Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant

Journal of Environmental Radioactivity xxx (2017) 1e10

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Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant Yoshifumi Wakiyama a, *, Alexei Konoplev a, Toshihiro Wada a, Tsugiko Takase a, Ian Byrnes b, Matthew Carradine b, Kenji Nanba a a b

Institute of Environmental Radioactivity, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan Department of Environmental and Radiological Health Sciences, Colorado State University, Fort Collins, CO 80521, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 December 2016 Received in revised form 19 July 2017 Accepted 26 July 2017 Available online xxx

137

Keywords: Dissolved 137Cs Distribution coefficient Fukushima Particulate 137Cs Pond

Cs activity concentration in the water of four ponds, Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ), and Kashiramori (KM), that are within 10 km of the Fukushima Dai-ichi nuclear power plant were observed from April 2015 to August 2016. 137Cs inventories in soils surrounding SU, FS, IZ, and KM were 6.4, 2.9, 2.1, and 0.9 MBq m
2, respectively. 137Cs inventories in the bottom sediments of SU, FS, IZ, and KM were 13, 8.9, 1.6, and 1.1 MBq m
2, respectively. Higher 137Cs inventories in bottom sediment than those of soil in SU and FS suggest that 137Cs was delivered to and accumulated in these ponds. Mean total 137Cs activity concentrations in SU, FS, IZ, and KM were 41, 13, 9.5, and 1.4 Bq L
1, respectively. Particulate 137Cs concentration accounted for 71e90% of total 137Cs in the water samples, on average. The mean distribution coefficient, Kd, in SU, FS, IZ, and KM was 1.3  105, 2.1  105, 1.7  105, and 6.2  105 L kg
1, respectively. These Kd values were higher than the Kd values observed in the Chernobyl area by 1e2 orders of magnitude. Although no significant decreasing trends were found, dissolved 137Cs activity concentration tended to be low during winter in all four ponds. Dissolved 137Cs activity concentrations were proportional to Kþ and DOC concentrations in all the ponds. The results from principal component analysis performed for 137Cs activity concentration and water chemistry data sets suggested that there were different mechanisms behind variability of dissolved 137Cs activity concentrations for each pond. Continuous monitoring is required to reveal temporal trends in 137Cs activity concentrations of these waters and controlling factors of such in closed water systems in Fukushima. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction The Fukushima Dai-ichi nuclear power plant (FDNPP) accident triggered by the Great East Japan earthquake and tsunami in March 2011 has led to radioactive contamination of extensive areas in northeastern Honshu Island, Japan (e.g., Chino et al., 2011; Steinhauser et al., 2014). Currently, radiocesium (mainly 134Cs and 137 Cs) is a major source of radiation in this area and its redistribution in the terrestrial environment and future predictions of such are important issues. In the years after the FDNPP accident, the distribution and transport of radiocesium in rivers in areas adjacent to the plant and in their catchments were studied quite extensively

* Corresponding author. Institute of Environmental Radioactivity, Fukushima University, 1 Kanayagawa, Fukushima City, Fukushima Prefecture 960-1296, Japan. E-mail addresses: [email protected], [email protected] (Y. Wakiyama).

(Nagao et al., 2013, 2015; Ueda et al., 2013; Yoshimura et al., 2015; Ochiai et al., 2015; Eyrolle-Boyer et al., 2016; Tsukada and Ohse, 2016). Understanding 137Cs behavior is crucial for dose assessment and predictions, for both humans and the environment. Following the Chernobyl accident, it was shown that the most sensitive environments to radiocesium contamination are closed and semi-closed water bodies, such as lakes and ponds, characterized by high organic matter content, and accordingly, increased ammonium concentrations in the water (e.g., Comans et al., 1989; Smith et al., 2005). For example, Konoplev et al. (1998) found high persistence of 137Cs in lakes Svyatoe and Kozhanovskoe in Russia after the Chernobyl accident. Zibold and Klemt (2005) presented results for more than ten years of observation of Lake Vorsee in Germany and proposed a model accounting for long-term trends and seasonal fluctuation in the 137Cs activity concentration of the lake water. However, the meteorological and geographical settings of those areas differ from those in the Fukushima area and the

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Please cite this article in press as: Wakiyama, Y., et al., Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant, Journal of Environmental Radioactivity (2017), http://dx.doi.org/10.1016/j.jenvrad.2017.07.017

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Y. Wakiyama et al. / Journal of Environmental Radioactivity xxx (2017) 1e10

