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Spatial variation in sedimentary radioactive cesium concentrations in Tokyo Bay following the Fukushima Daiichi Nuclear Power Plant accident
Chemosphere 235 (2019) 550e555
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Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Spatial variation in sedimentary radioactive cesium concentrations in Tokyo Bay following the Fukushima Daiichi Nuclear Power Plant accident Atsushi Kubo a, *, Kai Tanabe b, Yukari Ito b, Takashi Ishimaru b, Hisayuki Arakawa b, Jota Kanda b a b
Department of Geosciences, Shizuoka University, Shizuoka, Japan Department of Ocean Sciences, Tokyo University of Marine Science and Technology, Tokyo, Japan
h i g h l i g h t s The radiocesium concentrations in the sediment of river mouth and Tokyo Bay were measured at 26 stations in 2017. Radiocesium-bearing microparticles contributions to each 137Cs concentration of the bulk sediment sample were very low. The 137Cs inventory in the sediment was 0.67 TBq which is approximately 3.2 times higher than that of a previous estimate.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2019 Received in revised form 21 June 2019 Accepted 27 June 2019 Available online 28 June 2019
Cesium-137 concentrations in sediment (137Cs) at Tokyo Bay were measured at 26 stations during 2017. Average 137Cs concentrations at the Arakawa river mouth (117 ± 46 Bq kg1) were approximately six times higher than those of the other stations in the bay (20 ± 16 Bq kg1). There were radiocesiumbearing microparticles in the bay sediment as well as in suspended matter of Fukushima coastal waters. Radioactivity of radiocesium-bearing microparticles was estimated to be 0.12 Bq. However, the contributions of radiocesium-bearing microparticles to each 137Cs concentration of the bulk sample were low; 3% was the maximum. The 137Cs inventory in sediment at the entire bay was 0.67 TBq, showing that a large amount of 137Cs was supplied to the bay from the river following the Fukushima Daiichi Nuclear Power Plant accident. Approximately 9.2% of the 137Cs which was fallout in the drainage basin has already flowed into the bay from the watershed, which is approximately 3.2 times higher than that of a previous estimate. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Martine Leermakers Keywords: Cesium-137 Sediment FDNPP Tokyo bay CsMP
1. Introduction Significant radioactive material contamination of the environment occurred as a result of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011 (Yoshida and Kanda, 2012). Radiocesium has been detected in the hydrosphere, biosphere, and lithosphere (e.g., Yasunari et al., 2011; Buesseler et al., 2012; Buesseler et al., 2017). Field observations and model simulations of radiocesium concentrations in sediment at Tokyo Bay and its watershed since the FDNPP accident have been conducted (e.g.,
* Corresponding author. E-mail address: [email protected] (A. Kubo). https://doi.org/10.1016/j.chemosphere.2019.06.215 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
Oura and Ebihara, 2012; Otsuka et al., 2012; Koibuchi, 2013; Soemori et al., 2013; Nakamura et al., 2017; Yamazaki et al., 2018). Although radiocesium concentrations in the sediment of the river flowing into the north part of the bay were very high, concentrations have gradually decreased from 2013 to 2016 (Otsuka et al., 2017). The concentration was ranged from 3000 to 6000 Bq kg1 in 2013, but it was ranged from 500 to 1000 Bq kg1 in 2016 (Otsuka et al., 2017). In contrast, the radiocesium inventory in sediment increased at the river mouth from 2011 to 2016 as radiocesium deposited in the watershed was gradually supplied to the bay through rivers (Yamazaki et al., 2018). The radiocesium inventory in river mouth sediment during 2011 showed 20.1 kBq m2, but it increased to 104 kBq m2 in 2016 (Yamazaki et al., 2018). Radiocesium particles selectively accumulated at the river mouth where
A. Kubo et al. / Chemosphere 235 (2019) 550e555
the river flow velocity decreases. Therefore, it was considered that radiocesium did not flow into the central bay (Yamazaki et al., 2018). Although there have been a few reports of radiocesium in the sediment of the bay, most studies obtained sediment samples within only 5 cm from the surface (Koibuchi, 2013; Teishima et al., 2014; Otsuka et al., 2016; Yamazaki et al., 2018). Because radiocesium derived from FDNPP was detected at a depth about 15 cm near Tokyo Bay (Kitamura et al., 2019), it was difficult to estimate a detailed inventory in the sediment of the bay using only surface sediment data. The radiocesium concentrations in Manila clam (Ruditapes philippinarum) and Japanese whiting (Sillago japonica) from the bay were measured following the FDNPP accident. The maximum radiocesium concentrations of R. philippinarum and S. japonica were 0.16 Bq kg1-wet (Teishima et al., 2015) and 1.7 Bq kg1-wet (Teishima et al., 2017), respectively. Although the 137Cs concentrations in these species were very low, radiocesium concentrations may increase according to the influx of radiocesium-bearing microparticles (CsMPs) because R. philippinarum and S. japonica prey on benthic crustacea and polychaeta. In addition, the terrestrial and riverine radiocesium supply can eventually accumulate in Tokyo Bay over a long period of time because it is a semi-enclosed embayment. Therefore, understanding the radiocesium inventory in the sediment of the bay is important to estimate the radiocesium contamination in the ecosystem over the long term.
