Migration Behavior of Particulate 129I in the Niida River System
This study investigates the source and flux of particulate 129I in the downstream reaches of the Niida River system in Fukushima. The upper watershed is a relatively highly contaminated zone located 30–40 km northwest of the Fukushima Daiichi Nuclear Power Plant. Samples of total suspended substance (SS) were collected continuously at Haramachi (5.5 km upstream from the river mouth) from December 2012 to January 2014 using a time-integrative SS sampler. Activity of 129I and the 129I/127I ratio in SS were 0.9–4.1 mBq kg−1 and (2.5–4.4) × 10−8, respectively, and were strongly correlated with the total dry weight of SS samples with R2 of 0.79–0.88. High SS 129I activity and 129I/127I ratios were found in March, April, September, and October 2013. SS 129I activity and 129I/127I ratios are considered to reflect the SS source, i.e., the more contaminated upper watershed or the less contaminated downstream area. The flux of particulate 129I at the Haramachi site was estimated to be 7.6–9.0 kBq month−1 during September–October 2013. A relatively high amount of particulate 129I may have been transported from the upstream to the downstream reaches of the Niida River by high rainfall over this period.
KeywordsRiver system Suspended substance 129I activity 129I/127I ratio Rain event Migration behavior Accelerator mass spectrometry
The nuclear accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP), Japan, resulted in the massive release of high-volatility fission products to the environment, including 129I (T1/2 = 15.7 million years) and 131I (T1/2 = 8.02 days). The total amount of radionuclides discharged into the atmosphere was estimated to be 8.1 GBq for 129I  and 120–150 PBq for 131I [2, 3]. Approximately 13 % of the total amount of released 131I was deposited over Japan via radioactive plumes . Any short-lived 131I deposited in the soil decays after a few months, however, long-lived 129I derived from the FDNPP accident must be traced from land to the marine environment via river systems owing to its relatively high fission yield, high chemical reactivity, biological concentration in the marine ecosystem, and affinity for the thyroid gland although it is less radiologically harmful than 131I.
This study aims to elucidate the source and flux of particulate 129I in the downstream reaches of the Niida River, a small river in Fukushima Prefecture with an upper watershed located in a relatively high-contamination zone 30–40 km northwest of the FDNPP. We investigated temporal changes in 129I activity and 129I/127I ratios in total suspended substance (SS) collected during December 2012–January 2014 at Haramachi on the Niida River. Particular attention is given to quantifying the monthly flux of particulate 129I in the downstream reaches of the Niida River system.
6.2 Materials and Methods
Samples for measurements of 129I activity and 129I/127I ratios were prepared following Muramatsu et al.  and analyzed using an accelerator mass spectrometer (AMS) configured by Matsuzaki et al. . Dried SS samples (∼0.5 g) were combusted with V2O5 at 1000 °C in a quartz tube for 30 min under a constant flow of pure O2 and water vapor. Volatilized iodine was trapped in an organic alkaline solution. Stable iodine (127I) in trap solutions was measured using an inductively coupled plasma-mass spectrometer (ICP–MS, Agilent 8800). After adding 2 mg of iodine carrier to the trap solution, the iodine was isolated and precipitated as AgI. The 129I/127I ratio of AgI targets was measured using an AMS system at the Micro Analysis Laboratory Tandem Accelerator (MALT), University of Tokyo. A terminal voltage of 3.47 MV and a charge state of 5+ were chosen for acceleration and detection. Measurement ratios were normalized against the Purdue-2 reference material, which has an 129I/127I ratio of 6.54 × 10−11  and was obtained from the Purdue Rare Isotope Measurement Laboratory (PRIME Lab) at Purdue University. The overall precision of the system was better than 5 %, and the blank levels, which included the iodine carrier, were 2.2–4.9 × 10−13 during all experimental procedures. The original 129I/127I ratio and 129I activity in SS samples were calculated using the 127I concentration obtained by ICP–MS and the 129I/127I ratio obtained by AMS.
6.3 Results and Discussion
6.3.1 Source of Particulate 129I in the Niida River System
Suspend substance (SS) weight, 129I activity, and 129I/127I ratios measured in samples collected at Haramachi in the downstream reaches of the Niida River system
(g dry weight)
March and April
2.06 ± 0.05
4.02 ± 0.11
0.92 ± 0.03
2.52 ± 0.09
1.71 ± 0.06
3.24 ± 0.11
2.15 ± 0.07
4.14 ± 0.14
4.07 ± 0.20
4.39 ± 0.22
2.86 ± 0.11
4.83 ± 0.20
SS 129I activity and 129I/127I ratios in Niida River samples were 0.9–4.1 mBq kg−1 and (2.5–4.4) × 10−8, respectively. These values are 2–10 and 2–3 times larger, respectively, than the pre-accident level for 129I activity of 0.42 mBq kg−1 and 129I/127I ratio of 1.6 × 10−8 at Fukushima before the FDNPP accident . Higher SS 129I activity and 129I/127I ratios were found in March, April, September, and October 2013, when the SS weights were relatively high. The 129I activity and 129I/127I ratios are strongly correlated with SS weight (R2 = 0.79–0.88). As described in Sect. 6.2, the Niida River flows through highly contaminated areas in the upper watershed and medium–low contamination areas in the middle to lower reaches . Therefore, it is possible that SS 129I activity and 129I/127I ratios reflect the source of SS, i.e., either the more contaminated upper watershed or less contaminated downstream area. Further study is needed to clarify the differences in 129I activity, 129I/127I ratios, and the 129I inventory in soil between the upper watershed and downstream areas.
6.3.2 Flux of Particulate 129I in the Niida River System
Monthly suspended substance flux and associated 129I at Haramachi in the downstream reaches of the Niida River system
Suspended substance flux
Monthly SS 129I activity and 129I/127I ratios measured from March to October 2013 in the downstream reaches of the Niida River system were 0.9–4.1 mBq kg−1 and (2.5–4.4) × 10−8, respectively. These values are strongly correlated with the total SS dry weight (R2 = 0.79–0.88). The SS 129I activity and 129I/127I ratio are considered to reflect the source of SS, i.e., the more contaminated upper watershed or the less contaminated downstream area. The particulate 129I flux at Haramachi was estimated to be 7.6–9.0 kBq month−1 from September to October 2013. Relatively large amounts of particulate 129I were transported by heavy rain from the upstream to downstream reaches of the Niida River over this period.
We are grateful to the staff of MALT, The University of Tokyo, for technical assistance with 129I measurements. This work was supported by JSPS KAKENHI Grant Numbers 24246156 and 24110006.
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