applicability of these empirical models requires verification. Moreover, concerning Fukushima, understanding the temporal trends is required to develop site-specific parameterization for future prediction. Another important issue is the dynamics of dissolved radiocesium in the water column of the ponds. Dissolved radiocesium in the water column occurs in dynamic equilibrium with its exchangeable fraction on suspended solids, based on ion exchange mechanisms. This equilibrium can be characterized by the distribution coefficient Kd, which is originally the ratio of radionuclide steady-state activity concentrations in solid phase and in solution (at equilibrium). It is well documented that Kd values are influenced by the concentrations of major competitive ions such as Kþ and NHþ 4 (Cremers et al., 1988; Wauters et al., 1996). The behavior of dissolved radiocesium in river water has been documented previously by researchers in Fukushima and the controlling factors of 137 Cs activity concentration were discussed on basis of comparisons between dissolved 137Cs activity concentration and other water chemistry (e.g., Tsuji et al., 2016; Naulier et al., 2017). In the case of closed and/or semi closed water system such as reservoirs and lakes, it has been suggested that radiocesium can be mobilized from lacustrine anoxic sediments (e.g., Comans et al., 1989) and it resulted in seasonal cycling of 137Cs and hence Kd. Such processes may result in more complicated behavior of dissolved 137Cs in semiclosed and closed water systems. However, its behavior in the ponds in close proximity to the FDNPP in which fallout was the highest has yet to be investigated in detail. Fukushima Prefecture has more than 3700 individual ponds of different sizes, many of which are important for amelioration and are used for irrigation of rice paddies. 137Cs activity concentrations in water and bottom sediment have been tabulated by the Tohoku Regional Agricultural Administration Office of the Ministry of Agriculture, Forestry and Fishery (MAFF) (Tohoku Regional Agricultural Administration Office, 2016a; 2016b). The MAFF and Fukushima prefecture (2015) also investigated 2679 ponds in Fukushima Prefecture and reported their water and sediment contamination levels. However, only a few studies have described temporal trends and fluctuations in 137Cs activity concentration in the water of ponds in Fukushima (Kubota et al., 2015). Kubota et al. (2015) observed a heavily contaminated pond during the rainy season of 2013 and concluded that total 137Cs concentration in the pond water depends on particulate 137Cs, which depends on the concentration of suspended sediment. Understanding the mechanism of 137Cs behavior in these ponds is important not only for revitalization of the region but also preparedness for any possible, future nuclear emergencies. The objective of this study was to reveal 137Cs contamination levels and 137Cs behavior in the water of ponds in the most heavily contaminated area in Fukushima. 137Cs activity concentration in the soil, sediment, and water of four ponds located within 10 km of the FDNPP were measured. First, we present the contamination level of 137 Cs in the surrounding soils and bottom sediments of the ponds. After that, we provide results of 137Cs activity concentration in the water collected from these ponds from mid-2015 to mid-2016. Based on these results, we discuss the temporal variations in 137 Cs activity concentrations of the pond waters and the relevant controlling factors, by employing correlation analyses and Principal Component Analysis (PCA). 2. Materials and method 2.1. Study site We selected four irrigation ponds, Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ), and Kashiramori (KM), which are located within a

10 km radius of the FDNPP and in the town of Okuma (Fig. 1) (Konoplev et al., 2016b). The initial 137Cs deposition on Okuma reached 30 M Bq m
2, according to the fourth airborne survey by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) (MEXT, 2011). Mean annual precipitation and mean annual air temperature in this region were 1493 mm and 12.3  C, respectively, per records of the Namie weather station, which is the closest station to Okuma, from 1976 to 2015. The variabilities in precipitation and air temperature were quite high, both by year and by season. The maximum precipitation occurred in June to September, whereas in winter, the amount of precipitation was low. The temperature regime was characteristic of a monsoon climate with mild winters with mean monthly temperatures above zero and hot wet summers. Absence of soil freezing, high amount of precipitation, and significant diurnal temperature variations in the surface layer are conducive to active physicochemical processes in the soils of the region. The territory is a piedmont flat plain with separate highlands, which are edges of the massif that occurs west of the plain. The plain is made of alluvial-proluvial deposits that are partially reworked closer to the coast in the process of longshore transport. Sands dominate the alluvial soils formed on these deposits. Bedrock of this region consists of mainly two geologies, tephra beds of the Pliocene Dainenji formation and Quaternary deposits (Yanagisawa et al., 2003). The properties of the ponds are described in Table 1. Their water surface areas are relatively small. The maximum water depth of all the ponds was 5 m, found in KM, and therefore the ponds can be classified as shallow. Ambient dose rates measured in forests on banks of each pond ranged from 3.3 to 24 mSv h
1. SU had the highest ambient air dose rate among the catchments of the four ponds. An intermittent creek from the wetland near the pond flows during rainfall. FS is located in the center of Okuma and its banks and surrounding areas are paved. It is connected with the Kuma River system; with a creek flowing in from upstream and the pond water flowing out downstream to enter the Kuma River. IZ is the closest to the FDNPP and is surrounded by forest. No surface water flow into this pond was found. The KM catchment area is characterized by a relatively low ambient dose rate. An artificial dam is located on the north part of this pond and the other sides are surrounded by forest. At least one intermittent creek lies opposite the dam. 2.2. Soil and bottom sediment sampling and analyses For quantifying 137Cs contamination in the ponds, soils surrounding the ponds and sediment on the pond bottoms were collected. Soil cores were collected to a depth of 30 cm using a liner core sampler DIK-110C (DAIKI, Japan) with a plastic cylinder insert 5 cm in diameter. Additional soil sampling was conducted using a core sampler 11 cm in diameter and 5 cm in depth, to determine the 137 Cs distribution. Three cores of bottom sediment were obtained using 4 cm diameter polyvinyl core tubing in each pond. The sampling depth ranged from 33 to 56 cm. Sampling was conducted in June and July 2016. To compare the intensity of radioactive contamination with other ponds in Fukushima, sediments from the bottom surface were collected with an Ekman-Birge bottom sampler (Cat. No. 5141-B, Rigo Co. Ltd., Japan). The sampler is employed as part of an official method for bottom sediment sampling in Japan (Director of the Environmental Management Bureau, Ministry of the Environment, 2012). The upper layers of soil and sediment were sliced at every 1e2 cm and the lower layers at 3e5 cm. Each sample was dried at 50 С for at least three days, then ground and homogenized in a mortar. A representative fraction of each soil and sediment sample

Please cite this article in press as: Wakiyama, Y., et al., Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant, Journal of Environmental Radioactivity (2017), http://dx.doi.org/10.1016/j.jenvrad.2017.07.017

Y. Wakiyama et al. / Journal of Environmental Radioactivity xxx (2017) 1e10

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Fig. 1. Map of study site. This map was created using ArcGIS 10.2 software. Arrows on map of ponds indicate sampling point of bottom sediments. Gray lines show the boundaries of local governments.