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type; Rigo Co., Ltd., Tokyo, Japan) and were cut into 0.5- or 1-cm intervals. After collection, the samples were stored in a freezer (25 C). In the laboratory, the sample were freeze-dried and filled 100-mL plastic containers. The 134Cs and 137Cs concentrations were measured using a coaxial Ge gamma-ray spectrometer (GR2018; Canberra, Meriden, CT). The radiocesium concentrations in the sediment were decay-corrected based on the sampling date. An imaging plate (Typhoon FLA7000, Fujifilm, Tokyo, Japan) was used to make an autoradiograph. The exposure was performed for 20 h. The gray value for 24 h intensity was corrected using a linear regression between exposure time and the intensity. The autoradiograph was analyzed using the ImageJ software (https://imagej. nih.gov/ij/). The gray value of the particle (24 h) area (cm2) were used as a measure of the radioactivity magnitude of the autoradiography images. The 137Cs concentrations were decay-
2. Materials and methods Observations were conducted in Tokyo Bay using R/V Hiyodori from January 30, 2017 to February 1, 2017 at 20 stations (stations 2e21; Fig. 1) and using R/V Seiyo-maru on June 8, 2017 at 6 stations (stations 22e27; Fig. 1). Sediment samples for 134Cs and 137Cs measurements were collected using a gravity core sampler (HR
Fig. 1. Study area. Sampling locations are indicated by black circles.
Fig. 2. Vertical 137Cs concentration profiles (Bq kg1) at the river mouth stations (water depth < 10 m) as the decay-corrected value of the sampling date.
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corrected to each sampling date. 3. Results and discussion 3.1. Vertical
137
Cs concentration profiles
Vertical 137Cs concentration profiles at the river mouth (stations 2e7) are shown in Fig. 2. The 137Cs concentrations ranged from 50.0 to 343.6 Bq kg1. Because nearly the same values were observed
Fig. 3. Vertical
from the surface layer to approximately 15 cm depth, except for at station 3, the sediment at the river mouth were likely mixed evenly by physical disturbance and bioturbation. Continued influx of 137Cs from the watershed and selective sedimentation at the river mouth were also possible. The 137Cs concentrations at the river mouth were significantly lower than the 137Cs concentrations observed in 2011 (~1000 Bq kg1; Koibuchi, 2013; Yamazaki et al., 2018). Radiocesium in the sediment may be transported to deeper part of the sediments and/or offshore because the decline rate is faster
Cs concentration profiles (Bq kg1) in the entire Tokyo Bay (water depth > 10 m) as the decay-corrected value of the sampling date.
137
A. Kubo et al. / Chemosphere 235 (2019) 550e555
than the radioactive decay of 137Cs. In contrast, vertical 137Cs concentration profiles in the entire bay (stations 8e27) are shown in Fig. 3. The 137Cs concentrations ranged from 0.8 to 139.2 Bq kg1. The 137Cs concentrations in the entire bay (20 ± 16 Bq kg1; average ± standard deviation) were six times lower than those at the river mouth (117 ± 46 Bq kg1). The vertical profiles of 137Cs concentrations in the bay were not evenly mixed. In addition, there were several peaks of 137Cs concentrations because a pulse input of 137 Cs could have occurred from the watershed. Radiocesium flux during heavy rains in Fukushima rivers accounts for 30e50% of the annual flux from land to the coastal waters (Nagao et al., 2013). Therefore, heavy rain events are one factor contributing to the transport of radiocesium from land to river and coastal waters. The sources of radiocesium derived from FDNPP were estimated 134 Cs/137Cs ratio correlated to 11 March 2011 (Nishihara et al., 2012; Unit 1: 0.94, Unit 2: 1.05, Unit 3: 1.08). In this study, the 134Cs/137Cs ratio of all sediment samples was 1.11 ± 0.16 corrected to March 11, 2011. The 134Cs/137Cs ratio of the samples in this study was mostly derived from unit 2 and/or unit 3. According to the result of soil observation, radiocesium derived from unit 2 and/or 3 were spread into the southward direction (Satou et al., 2018). Therefore, the accident-derived radiocesium from unit 2 and 3 had transported to the bays. The vertical profile of the 134Cs/137Cs ratio at station 8 is shown in Fig. 4. The 134Cs/137Cs ratio at station 8 was 1.13 ± 0.13 corrected to March 11, 2011. Even 30 cm from the surface layer, the 137 Cs concentration was 50 Bq kg1 (Fig. 3) and the 134Cs/137Cs ratio was approximately 1. Consequently, the deep parts of the sediment samples were affected by the radiocesium derived from the FDNPP. The sedimentation rate of the particles may be several times faster than sedimentation rates which was estimated to be 1.0 cm year1 using the 210Pb method in the bay (Matsumoto, 1983; Shimizu et al., 2005). The effect of bioturbation in sediment has been reported to affect downward to approximately 20 cm depth in coastal waters (Seike et al., 2013, 2016). Although a numerical model of Cs concentration considers mass transport occurring in Tokyo Bay sediment, the effect of bioturbation in the bay sediment calculated to only 10 cm depth (Nakamura et al., 2017). Therefore, the effect of bioturbation and/or more rapid sedimentation rate in the bay may transport 137Cs to a deeper part of sediment.
