11:15 AM - 11:30 AM
[MAG34-08] Temporal variation and control factors of 137Cs flux at the mouth of the Tokyo Bay
Keywords:Tokyo Bay, Fukushima Daiichi Nuclear Power Plant accident, Sinking particle, Radiocaesium
1. Introduction
Tokyo Bay is located at 250 km southwest of Fukushima Daiichi Nuclear Power Plant. Although the amount of initial accumulation of radionuclide were not so much immediately after the accident in March 2011, radionuclide which deposited in the catchments were gathered by rivers and have flowed into the bay[1]. Sediments in Edogawa River and its estuary shows a higher radiocesium activity concentration[1, 2]. On the other hand, there is a possibility that radionuclide in the bay have transported to the open ocean through the mouth of the bay, and the transportation process had not been clarified. Time-series sediment trap is an effective tool to observe temporal variation of vertical flow of particles. In this study, temporal variations of particulate flux and 137Cs activity concentration in sinking particles were observed at the mouth of the Tokyo Bay and transportation process of 137Cs from coastal area to open ocean is discussed.
2. Methods
A sediment trap was deployed at 100 m above the sea bottom, the south end of Uraga channel, mouth of the Tokyo Bay (35°02.2' N, 139°38.9' E, 850 m water depth). The sampling of sinking particles was carried out from December 2018 to December 2019 and 52 samples were collected with 7 days intervals. Sinking particles were filtered by a 0.6-µm pore size membrane filter after zooplankton was removed. Total mass flux was calculated by the weight of dried sample. Concentration of lithogenic aluminosilicate and biogenic SiO2 which are major components in the sinking particles were measured following Sukigara et al. (2022).[3] 137Cs activity concentration was measured by gamma-ray spectrometry.
3. Result and discussion
33 samples out of 52 samples showed a higher 137Cs activity concentration than its detection limit (~11 mBq/g, depends on measurement condition). Measured 137Cs activity concentrations ranged from 2 to 10 mBq/g, and the average value was 5.9±1.7 mBq/g. Compared with the averaged 137Cs activity concentrations of sediment in the Tokyo Bay (117±46 mBq/g at the mouth of Arakawa River and 20±16 mBq/g at the center of the bay, respectively[2]), sinking particles showed lower values.
Particulate 137Cs flux ranged from 12 to 123 mBq/m2/day and the average value was 66±28 mBq/m2/day. In most periods, particulate 137Cs fluxes were significantly higher than those of the atmospheric deposition. This result indicates that the 137Cs was supplied from inner bay. 137Cs flux shows several peaks during observation period.
137Cs activity concentration of sinking particles and biogenic SiO2 concentration which accounts for 10% of sinking particles showed positive correlation (correlation coefficient: 0.32). This result implied that particulate 137Cs was transported by diatoms which produced inner bay. Recent studies reported that 137Cs adsorbed on the surface of mineral particles in sediment desorbs and increases 137Cs concentration in sea water[4]. Transition of 137Cs from sediment of high 137Cs activity concentration in the bay to sea water might occurs in the Tokyo Bay which has semi-closed condition. Uptake of 137Cs by diatoms will likely affect the 137Cs transportation system and its bioconcentration.
In conclusion, factors affecting temporal variation of 137Cs flux at the mouth of the Tokyo Bay were inferred as follows: (1) the tentative large transport caused by severe weather like a typhoon, (2) lateral transport of resuspended sediment in winter, and (3) uptake of 137Cs by diatoms in surface water and subsequent vertical transport.