Table 1 Properties of ponds. Parameter

SU

FS

IZ

KM

Distance from FDNPP, kma Dose rate (03/2014), mSv/h Catchment soil type Surface area, m2 Maximum depth, m

3.75 24 Fluviosol 4100 1

3.5 6 Rigosol 10,700 2.5

0.24 11 Fluviosol 6500 2

7 3.3 Andosol 8100 5

a Distances were determined from the study site to the nearest point on the boundary of the FDNPP industrial site.

was placed in a 100 mL plastic container, and then measured to determine its 137Cs activity concentration (Bq kg
1) using standard electrode coaxial Ge detectors (CANBERRA GC4020, Canberra, USA) with relative efficiency of 42.6% at the Institute of Environmental Radioactivity, Fukushima University. Measurements were taken up to 85,600 s to ensure a minimal statistical error of <5%. Of 116 sectioned core samples, the 8 samples taken from the deep portion of the bottom sediment had low 137Cs activity concentrations, at a minimum of 1.83 Bq kg
1, and high relative error, at 12.8% maximum. However, the contributions of 137Cs from these samples were small, at 0.48% maximum. The measured 137Cs activity concentration of the sediments and 137Cs inventories in the soils (Bq m
2) were calculated using the weight and sampling area of the samples.

2.3. Water sampling and analysis We collected water samples from the four ponds using a 2 L bucket on a 1 m rod at thirteen occasions (April 22, August 29, October 6, November 10, and December 7 in 2015 and January 16, February 16, March 16, April 13, May 20, June 6, July 6, and August 8 in 2016). Approximately 2 L of sampled water was filtered using 0.45-mm-pore cellulose acetate membrane filter units. Suspended

solids (SSs) were measured to calculate the suspended sediment concentration (SSC; mg L
1) of the pond water. To quantify the 137Cs activity concentration (Bq kg
1) and dissolved 137Cs activity concentration (Bq L
1) of the SSs and filtrates, respectively, standard electrode coaxial Ge detectors (CANBERRA GC4020, Canberra, USA) with relative efficiency of 42.6% were used at the Institute of Environmental Radioactivity, Fukushima University. Measurements were taken for 90,000 s for 137Cs activity concentrations in SSs and 240,000 s for dissolved 137Cs activity concentrations to ensure minimal statistical errors, which were <5% and <10%, respectively. Particulate 137Cs activity concentration (Bq L
1) was calculated by multiplying the SSC by the137Cs activity concentration of the SSs. Total 137Cs activity concentration is equal to the sum of the particulate and dissolved 137Cs activity concentrations. We calculated the apparent distribution coefficient Kd (L kg
1) by dividing the 137 Cs activity concentration of the SSs by the dissolved 137Cs activity concentration. Water samples were subjected to analyses of major ion water chemistry. After their pH was measured, the water samples were passed through a 0.45 mm mesh syringe filter. The concentrations of þ 2þ 2þ the major cations Kþ, NHþ were determined 4 , Na , Ca , and Mg by ion chromatography (DIONEX 1100, Thermo Scientific, USA). These concentrations were quantified based on calibration curves obtained using a mixed cation standard solution. The concentration of standard solution used for the calibration curves of Naþ and NHþ 4 ranged from 0.2 to 2.0 mg L
1 and those of Kþ, Ca2þ, and Mg2þ ranged from 0.5 to 5.0 mg L
1. If the concentration of a cation exceeded the range of its calibration curve, we diluted the sample with pure water, then reanalyzed. The detection limit of these cations differed among the samples and ranged from 0.03 to 0.05 mg L
1. Dissolved organic carbon (DOC) concentration was determined using a total organic carbon analyzer (TOC-L CSH, Shimadzu, CO., Japan). All samples for DOC measurement were pretreated with a 1 M HCl solution to remove inorganic carbon. DOC concentrations were quantified based on calibration curves

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Y. Wakiyama et al. / Journal of Environmental Radioactivity xxx (2017) 1e10

made using a potassium hydrogen phthalate solution at concentrations ranging from 0.3 to 3 mg C L
1. Stable 133Cs was determined by using inductively coupled plasma mass spectrometry (ICP-MS) (ELAN DRC 2, Perkin Elmer, USA). The samples were prepared by adding HNO3 to a volumetric concentration of 5%. The detection limit of 133Cs differed among the samples and ranged from 0.11 to 0.19 ppt. To reveal the factors controlling the variability of 137Cs activity concentrations in water, we investigated the relationship between 137 Cs activity concentration and water chemistry. Since there are many factors that affect the environmental behavior of 137Cs, we also conducted principal component analysis (PCA), based on the water chemistry data. The PCA was performed by using a free software R for statistical analyses (R core team, 2017). 3. Results and discussion 3.1.