3.2. Radiocesium-bearing microparticles in sediment Following the FDNPP accident, highly radioactive particles have been found in the environment (Adachi et al., 2013; Itoh et al., 2014; Satou et al., 2016; Kubo et al., 2018; Miura et al., 2018). In the
Fig. 4. Vertical profile of the 134Cs/137Cs ratio corrected to March 11, 2011, at station 8.
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Fukushima coastal waters, CsMPs in suspended matter contributed a maximum of 54% (Kubo et al., 2018). If there are many CsMPs in the sediments, there is a possibility that the radioactivity of the sediment is heterogeneous in the bay. There were several spots on the autoradiographic image (Fig. 5 and S1). These indicated CsMPs in the sediment samples of the bay as well as the suspended matter of the Fukushima coastal waters (Kubo et al., 2018). Comparing the GVA of each sample and the radioactivity, the radiocesium and GVA coefficient was 5.2 105. This was comparable to previous estimations (4.8e6.0 105, Itoh et al., 2014; Kubo et al., 2018). Using this coefficient, we estimated the particle radioactivity. The particle radioactivity in a geometrical average was estimated to be a maximum of 0.26 Bq and 0.12 Bq on average (Fig. 5 and S1; n ¼ 116), comparable to the samples from the soil, air dust, and suspended matter (spherical CsMPs; approximately 0.1 Bq per particle; Itoh et al., 2014; Kubo et al., 2018). However, particles of several dozen Bq obtained from the soils near the FDNPP (non-spherical CsMPs) were not found (Satou et al., 2016). The contribution ratio of the CsMPs to each 137Cs concentration was calculated from the 137Cs concentrations in the bulk sediment and the total concentrations of CsMPs as estimated from the GVA. These were a maximum of 3% and <1% on average. Spherical CsMPs (few mm diameter, few Bq particle1) have found in soils at the watershed of the bay (Ustunomiya et al., 2019). These particles may gradually flow into the bay by rain runoff events. However, all sediment samples in the bay were not influenced by the CsMPs as of 2017. In addition, nonspherical CsMPs (>50 mm, several dozen Bq particle1) were discovered by only the vicinity of north part of the FDNPP within a few km ranges because of large size (Igarashi et al., 2019). Therefore, the supply of the non-spherical particles to the bay is small in the future. 3.3. Inventory of
137
Cs in sediment
The inventories of 137Cs at each station are shown in Table 1. Averaged inventories at the river mouth and in the entire bay were 6.9 and 1.2 kBq m2 for the decay-corrected value of the sampling date, respectively. The inventories of 137Cs at the river mouth was lower than the 131 kBq m2 reported by Yamazaki et al. (2018) using data from October 2011 to July 2016. This was because we did not perform sediment sampling in the old Edogawa river mouth where the 137Cs concentration was high. In contrast, the inventory value of the entire bay was higher than the 0.73 kBq m2 reported by Yamazaki et al. (2018). This was because the deep parts of the sediment samples in the bay were already affected by the radiocesium derived from the FDNPP. There was a strong inverse correlation between inventory of 137Cs and water depth at each observation stations (Yinventory ¼ 27 X1.06 r2 ¼ 0.66, water depth, p < 0.001). Therefore, radiocesium gradually flowed from the river and river mouth area into the bay. Radiocesium is barely detected in the bay where the water depth is 30 m or greater even now (Fig. 3; Stations 24 and 27). Assuming that radiocesium was uniformly distributed along the water depth in the bay, the inventory of 137Cs in the bay (area of water depth over 10 m) was estimated to be 0.51 TBq, as calculated using the same area (330 km2) of Yamazaki et al. (2018); the decay-corrected value of the sampling date was 0.58 TBq for March 16, 2011. Actually, the inventoried 137Cs value was estimated to be 0.67 TBq as the decay-corrected value of the sampling date and 0.77 TBq as the decay-corrected value for March 16, 2011 (calculated area: 500 km2), respectively, because 137Cs was observed in a wider range. This value was 3.2 times higher than the 0.24 TBq decay-corrected radiocesium (134Csþ 137Cs) value of March 16, 2011 reported by Yamazaki et al. (2018). The quantity of radiocesium precipitated in the watershed of the Edogawa river
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A. Kubo et al. / Chemosphere 235 (2019) 550e555
Fig. 5. Autoradiographic images of the Tokyo Bay sediments. Black points indicate CsMPs.
