolved form for understanding environmental behavior of radioiodine.&lt;/p&gt;