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Cs in soil and bottom sediments

Table 2 shows the mean 137Cs inventories in the soils and bottom sediments and the 137Cs activity concentration in the bottom sediments. The 137Cs inventories of the bottom sediment were on the order of 105e106 Bq m
2. Considering that mean 137Cs inventory of a cooling pond next to the Chernobyl nuclear power plant was 7.7  106 Bq m
2 (Bulgakov et al., 2009), the 137Cs contamination of these ponds appeared to be very high. Independent T-test indicated that there was no significant difference in 137 Cs inventories between the bottom sediments and the respective soil samples for all four ponds. P values obtained by the independent T-test for SU, FS, IZ, and KM were 0.18, 0.08, 0.478, and 0.227, respectively. The enrichment factors, 137Cs inventories of the bottom sediment divided by those of the surrounding soils, in SU, FS, IZ, and KM were 2.0, 3.1, 0.76, and 1.2, respectively. The MAFF and Fukushima prefecture (2015) showed that the ratios of 137Cs inventories between catchment and bottom sediment were approximately 1.3. Assuming that our 137Cs inventories of the surrounding soils are equal to those of the corresponding catchment, it can be said that the enrichments of 137Cs in SU and FS were very high. Inflows from the pond next to SU and 137Cs wash-off from surrounding soils during rainfall appeared to have result in enrichment of 137Cs in SU. The high enrichment of FS can be attributed to the input of river water from upstream, a highly contaminated area. In contrast, IZ and KM did not show significant 137Cs enrichment. As shown in Fig. 1, IZ is not connected to any creeks or irrigation flow systems and its bank was flat and covered by forest. This situation resulted in the small input of 137Cs associated with inflow via creeks and 137 Cs wash-off from the bank in IZ. These results suggest that magnitudes of 137Cs enrichment in a pond depends on the drainage system and erodibility on the bank were important factors for controlling the abundance of 137Cs in the ponds.

Table 2 137 Cs inventories in soils and sediments. Pond

SU FS IZ KM

137

Cs activity concentration (kBq kg
1)

137

Bottom sediment

Soil

Cs inventory (MBq m-2) Bottom sediment

n

Mean ± SD*

n

Mean ± SD*

4 4 4 3

130 ± 24 170 ± 13 19 ± 6.4 38 ± 6.1

8 7 9 8

6.4 2.9 2.1 0.9

*Standard deviation (s).

± ± ± ±

2.2 0.9 1.1 0.04

n

Mean ± SD*

3 3 3 3

13 ± 6.5 8.9 ± 6.3 1.6 ± 0.67 1.1 ± 0.46

Fig. 2. Histogram of 137Cs activity concentration of bottom sediment (total 88 ponds including our four ponds). The data are based on Tohoku Regional Agricultural Administration Office (2016a) and Tohoku Regional Agricultural Administration Office (2016b) listing 137Cs activity concentrations of bottom sediments obtained in 67 ponds in evacuated zone and 17 ponds out of evacuated zone, respectively.

137 Cs activity concentrations in the bottom sediments ranged from 19 to 170 kBq kg
1. To compare these values with other Fukushima ponds, we made a histogram of frequency of 137Cs activity concentration of the bottom sediment based on the Tohoku Regional Agricultural Administration Office (2016a, 2016b), which lists values obtained in 67 ponds inside the evacuated zone and in 17 ponds outside the evacuated zone (Fig. 2). The MAFF and Fukushima Prefecture (2015) also reported radiocesium (137Cs þ134Cs) activity concentrations of bottom sediments collected with Ekman-Berge bottom samplers and indicated the number of ponds exceeding 100 kBq kg
1 was 33 out of 306. Comparing to these results, SU and FS were classified as the most contaminated ponds and IZ and KM were classified as moderately contaminated ponds in Fukushima.

3.2. Mean values of chemistry

137

Cs activity concentration and other water

Table 3 summarizes the observation of 137Cs activity concentration and other water chemistry results. SSC tended to be higher in shallower ponds. The mean 137Cs activity concentration of SSs was higher than the mean 137Cs activity concentration in the surface layer of the bottom sediments in all four ponds. This is because the 137Cs activity concentration in sediment can be underestimated when using an Ekman-Birge bottom sediment sampler. Hinokio et al. (2015) compared 137Cs activity concentrations in bottom sediment obtained using core samplers with those obtained using an Ekman-Birge sampler, and estimated that the sampling depth with the Ekman-Birge sampler ranges from 5 to 11 cm. As SSs are expected to originate on the bottom surface, where 137Cs activity concentrations are high, deeper sampling using our sampling method with the Ekman-Birge sampler may have underestimated 137 Cs activity concentrations. A lower mean particulate 137Cs activity concentration in FS compared to SU was due to a lower mean SSC rather than the 137Cs activity concentration of SSs. Composition and amounts of dissolved cation were similar among SU, FS and KS. In the ponds, Ca2þ was the main cation and ranging from 0.60 to 0.83 meq L
1, whereas Naþ is the main cation in IZ. These results suggest that water resource in IZ is different from the other four ponds. . The highest total 137Cs activity concentration was observed in water of pond SU. This value was higher than for the other systems

Please cite this article in press as: Wakiyama, Y., et al., Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant, Journal of Environmental Radioactivity (2017), http://dx.doi.org/10.1016/j.jenvrad.2017.07.017

Y. Wakiyama et al. / Journal of Environmental Radioactivity xxx (2017) 1e10 Table 3 Mean values of

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Cs activity concentration of water, Kd, and water chemistry.