Table 1 Inventories of
137
Cs in each station corrected to sampling day and March 16, 2011. Cs inventory at sampling day (kBq m2)
Station
Water Depth (m)
Core Length (cm)
137
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
3.8 3.7 2.8 6.1 11.8 10.0 14.4 15.0 14.6 13.8 15.7 19.0 18.6 19.1 16.5 25.4 26.0 23.2 20.7 23.0 29.3 21.6 33.9 25.4 27.4 35.4
11 4 10 14 18 7 30 12 20 15 25 30 11 30 30 30 28 30 30 30 23 30 10 26 29 12
5.30 2.72 7.77 10.06 4.36 1.42 2.91 0.62 1.52 0.86 1.80 0.94 0.48 0.88 1.00 1.47 0.55 0.73 0.71 0.98 0.98 1.43 0.29 1.18 0.78 0.41
was 8.33 TBq (134Csþ 137Cs) corrected to March 16, 2011 (Yamazaki et al., 2018). Approximately 9.2% of the 137Cs has already flowed into the bay from the watershed. The radiocesium in the river mouth sediment will gradually enter the bay, particularly after heavy rain events. Therefore, continued observation of 137Cs is needed at the river mouth and in the entire bay.
137
Cs inventory at March 16, 2011 (kBq m2)
6.06 3.12 8.90 11.52 4.99 1.63 3.34 0.71 1.74 0.98 2.06 1.07 0.55 1.01 1.15 1.68 0.62 0.83 0.81 1.12 1.12 1.64 0.33 1.35 0.89 0.47
addition, approximately 9.2% of the 137Cs which was fallout in the drainage basin has already flowed into the bay.
Acknowledgements 4. Conclusions The radiocesium concentrations in the sediment of river mouth and Tokyo Bay were measured at 26 stations during 2017. Radiocesium concentrations in the bay (20 ± 16 Bq kg1) were six times lower than those in the river mouth (117 ± 46 Bq kg1). Although there were CsMPs in the bay sediment, the contributions of CsMPs to each 137Cs concentration of the bulk sample were very low. The 137 Cs inventory in sediment at the entire bay was 0.67 TBq which is approximately 3.2 times higher than that of a previous estimate. In
We are grateful to captain Kentaro Takeuchi and chief engineer Nobuyuki Suga of R/V Hiyodori. We also would like to thank the crewmembers and scientists on board the R/V Hiyodori and Seiyomaru for their help in sampling. This work was supported by a grant-in-aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan (KAKEN), Interdisciplinary Study on Environmental Transfer of Radionuclides from the Fukushima Daiichi NPP Accident (Grant Number is 24110005).
A. Kubo et al. / Chemosphere 235 (2019) 550e555
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Otsuka, F., Yoshimatsu, R., Kawanishi, T., Masuda, K., 2016. A study of estimating the amount of radioactive cesium flowing into Tokyo Bay from Edo-River. Journal of JSCE B2 72, 1303e1308. Otsuka, F., Nakamoto, S., Yoshida, S., Kawanishi, T., Masuda, K., 2017. A study of the radioactive material in the front sea area of the Mama-River mouth. Journal of JSCE B3 73, 917e922. Oura, Y., Ebihara, M., 2012. Radioactivity concentrations of 131I, 134Cs and 137Cs in river water in the greater Tokyo Metropolitan area after the Fukushima Daiichi nuclear power plant accident. Geochem. J. 46, 303e309. Satou, Y., Sueki, K., Sasa, K., Adachi, K., Igarashi, Y., 2016. First successful isolation of radioactive particles from soil near the Fukushima Daiichi Nuclear Power Plant. Anthropocene 14, 71e76. Satou, Y., Sueki, K., Sasa, K., Yoshikawa, H., Nakama, S., Minowa, H., Abe, Y., Nakai, I., Ono, T., Adachi, K., Igarashi, Y., 2018. Analysis of two forms of radioactive particles emitted during the early stages of the Fukushima Dai-ichi Nuclear Power Station accident. Geochem. J. 52, 137e143. Seike, K., Shirai, K., Kogure, Y., 2013. Disturbance of shallow marine soft-bottom environments and megabenthos assemblages by a huge tsunami induced by the 2011 M9.0 Tohoku-Oki Earthquake. PLoS One 8, e65417. Seike, K., Kitahashi, T., Noguchi, T., 2016. Sedimentary features of Onagawa Bay, northwestern Japan after the 2011 off the Pacific coast of Tohoku Earthquake: Sediment mixing by recolonized benthic animals decreases the preservation potential of tsunami deposits. J. Oceanogr. 72, 141e149. Shimizu, J., Yamao, S., Nosaka, T., Noguchi, K., Mine, M., Matsumoto, K., Oichi, K., Tsutsumi, K., Miyamoto, T., Ikeda, Y., 2005. Distribution of sedimentary POPs in Tokyo bay -results from the survey of POPs in Tokyo bay in 2002. Rep. Hydro. Oceanogr. Res. 41, 35e49 (in Japanese with English Abstract). Soemori, A., Shozugawa, K., Nogawa, N., Higaki, S., Matsuo, M., 2013. A change in the concentrations of radioactive cesium in Tokyo-Bay’s sediments released by the Fukushima Daiichi Nuclear Power Station accident. Bunseki Kagaku 62, 1079e1086. Teishima, H., Eriguchi, T., Yanagita, K., Horiguchi, F., 2014. Distribution of radiocesium in Tokyo bay after the Fukushima Daiichi nuclear power station accident. J. Adv. Mar. Sci. Technol. Soc. 19, 1e11 (in