Keywords

1 Introduction

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 (T 1/2 = 15.7 million years) and 131I (T 1/2 = 8.02 days). The total amount of radionuclides discharged into the atmosphere was estimated to be 8.1 GBq for 129I [1] and 120–150 PBq for 131I [2, 3]. Approximately 13 % of the total amount of released 131I was deposited over Japan via radioactive plumes [4]. 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.

2 Materials and Methods

The Niida River system in northeastern Fukushima Prefecture has a 78-km-long watershed with an area of 585 km2 [5]. The upper watershed is located 30–40 km northwest of the FDNPP and is affected by relatively high levels of radioactive contamination, with 137Cs inventory of >300 kBq m−2 (Fig. 6.1) [6]. In contrast, the downstream reaches are characterized by low levels of contamination, with 137Cs inventories of 10–300 kBq m−2 [6]. Mean annual precipitation is ∼1900 mm at Haramachi station in the downstream reaches and 1700–1800 mm at Iitate and Tsushima stations located near the upper watershed [7].

Fig. 6.1
figure 1

Map showing the sampling site (Haramachi) located 5.5 km upstream from the Niida river mouth along with the spatial 137Cs inventory from MEXT [6]. Total SS was collected continuously from December 2012 to January 2014 using a time-integrative SS sampler

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SS was collected continuously at Haramachi (5.5 km upstream from the river mouth) from December 2012 to January 2014 using a time-integrative SS sampler with an intake diameter of 4 mm (Fig. 6.2). This sampler was used successfully in a previous study of particulate 137Cs flux in the Fukushima river system [8, 9]. Monthly turbidity (kg m−3) and water discharge (m3 month−1) were calculated at sampling sites using data from a tuner turbidity sensor and water level sensor (Fig. 6.2). The flux of particulate 129I at Haramachi can be estimated using the following function:

Fig. 6.2
figure 2

(a) Schematic diagram of the time-integrative SS sampler. (b) Photographs of the tuner turbidity sensor, and (c) water level sensor for measuring discharge

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$$ \mathrm{F}\ \left({}^{129}\mathrm{I}\right) = \mathrm{A}\ \left({}^{129}\mathrm{I}\right)\cdot \mathrm{T}\cdot \mathrm{D} $$
(6.1)

where F (129I) is the flux of 129I (Bq month−1), A (129I) is the SS 129I activity (Bq kg−1), T is turbidity (kg m−3), and D is water discharge (m3 month−1).

Samples for measurements of 129I activity and 129I/127I ratios were prepared following Muramatsu et al. [10] and analyzed using an accelerator mass spectrometer (AMS) configured by Matsuzaki et al. [11]. 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 [12] 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.

3 Results and Discussion

3.1 Source of Particulate 129I in the Niida River System

Table 6.1 lists the total dry weight, 129I activity, and 129I/127I ratio for SS samples collected at Haramachi. As shown in Fig. 6.3, SS weights increased abruptly in March and April 2013, then increased continuously from May to October 2013. Based on meteorological data provided by the Japan Meteorological Agency, monthly precipitation was relatively high at Haramachi, Iitate, and Tsushima stations in April (140–160 mm), September (150–220 mm), and October (240–350 mm) [7]. Thus, the increased SS dry weights are thought to be related to higher-than-average precipitation in 2013.

Table 6.1 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
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Fig. 6.3
figure 3

(a) Temporal changes in monthly SS weight (black bars), 129I activity (open diamonds), and 129I/127I ratio (gray circles) at Haramachi. (b) Correlations between 129I activity and SS, and (C) the 129I/127I ratio and SS

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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 [13]. 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 [6]. 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.

3.2 Flux of Particulate 129I in the Niida River System

Table 6.2 lists the monthly flux of SS and associated 129I at Haramachi. The SS flux and particulate 129I are estimated to be 30–3200 ton month−1 and 0.1–9.0 kBq month−1 from March to October 2013, respectively. A higher 129I flux of 7.6–9.0 kBq month−1 was recorded in September and October 2013, when high monthly precipitation (150–350 mm) was observed in the Niida River watershed [7]. As discussed in Sect. 6.3.1, particulate 129I in September–October 2013 is considered to have originated mainly from the more highly contaminated upper watershed area. Therefore, a relatively high amount of particulate 129I was transported from the upstream to downstream reaches of the Niida River by a rain event during September and October 2013. Further investigation is needed to better understand the 129I flux in the river system.

Table 6.2 Monthly suspended substance flux and associated 129I at Haramachi in the downstream reaches of the Niida River system
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4 Conclusions

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.