Parameter

unit

n

SU

FS

IZ

KM

mg L
1 kBq kg
1 Bq L
1 Bq L
1 Bq L
1 % (  105) L Kg
1

13 13 13 13 13 13 13

99 ± 43 333 ± 79 37.6 ± 20.1 3.0 ± 1.4 40.6 ± 20.8 90 ± 7.6 1.3 ± 0.50

32 ± 24 337 ± 120 11.4 ± 9.5 1.8 ± 0.51 13.3 ± 9.4 79 ± 15 2.1 ± 0.93

31 ± 14 263 ± 136 7.9 ± 4.9 1.6 ± 0.22 9.5 ± 5.0 79 ± 14 1.7 ± 0.80

10 ± 7.4 133 ± 112 1.1 ± 1.1 0.24 ± 0.10 1.4 ± 1.1 71 ± 16 6.2 ± 0.68

-

11 13 13 13 13 13 13 13 10

7.3 ± 0.23 0.064 ± 0.020 0.012 ± 0.0074 0.26 ± 0.040 0.60 ± 0.010 0.37 ± 0.77 1.30 ± 0.24 1.12 ± 0.41 4.5 ± 1.1

7.7 ± 0.25 0.040 ± 0.016 0.017 ± 0.018 0.24 ± 0.062 0.83 ± 0.19 0.23 ± 0.049 1.36 ± 0.33 1.36 ± 0.45 3.3 ± 0.70

7.3 ± 0.41 0.018 ± 0.0026 0.011 ± 0.004 0.21 ± 0.070 0.10 ± 0.011 0.073 ± 0.063 0.42 ± 0.26 1.47 ± 0.36 3.26 ± 0.29

7.9 ± 0.47 0.026 ± 0.0037 0.032 ± 0.022 0.25 ± 0.022 0.63 ± 0.042 0.20 ± 0.010 1.14 ± 0.12 1.28 ± 0.45 2.5 ± 0.33

137

Cs activity concentration SSC 137 Cs activity concentration of SS Particulate 137Cs activity concentration Dissolved 137Cs activity concentration Total 137Cs activity concentration Particulate/Total Kd Water chemistry pH Kþ NHþ 4 Naþ 2þ Ca Mg2þ Total cation 133 Cs DOC

meq L
1 meq L
1 meq L
1 meq L
1 meq L
1 meq L
1 (  10
7) meq L
1 mg L
1

Mean ± Standard deviation (s).

in Fukushima. According to Tohoku Regional Agricultural Administration Office (2016b), the maximum total 137Cs activity concentration in 201 ponds in the exclusion zone in summer 2015 was 20 Bq L
1. It appeared to be much higher as compared to rivers on the territories contaminated following the FDNPP accident (e.g., Ueda et al., 2013; Nagao et al., 2013). For example, Ueda et al. (2013) presented mean total 137Cs activity concentration of river water from the Hiso River and Wariki River during the summer of 2011 as 5.1 and 6.0 Bq L
1, respectively. As reported that 137Cs activity concentration in aquatic environment decline (Smith et al., 1999), it can be said that 137Cs activity concentrations in the ponds may have been maintained very high even in Fukushima. In Chernobyl, Matsunaga et al. (1998) indicated that total 137Cs activity concentration in Lake Glubokoye was approximately 15 Bq L
1 in average and more than 10 times higher than those of water in the Prypyat river system in 1995 and 1996, ten years after the Chernobyl disaster. In this study, for comparing our results with the results for Chernobyl, we estimated normalized dissolved 137Cs activity concentration (m
1) in the ponds, by dividing dissolved 137Cs activity concentration by the mean 137Cs inventory in soil from each pond. Although normalized 137Cs activity concentration was originally calculated by dividing 137Cs activity concentration in the pond (or river) with mean 137Cs inventory in its catchment (e.g., Smith et al., 2005), it was difficult to estimate them in this study due to the complex drainage system of the ponds. The values in SU, FS, IZ and KM were 4.7  10
4, 6.2  10
4, 7.6  10
4 and 2.6  10
4 m-1, respectively. In Chernobyl and adjacent areas, normalized dissolved 137 Cs activity concentrations were 1.3  10
2 m-1 in Lake Kozhanovskoe (Konoplev et al., 1998) and 1.3  10
2 m-1 in Lake Svyatoe (Konoplev et al., 1992). The Kd values are on the order of 105 L kg
1. These values are comparable to Kd values obtained by observation of river water in Fukushima (Ueda et al., 2013; Ochiai et al., 2015; Yoshimura et al., 2015; Eyrolle-Boyer et al., 2016). As well documented, Kd values are higher in Fukushima than in Chernobyl (e.g., Evrard et al., 2015; Yoshimura et al., 2015; Konoplev et al., 2016a). According to Konoplev et al. (2016a), the Kd values in Fukushima ranged from 1.1  105 to 11  105 L kg
1 whereas those in Chernobyl were 0.15  105 and 0.32  105 L kg-1. They also suggested that these high values in Fukushima could be attributed to relatively high values of Radiocesium Interception Potential (RIP) of soils in Fukushima, as shown in Nakao et al. (2014).