Japanese with English Abstract). Teishima, H., Eriguchi, T., Yanagita, K., Horiguchi, F., 2015. Concentration of radiocesium in Manila clam Ruditapes philippinarum living in Tokyo bay after the Fukushima Daiichi nuclear power station accident. J. Adv. Mar. Sci. Technol. Soc. 21, 31e36 (in Japanese with English Abstract). Teishima, H., Eriguchi, T., Yanagita, K., Ishikawa, Y., Horiguchi, F., 2017. Concentration of radiocesium in Japanese whiting Sillago japonica living in Tokyo bay after the Fukushima Daiichi nuclear power station accident. J. Adv. Mar. Sci. Technol. Soc. 23, 1e9 (in Japanese with English Abstract). Utsunomiya, S., Furuki, G., Ochiai, A., Yamasaki, S., Nanba, K., Grambow, B., Ewing, R.C., 2019. Caesium Fallout in Tokyo on 15th March, 2011 Is Dominated by Highly Radioactive, Caesium-Rich Microparticles arXiv:1906.00212[physics.med-ph]. Yamazaki, H., Ishida, M., Hinokio, R., Yamashiki, Y.A., Azuma, R., 2018. Spatiotemporal distribution and fluctuation of radiocesium in Tokyo bay in the five years following the Fukushima Daiichi nuclear power plant (FDNPP) accident. PLoS One 13, e0193414. Yasunari, T., Stohl, A., Hayano, R.S., Burkhart, J.F., Eckhardt, S., Yasunari, T., 2011. Cesium-137 deposition and contamination of Japanese soils due to the Fukushima nuclear accident. Proc. Natl. Acad. Sci. U.S.A. 108, 19530e19534. Yoshida, N., Kanda, J., 2012. Tracking the Fukushima radionuclides. Science 336, 1115e1116.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Spatial variation in sedimentary radioactive cesium concentrations in Tokyo Bay following the Fukushima Daiichi Nuclear Power Plant accident Atsushi Kubo a, *, Kai Tanabe b, Yukari Ito b, Takashi Ishimaru b, Hisayuki Arakawa b, Jota Kanda b a b
Department of Geosciences, Shizuoka University, Shizuoka, Japan Department of Ocean Sciences, Tokyo University of Marine Science and Technology, Tokyo, Japan
h i g h l i g h t s The radiocesium concentrations in the sediment of river mouth and Tokyo Bay were measured at 26 stations in 2017. Radiocesium-bearing microparticles contributions to each 137Cs concentration of the bulk sediment sample were very low. The 137Cs inventory in the sediment was 0.67 TBq which is approximately 3.2 times higher than that of a previous estimate.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2019 Received in revised form 21 June 2019 Accepted 27 June 2019 Available online 28 June 2019
Cesium-137 concentrations in sediment (137Cs) at Tokyo Bay were measured at 26 stations during 2017. Average 137Cs concentrations at the Arakawa river mouth (117 ± 46 Bq kg1) were approximately six times higher than those of the other stations in the bay (20 ± 16 Bq kg1). There were radiocesiumbearing microparticles in the bay sediment as well as in suspended matter of Fukushima coastal waters. Radioactivity of radiocesium-bearing microparticles was estimated to be 0.12 Bq. However, the contributions of radiocesium-bearing microparticles to each 137Cs concentration of the bulk sample were low; 3% was the maximum. The 137Cs inventory in sediment at the entire bay was 0.67 TBq, showing that a large amount of 137Cs was supplied to the bay from the river following the Fukushima Daiichi Nuclear Power Plant accident. Approximately 9.2% of the 137Cs which was fallout in the drainage basin has already flowed into the bay from the watershed, which is approximately 3.2 times higher than that of a previous estimate. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Martine Leermakers Keywords: Cesium-137 Sediment FDNPP Tokyo bay CsMP
1. Introduction Significant radioactive material contamination of the environment occurred as a result of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011 (Yoshida and Kanda, 2012). Radiocesium has been detected in the hydrosphere, biosphere, and lithosphere (e.g., Yasunari et al., 2011; Buesseler et al., 2012; Buesseler et al., 2017). Field observations and model simulations of radiocesium concentrations in sediment at Tokyo Bay and its watershed since the FDNPP accident have been conducted (e.g.,
* Corresponding author. E-mail address: [email protected] (A. Kubo). https://doi.org/10.1016/j.chemosphere.2019.06.215 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
Oura and Ebihara, 2012; Otsuka et al., 2012; Koibuchi, 2013; Soemori et al., 2013; Nakamura et al., 2017; Yamazaki et al., 2018). Although radiocesium concentrations in the sediment of the river flowing into the north part of the bay were very high, concentrations have gradually decreased from 2013 to 2016 (Otsuka et al., 2017). The concentration was ranged from 3000 to 6000 Bq kg1 in 2013, but it was ranged from 500 to 1000 Bq kg1 in 2016 (Otsuka et al., 2017). In contrast, the radiocesium inventory in sediment increased at the river mouth from 2011 to 2016 as radiocesium deposited in the watershed was gradually supplied to the bay through rivers (Yamazaki et al., 2018). The radiocesium inventory in river mouth sediment during 2011 showed 20.1 kBq m2, but it increased to 104 kBq m2 in 2016 (Yamazaki et al., 2018). Radiocesium particles selectively accumulated at the river mouth where
A. Kubo et al. / Chemosphere 235 (2019) 550e555
the river flow velocity decreases. Therefore, it was considered that radiocesium did not flow into the central bay (Yamazaki et al., 2018). Although there have been a few reports of radiocesium in the sediment of the bay, most studies obtained sediment samples within only 5 cm from the surface (Koibuchi, 2013; Teishima et al., 2014; Otsuka et al., 2016; Yamazaki et al., 2018). Because radiocesium derived from FDNPP was detected at a depth about 15 cm near Tokyo Bay (Kitamura et al., 2019), it was difficult to estimate a detailed inventory in the sediment of the bay using only surface sediment data. The radiocesium concentrations in Manila clam (Ruditapes philippinarum) and Japanese whiting (Sillago japonica) from the bay were measured following the FDNPP accident. The maximum radiocesium concentrations of R. philippinarum and S. japonica were 0.16 Bq kg1-wet (Teishima et al., 2015) and 1.7 Bq kg1-wet (Teishima et al., 2017), respectively. Although the 137Cs concentrations in these species were very low, radiocesium concentrations may increase according to the influx of radiocesium-bearing microparticles (CsMPs) because R. philippinarum and S. japonica prey on benthic crustacea and polychaeta. In addition, the terrestrial and riverine radiocesium supply can eventually accumulate in Tokyo Bay over a long period of time because it is a semi-enclosed embayment. Therefore, understanding the radiocesium inventory in the sediment of the bay is important to estimate the radiocesium contamination in the ecosystem over the long term.
551
type; Rigo Co., Ltd., Tokyo, Japan) and were cut into 0.5- or 1-cm intervals. After collection, the samples were stored in a freezer (25 C). In the laboratory, the sample were freeze-dried and filled 100-mL plastic containers. The 134Cs and 137Cs concentrations were measured using a coaxial Ge gamma-ray spectrometer (GR2018; Canberra, Meriden, CT). The radiocesium concentrations in the sediment were decay-corrected based on the sampling date. An imaging plate (Typhoon FLA7000, Fujifilm, Tokyo, Japan) was used to make an autoradiograph. The exposure was performed for 20 h. The gray value for 24 h intensity was corrected using a linear regression between exposure time and the intensity. The autoradiograph was analyzed using the ImageJ software (https://imagej. nih.gov/ij/). The gray value of the particle (24 h) area (cm2) were used as a measure of the radioactivity magnitude of the autoradiography images. The 137Cs concentrations were decay-
2. Materials and methods Observations were conducted in Tokyo Bay using R/V Hiyodori from January 30, 2017 to February 1, 2017 at 20 stations (stations 2e21; Fig. 1) and using R/V Seiyo-maru on June 8, 2017 at 6 stations (stations 22e27; Fig. 1). Sediment samples for 134Cs and 137Cs measurements were collected using a gravity core sampler (HR
Fig. 1. Study area. Sampling locations are indicated by black circles.
Fig. 2. Vertical 137Cs concentration profiles (Bq kg1) at the river mouth stations (water depth < 10 m) as the decay-corrected value of the sampling date.
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corrected to each sampling date. 3. Results and discussion 3.1. Vertical
137
Cs concentration profiles
Vertical 137Cs concentration profiles at the river mouth (stations 2e7) are shown in Fig. 2. The 137Cs concentrations ranged from 50.0 to 343.6 Bq kg1. Because nearly the same values were observed
Fig. 3. Vertical
from the surface layer to approximately 15 cm depth, except for at station 3, the sediment at the river mouth were likely mixed evenly by physical disturbance and bioturbation. Continued influx of 137Cs from the watershed and selective sedimentation at the river mouth were also possible. The 137Cs concentrations at the river mouth were significantly lower than the 137Cs concentrations observed in 2011 (~1000 Bq kg1; Koibuchi, 2013; Yamazaki et al., 2018). Radiocesium in the sediment may be transported to deeper part of the sediments and/or offshore because the decline rate is faster
Cs concentration profiles (Bq kg1) in the entire Tokyo Bay (water depth > 10 m) as the decay-corrected value of the sampling date.