Interestingly, Kd values in the ponds of Okuma were comparable with Kd values observed for rivers and large lakes in the Fukushima area (Yoshimura et al., 2015; Konoplev, 2015), whereas Kd values in small closed lakes and ponds in the Chernobyl area were much lower than those observed in rivers and large flow-through lakes (e.g., Konoplev et al., 2002; Smith et al., 2005). For instance, Konoplev et al. (2002) estimated the Kd value for Lake Vorsee with 0.09 km2 of surface area as 8.1  103 L kg
1, whereas the Kd values in river systems in Chernobyl area ranged from 1.5  104 and 3.2  104 L/kg in the Dnieper River and Pripyat River, respectively. This discrepancy can be attributed to the differences in binding ability of SSs between the Chernobyl and Fukushima areas as described by Konoplev et al. (2016a). The mean values of pH and other water chemical components indicate that the water in these ponds was neutral in pH and comparable to other water systems in the Fukushima area. The major cation in SU, FS, and KM was Ca2þ, whereas that in IZ was Naþ. The mean values of Naþ and DOC were relatively similar among the four ponds. Composition of water chemistry of SU, FS, and KM were similar to those of spring water taken in the area close to KM (Yabusaki and Shimano, 2015). Because a granite rock underlies the spring, water in these three ponds might be partly provided from upstream area. By contrast, the difference in compositions of dissolved cation suggests that water sources for IZ differ from those of the other ponds. Because IZ was not connected inflow creeks as shown in Fig. 1, we speculated that IZ might be mainly fed by groundwater. The 133Cs activity concentrations were much lower than those of Kþ and NHþ 4 by 7 orders of magnitude. Although stable 133Cs is a 1000 fold stronger competitor for radiocesium on FESs of clay minerals than potassium and 200 times stronger than ammonium (Cremers et al., 1988; Wauters et al., 1996), its role in 137Cs remobilization is expected to be negligible in comparison with Kþ and NHþ 4 in terms of abundance in a waterbody. However, NHþ 4 concentrations of most samples were lower than the detection limit. Pinder et al. (2010) suggested that shallow ponds hardly support a persistent anoxic hypolimnion. Hence, we speculated that shallowness of our ponds resulted in a low concentration of NH4þ. 3.3. Temporal variations in water

137

Cs activity concentrations in pond

Fig. 3 shows the temporal variations in total, particulate, and

Please cite this article in press as: Wakiyama, Y., et al., Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant, Journal of Environmental Radioactivity (2017), http://dx.doi.org/10.1016/j.jenvrad.2017.07.017

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Y. Wakiyama et al. / Journal of Environmental Radioactivity xxx (2017) 1e10

Fig. 3. Temporal variations in total, particulate, and dissolved 137Cs activity concentration. Error bars present measurement errors. Measurement errors of total concentration were calculated by considering propagation of error.

dissolved 137Cs activity concentrations. The maximum values of total 137Cs activity concentrations was 77 Bq L
1 observed in SU and of its activity concentrations in dissolved state was 5 Bq L
1. In terms of temporal variation, total 137Cs activity concentration varied enormously and the temporal variations coincided with those of particulate 137Cs activity concentration. As described in previous works on Chernobyl, dissolved 137Cs activity concentration in the lakes decreased by 3 orders of magnitude 5 years after the Chernobyl accident (Zibold and Klemt, 2005). No seasonal variability of particulate 137Cs activity concentration was found for these four ponds. Dissolved 137Cs activity concentration commonly showed low values during the winter season. These seasonal fluctuations

137

Cs activity

also were found in the Chernobyl-affected area (e.g., Smith et al., 2005; Zibold and Klemt, 2005). Although our sampling period is quite short for detecting long term trends, the results suggested the seasonal variations in dissolved 137Cs activity concentration in ponds. Fig. 4 shows the temporal variability in 137Cs activity concentration of the SSs and Kd values. 137Cs activity concentrations of SSs varied significantly but showed neither a seasonal variability nor a decreasing trend. This is in contrast to previous studies in river water systems in Fukushima, which indicated that 137Cs activity concentration of SSs shows significant decreasing trends (e.g., Murakami et al., 2016). Only pond IZ showed a decreasing trend in

Fig. 4. Temporal variations of 137Cs activity concentration of suspended sediment (SS) and Kd. Error bars present measurement error. Measurement errors of Kd values were calculated by considering propagation of error.

Please cite this article in press as: Wakiyama, Y., et al., Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant, Journal of Environmental Radioactivity (2017), http://dx.doi.org/10.1016/j.jenvrad.2017.07.017

Y. Wakiyama et al. / Journal of Environmental Radioactivity xxx (2017) 1e10 137 Cs activity concentrations of SSs after August 2015. One possible explanation for this decreasing trend is depletion of SSs with high 137 Cs activity concentration due to small input of sediment-borne 137 Cs, as depicted in Table 2. However, it is possible that high 137 Cs activity concentration of SS may be observed again in the future. Continuous monitoring is required for testing the hypothesis on IZ. Neither an increasing trend nor seasonal fluctuation were found in the Kd values. As described in the Chernobyl literature, Kd values gradually increased because of long-term fixation by clay minerals and their movement in depth, down bottom sediments and catchment soils (Smith et al., 2005). Such a trend cannot be identified because observation periods are still short. However, it is worth noting that distribution coefficients in the ponds SU, FS, and IZ were quite stable over the observational period, whereas for the KM that was less contaminated, Kd values varied by an order of magnitude. The variability of Kd appeared to depend on the variability of 137Cs activity concentration of SSs rather than those of dissolved 137Cs activity concentration.