137
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than the radioactive decay of 137Cs. In contrast, vertical 137Cs concentration profiles in the entire bay (stations 8e27) are shown in Fig. 3. The 137Cs concentrations ranged from 0.8 to 139.2 Bq kg1. The 137Cs concentrations in the entire bay (20 ± 16 Bq kg1; average ± standard deviation) were six times lower than those at the river mouth (117 ± 46 Bq kg1). The vertical profiles of 137Cs concentrations in the bay were not evenly mixed. In addition, there were several peaks of 137Cs concentrations because a pulse input of 137 Cs could have occurred from the watershed. Radiocesium flux during heavy rains in Fukushima rivers accounts for 30e50% of the annual flux from land to the coastal waters (Nagao et al., 2013). Therefore, heavy rain events are one factor contributing to the transport of radiocesium from land to river and coastal waters. The sources of radiocesium derived from FDNPP were estimated 134 Cs/137Cs ratio correlated to 11 March 2011 (Nishihara et al., 2012; Unit 1: 0.94, Unit 2: 1.05, Unit 3: 1.08). In this study, the 134Cs/137Cs ratio of all sediment samples was 1.11 ± 0.16 corrected to March 11, 2011. The 134Cs/137Cs ratio of the samples in this study was mostly derived from unit 2 and/or unit 3. According to the result of soil observation, radiocesium derived from unit 2 and/or 3 were spread into the southward direction (Satou et al., 2018). Therefore, the accident-derived radiocesium from unit 2 and 3 had transported to the bays. The vertical profile of the 134Cs/137Cs ratio at station 8 is shown in Fig. 4. The 134Cs/137Cs ratio at station 8 was 1.13 ± 0.13 corrected to March 11, 2011. Even 30 cm from the surface layer, the 137 Cs concentration was 50 Bq kg1 (Fig. 3) and the 134Cs/137Cs ratio was approximately 1. Consequently, the deep parts of the sediment samples were affected by the radiocesium derived from the FDNPP. The sedimentation rate of the particles may be several times faster than sedimentation rates which was estimated to be 1.0 cm year1 using the 210Pb method in the bay (Matsumoto, 1983; Shimizu et al., 2005). The effect of bioturbation in sediment has been reported to affect downward to approximately 20 cm depth in coastal waters (Seike et al., 2013, 2016). Although a numerical model of Cs concentration considers mass transport occurring in Tokyo Bay sediment, the effect of bioturbation in the bay sediment calculated to only 10 cm depth (Nakamura et al., 2017). Therefore, the effect of bioturbation and/or more rapid sedimentation rate in the bay may transport 137Cs to a deeper part of sediment.
3.2. Radiocesium-bearing microparticles in sediment Following the FDNPP accident, highly radioactive particles have been found in the environment (Adachi et al., 2013; Itoh et al., 2014; Satou et al., 2016; Kubo et al., 2018; Miura et al., 2018). In the
Fig. 4. Vertical profile of the 134Cs/137Cs ratio corrected to March 11, 2011, at station 8.
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Fukushima coastal waters, CsMPs in suspended matter contributed a maximum of 54% (Kubo et al., 2018). If there are many CsMPs in the sediments, there is a possibility that the radioactivity of the sediment is heterogeneous in the bay. There were several spots on the autoradiographic image (Fig. 5 and S1). These indicated CsMPs in the sediment samples of the bay as well as the suspended matter of the Fukushima coastal waters (Kubo et al., 2018). Comparing the GVA of each sample and the radioactivity, the radiocesium and GVA coefficient was 5.2 105. This was comparable to previous estimations (4.8e6.0 105, Itoh et al., 2014; Kubo et al., 2018). Using this coefficient, we estimated the particle radioactivity. The particle radioactivity in a geometrical average was estimated to be a maximum of 0.26 Bq and 0.12 Bq on average (Fig. 5 and S1; n ¼ 116), comparable to the samples from the soil, air dust, and suspended matter (spherical CsMPs; approximately 0.1 Bq per particle; Itoh et al., 2014; Kubo et al., 2018). However, particles of several dozen Bq obtained from the soils near the FDNPP (non-spherical CsMPs) were not found (Satou et al., 2016). The contribution ratio of the CsMPs to each 137Cs concentration was calculated from the 137Cs concentrations in the bulk sediment and the total concentrations of CsMPs as estimated from the GVA. These were a maximum of 3% and <1% on average. Spherical CsMPs (few mm diameter, few Bq particle1) have found in soils at the watershed of the bay (Ustunomiya et al., 2019). These particles may gradually flow into the bay by rain runoff events. However, all sediment samples in the bay were not influenced by the CsMPs as of 2017. In addition, nonspherical CsMPs (>50 mm, several dozen Bq particle1) were discovered by only the vicinity of north part of the FDNPP within a few km ranges because of large size (Igarashi et al., 2019). Therefore, the supply of the non-spherical particles to the bay is small in the future. 3.3. Inventory of
137
Cs in sediment
The inventories of 137Cs at each station are shown in Table 1. Averaged inventories at the river mouth and in the entire bay were 6.9 and 1.2 kBq m2 for the decay-corrected value of the sampling date, respectively. The inventories of 137Cs at the river mouth was lower than the 131 kBq m2 reported by Yamazaki et al. (2018) using data from October 2011 to July 2016. This was because we did not perform sediment sampling in the old Edogawa river mouth where the 137Cs concentration was high. In contrast, the inventory value of the entire bay was higher than the 0.73 kBq m2 reported by Yamazaki et al. (2018). This was because the deep parts of the sediment samples in the bay were already affected by the radiocesium derived from the FDNPP. There was a strong inverse correlation between inventory of 137Cs and water depth at each observation stations (Yinventory ¼ 27 X1.06 r2 ¼ 0.66, water depth, p < 0.001). Therefore, radiocesium gradually flowed from the river and river mouth area into the bay. Radiocesium is barely detected in the bay where the water depth is 30 m or greater even now (Fig. 3; Stations 24 and 27). Assuming that radiocesium was uniformly distributed along the water depth in the bay, the inventory of 137Cs in the bay (area of water depth over 10 m) was estimated to be 0.51 TBq, as calculated using the same area (330 km2) of Yamazaki et al. (2018); the decay-corrected value of the sampling date was 0.58 TBq for March 16, 2011. Actually, the inventoried 137Cs value was estimated to be 0.67 TBq as the decay-corrected value of the sampling date and 0.77 TBq as the decay-corrected value for March 16, 2011 (calculated area: 500 km2), respectively, because 137Cs was observed in a wider range. This value was 3.2 times higher than the 0.24 TBq decay-corrected radiocesium (134Csþ 137Cs) value of March 16, 2011 reported by Yamazaki et al. (2018). The quantity of radiocesium precipitated in the watershed of the Edogawa river
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Fig. 5. Autoradiographic images of the Tokyo Bay sediments. Black points indicate CsMPs.
Table 1 Inventories of
137
Cs in each station corrected to sampling day and March 16, 2011. Cs inventory at sampling day (kBq m2)
Station
Water Depth (m)
Core Length (cm)
137
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
3.8 3.7 2.8 6.1 11.8 10.0 14.4 15.0 14.6 13.8 15.7 19.0 18.6 19.1 16.5 25.4 26.0 23.2 20.7 23.0 29.3 21.6 33.9 25.4 27.4 35.4
11 4 10 14 18 7 30 12 20 15 25 30 11 30 30 30 28 30 30 30 23 30 10 26 29 12
5.30 2.72 7.77 10.06 4.36 1.42 2.91 0.62 1.52 0.86 1.80 0.94 0.48 0.88 1.00 1.47 0.55 0.73 0.71 0.98 0.98 1.43 0.29 1.18 0.78 0.41
was 8.33 TBq (134Csþ 137Cs) corrected to March 16, 2011 (Yamazaki et al., 2018). Approximately 9.2% of the 137Cs has already flowed into the bay from the watershed. The radiocesium in the river mouth sediment will gradually enter the bay, particularly after heavy rain events. Therefore, continued observation of 137Cs is needed at the river mouth and in the entire bay.
137
Cs inventory at March 16, 2011 (kBq m2)
6.06 3.12 8.90 11.52 4.99 1.63 3.34 0.71 1.74 0.98 2.06 1.07 0.55 1.01 1.15 1.68 0.62 0.83 0.81 1.12 1.12 1.64 0.33 1.35 0.89 0.47
addition, approximately 9.2% of the 137Cs which was fallout in the drainage basin has already flowed into the bay.
Acknowledgements 4. Conclusions The radiocesium concentrations in the sediment of river mouth and Tokyo Bay were measured at 26 stations during 2017. Radiocesium concentrations in the bay (20 ± 16 Bq kg1) were six times lower than those in the river mouth (117 ± 46 Bq kg1). Although there were CsMPs in the bay sediment, the contributions of CsMPs to each 137Cs concentration of the bulk sample were very low. The 137 Cs inventory in sediment at the entire bay was 0.67 TBq which is approximately 3.2 times higher than that of a previous estimate. In
We are grateful to captain Kentaro Takeuchi and chief engineer Nobuyuki Suga of R/V Hiyodori. We also would like to thank the crewmembers and scientists on board the R/V Hiyodori and Seiyomaru for their help in sampling. This work was supported by a grant-in-aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan (KAKEN), Interdisciplinary Study on Environmental Transfer of Radionuclides from the Fukushima Daiichi NPP Accident (Grant Number is 24110005).
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