3.4. Factors affecting

137

Cs activity concentration

We investigated the relationships of particulate 137Cs activity concentration with SSC and with 137Cs activity concentration of SSs (Fig. 5). Significant correlations between 137Cs activity concentration of SSs and particulate 137Cs activity concentration were found for four ponds (p < 0.05). Particulate 137Cs activity concentrations in SU and FS showed significant positive correlations with both SSC and 137Cs activity concentration of SSs (p < 0.01). No significant correlation was found with SSC in IZ and KM (p > 0.05). These differences appeared to depend on the variations of SSC. IZ and KM showed relatively constant SSC and therefore the fluctuations of 137 Cs activity concentration of SSs depended on the fluctuations in particulate 137Cs activity concentration in these ponds. Table 4 shows the correlation coefficient between dissolved 137 Cs activity concentrations and water chemistry. Significant correlation was rarely found. These results suggest that behavior of dissolved 137Cs in pond waters were complicated and affected by many environmental factor. However, correlation coefficients with Kþ and DOC are positive through all four ponds although significance was variable from pond to pond. We would exclude ammonium from correlation analysis because our ammonium data are not stable and our ion chromatography has a high detection limit for ammonium. However, it is possible that remobilization of 137Cs

7

Table 4 Correlation coefficient of dissolved 137Cs activity concentration with concentrations of other water chemistry. Parameters

Unit

n

Correlation coefficient with dissolved 137Cs activity concentration SU

FS

IZ

KM

SSC 137 Cs activity concentration of SS1 Kþ Naþ Ca2þ Mg2þ 133 Cs DOC

mg L
1 kBq kg
1

13 13

0.39 0.61*


0.37
0.02


0.17 0.16

0.32
0.01

meq L
1 meq L
1 meq L
1 meq L
1 meq L
1 mg L
1

13 13 13 13 13 10

0.40
0.01
0.23
0.29 0.61* 0.40

0.21 0.31 0.50 0.47 0.22 0.63*

0.60* 0.20 0.34 0.32 0.12 0.44

0.23 0.53 0.27 0.29 0.26 0.56

*p < 0.05.

due to ion exchange with NHþ 4 in pore water of bottom sediment resulted in increase of dissolved 137Cs activity concentration during 137 summer. Measurements of both NHþ Cs activity 4 and dissolved concentration in future can offer an opportunity for testing the hypothesis. The positive relation with Kþ is in agreement with the previous works because Kþ is one of major competitive ions with dissolved 137 Cs in a water body (Comans et al., 1989; Wauters et al., 1996; Konoplev et al., 2002). Correlations with DOC are also reasonable. Decomposition of organic matter and subsequent dissolution of 137 Cs in aquatic environment can be postulated as a main reason for the positive relations with DOC concentration. For example, Smith et al. (2005) presented the positive correlation between normalized 137 Cs activity concentration in water and the percentage of organic soils in the catchment. In Fukushima, there are some field evidences that decomposition of organic matter resulted in an increase of dissolved 137Cs activity concentration (e.g., Iwagami et al., 2016; Nakanishi et al., 2014; Tsuji et al., 2016). Tsuji et al. (2016) observed an increase in dissolved 137Cs activity concentration in river water of a forested catchment during summer, and suggested that leaching from organic matter was the primary mechanism behind the increase. Furthermore, they suggested that increase temperature enhance the decomposition of organic matter and that surface runoff transport the dissolved 137Cs into river water. Thus, we can argue that a similar mechanism is possibly acting behind the variability of dissolved 137Cs activity concentration in the ponds.

Fig. 5. Scatter plot of particulate 137Cs activity concentration against suspended sediment concentration (SSC) and against 137Cs activity concentration of suspended sediment (SS). The numbers on each graph show the correlation coefficients. Double asterisks (**) and single asterisk (*) following the correlation coefficient mean p < 0.01 and p < 0.05, respectively. Error bars present measurement error as same to Fig. 3.

Please cite this article in press as: Wakiyama, Y., et al., Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant, Journal of Environmental Radioactivity (2017), http://dx.doi.org/10.1016/j.jenvrad.2017.07.017

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Y. Wakiyama et al. / Journal of Environmental Radioactivity xxx (2017) 1e10

On the other hand, it is often documented that vertical mixing of water occurs in closed water system during winter (e.g., Sato, 2007) and it is expected to result in increase of dissolved 137Cs activity concentration of surface water. However, our results did not provided evidences of the process. One possible explanation for this is too shallow depth of these four ponds for stratification of water. In such a situation, the temperature and abundance of fresh litter might be limiting factors for dissolution of 137Cs into water body. Comparisons of seasonal change in depth distribution of dissolved 137 Cs in pond water could offer an opportunity to test the hypothesis in future. To confirm the relationship of dissolved 137Cs with water chemistry, we plotted normalized dissolved 137Cs activity concentration and Kd values against Kþ and DOC (Fig. 6). Similar relationships between DOC and dissolved 137Cs activity concentration were found among SU, FS and KM. However, normalized dissolved 137 Cs activity concentration in IZ showed a different relationship with Kþ and DOC compared to the other three ponds. One possible explanation of this difference is that water resources in IZ are different from other ponds as discussed for Table 3. By excluding IZ data, a correlation coefficient of normalized dissolved 137Cs activity concentration with Kþ and DOC was 0.37 (n ¼ 39, p ¼ 0.04), and 0.45 (n ¼ 33, p ¼ 0.02), respectively. Similarly, the correlation factor of Kd values with Kþ and DOC was
0.42 (n ¼ 39, p ¼ 0.01) and
0.51 (n ¼ 33, p ¼ 0.01), respectively. These results suggest that temporal variation of DOC affects those of 137Cs in water more significantly than Kþ. To evaluate the factors affecting dissolved 137Cs activity concentration, we conducted PCA with scaling for each pond (Fig. 7). Cumulative variance explained by the first and second principal

components for SU, FS, IZ, and KM were 69.6%, 68.3%, 62.3%, and 65.1%, respectively. The compositions of factors of the first and second principal components varied from pond to pond. The principal component loadings of Ca2þ and Mg2þ were similar in the four ponds. This finding can be attributed to the fact that behavior of these cations are expected to be similar in aquatic system. In this context, strong relationship of dissolved 137Cs activity concentration with Kþ and DOC were not evident. In SU, dissolved 137Cs activity concentration was associated with sediment born 137Cs (SSC, 137 Cs activity concentration of SS and DOC). The mechanism of the variability of dissolved 137Cs activity concentration might be due to a combination of resuspension of fine sediments and decomposition of organic matter in SU. In FS, the first principal component was dominated by DOC and dissolved cations; the dissolved 137Cs activity concentrations showed similarities to the dissolved cations and DOC. The principal component scores during summer (June, July, and August) in FS tended to show negative values for the first principal component. Similar distribution was found on KM. These results suggested that variability of dissolved 137Cs activity concentration were controlled by dissolved cations and DOC in FS and KM during summer. In IZ, DOC dominated the second principal component, and its principal component loading was similar to those for dissolved 137Cs activity concentrations. The first principal component scores of each sample during fall-winter season (October, November, and December) showed negative values, and were opposite to the first principal component loadings of dissolved cations. The low concentration of competitive ions and DOC during winter might have resulted in low dissolved 137Cs activity concentration in IZ. The results of PCA suggested that there were different mechanisms behind variability of dissolved 137Cs activity

Fig. 6. Scatter plots of normalized dissolved 137Cs activity concentration and Kd value against Kþ concentration and against dissolved organic carbon (DOC) concentration. The numbers in each graph shows correlation coefficient of ponds. Double asterisks (**) and single asterisk (*) following the correlation coefficient mean p < 0.01 and p < 0.05, respectively. Error bars present measurement error as same to Fig. 4.

Please cite this article in press as: Wakiyama, Y., et al., Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant, Journal of Environmental Radioactivity (2017), http://dx.doi.org/10.1016/j.jenvrad.2017.07.017

Y. Wakiyama et al. / Journal of Environmental Radioactivity xxx (2017) 1e10

9

Fig. 7. Principal component analysis (PCA) biplot for data set of SU (a), FS (b), IZ (c), and KM (d). Projection of variables (bottom X axis and left Y axis) and samples (upper X axis and right Y axis) are given on the plane for two principal components. The values in parenthesis following the title of the axes indicate the variance explained by the factor.

concentrations in these ponds. Continuous monitoring of 137Cs activity concentration in pond water is essential to test the hypothesis for 137Cs behavior in ponds. 4. Conclusions This study describes the contamination situation in and around four ponds of Okuma town located near the FDNPP. Maximum 137Cs inventory in surrounding soils and bottom sediment were 6.4 and 13 MBq m
2, respectively. 137Cs inventories in bottom sediments were up to 3 times higher than those of surrounding soils. These higher inventories in bottom sediments suggested that 137Cs accumulated in the ponds due to wash-off from surrounding soils and upstream catchment. The highest enrichment of 137Cs in bottom sediment was found in the pond connected with surface water flows and suggest significant input of 137Cs into the pond. Mean total 137Cs activity concentration of water was very high and up to 10 Bq L
1. In addition, particulate 137Cs accounted for 71e90% of total 137Cs in water of this study in average. Mean Kd values were in order of 105 L kg
1 and these values are higher by one to two orders of magnitude than in Chernobyl. These results were in agreement with results of previous research in Fukushima. These results also emphasize the importance of sediment dynamics for controlling 137 Cs behavior in water bodies. In terms of temporal variations, no significant decreasing trends were found for 137Cs activity concentration in all four ponds.

Dissolved 137Cs activity concentration tended to be lower during winter and higher in summer eautumn for all four ponds. A positive relationship was found between Kþ and 137Cs activity concentration through all four ponds. Positive correlation between DOC and dissolved 137Cs activity concentrations suggested that the decomposition of organic matter resulted in dissolution 137Cs in water bodies. Temporal variation of DOC appeared to affect 137Cs in water more significantly than Kþ. PCA results suggested that there were different mechanisms behind variability of dissolved 137Cs activity concentrations for each pond. Although our observation duration is still short for describing temporal trend of 137Cs activity concentration, the results suggested seasonality of dissolved 137Cs activity concentration and some future research directions such as influences of glassy hot particle and dissolved organic matter on behavior of 137Cs in water of ponds. Continuous observations are required for revealing temporal trends of 137Cs activity concentrations in closed water systems and their controlling factors.

Acknowledgement This study was supported by a grant-in-aid from the Kurita Water and Environment Foundation (No. 15B098 and 16K023) and partly by JSPS KAKENHI projects (15H0403901 and 15H0462101). The authors would like to thank Professor Thomas Johnson from Colorado State University for his constructive comments.

Please cite this article in press as: Wakiyama, Y., et al., Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant, Journal of Environmental Radioactivity (2017), http://dx.doi.org/10.1016/j.jenvrad.2017.07.017

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Please cite this article in press as: Wakiyama, Y., et al., Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant, Journal of Environmental Radioactivity (2017), http://dx.doi.org/10.1016/j.jenvrad.2017.07.017