[1] H. Yamazaki, M. Ishida, R. Hinokio, YA. Yamashiki, and R. Azuma, PLoS One. 13, 3, e0193414 (2018)
[2] A. Kubo, K. Tanabe, Y. Ito, T. Ishimaru, H. Arakawa and J. Kanda, Chemosphere. 235, 550-555 (2019)
[3] C. Sukigara, S. Otosaka, N. Horimoto-Miyazaki, and M. Yoshihisa, J Oceanogr. (2022) DOI: 10.1007/s10872-022-00660-7
[4] S. Otosaka, S. Kambayashi, M. Fukuda, T. Tsuruta, T. Misonou, T. Suzuki and T. Aono, Environ. Sci. Technol. 54, 21, 13778-13785 (2020)
Tokyo Bay is located at 250 km southwest of Fukushima Daiichi Nuclear Power Plant. Although the amount of initial accumulation of radionuclide were not so much immediately after the accident in March 2011, radionuclide which deposited in the catchments were gathered by rivers and have flowed into the bay[1]. Sediments in Edogawa River and its estuary shows a higher radiocesium activity concentration[1, 2]. On the other hand, there is a possibility that radionuclide in the bay have transported to the open ocean through the mouth of the bay, and the transportation process had not been clarified. Time-series sediment trap is an effective tool to observe temporal variation of vertical flow of particles. In this study, temporal variations of particulate flux and 137Cs activity concentration in sinking particles were observed at the mouth of the Tokyo Bay and transportation process of 137Cs from coastal area to open ocean is discussed.
2. Methods
A sediment trap was deployed at 100 m above the sea bottom, the south end of Uraga channel, mouth of the Tokyo Bay (35°02.2' N, 139°38.9' E, 850 m water depth). The sampling of sinking particles was carried out from December 2018 to December 2019 and 52 samples were collected with 7 days intervals. Sinking particles were filtered by a 0.6-µm pore size membrane filter after zooplankton was removed. Total mass flux was calculated by the weight of dried sample. Concentration of lithogenic aluminosilicate and biogenic SiO2 which are major components in the sinking particles were measured following Sukigara et al. (2022).[3] 137Cs activity concentration was measured by gamma-ray spectrometry.
3. Result and discussion
33 samples out of 52 samples showed a higher 137Cs activity concentration than its detection limit (~11 mBq/g, depends on measurement condition). Measured 137Cs activity concentrations ranged from 2 to 10 mBq/g, and the average value was 5.9±1.7 mBq/g. Compared with the averaged 137Cs activity concentrations of sediment in the Tokyo Bay (117±46 mBq/g at the mouth of Arakawa River and 20±16 mBq/g at the center of the bay, respectively[2]), sinking particles showed lower values.
Particulate 137Cs flux ranged from 12 to 123 mBq/m2/day and the average value was 66±28 mBq/m2/day. In most periods, particulate 137Cs fluxes were significantly higher than those of the atmospheric deposition. This result indicates that the 137Cs was supplied from inner bay. 137Cs flux shows several peaks during observation period.
137Cs activity concentration of sinking particles and biogenic SiO2 concentration which accounts for 10% of sinking particles showed positive correlation (correlation coefficient: 0.32). This result implied that particulate 137Cs was transported by diatoms which produced inner bay. Recent studies reported that 137Cs adsorbed on the surface of mineral particles in sediment desorbs and increases 137Cs concentration in sea water[4]. Transition of 137Cs from sediment of high 137Cs activity concentration in the bay to sea water might occurs in the Tokyo Bay which has semi-closed condition. Uptake of 137Cs by diatoms will likely affect the 137Cs transportation system and its bioconcentration.
In conclusion, factors affecting temporal variation of 137Cs flux at the mouth of the Tokyo Bay were inferred as follows: (1) the tentative large transport caused by severe weather like a typhoon, (2) lateral transport of resuspended sediment in winter, and (3) uptake of 137Cs by diatoms in surface water and subsequent vertical transport.
[1] H. Yamazaki, M. Ishida, R. Hinokio, YA. Yamashiki, and R. Azuma, PLoS One. 13, 3, e0193414 (2018)
[2] A. Kubo, K. Tanabe, Y. Ito, T. Ishimaru, H. Arakawa and J. Kanda, Chemosphere. 235, 550-555 (2019)
[3] C. Sukigara, S. Otosaka, N. Horimoto-Miyazaki, and M. Yoshihisa, J Oceanogr. (2022) DOI: 10.1007/s10872-022-00660-7
[4] S. Otosaka, S. Kambayashi, M. Fukuda, T. Tsuruta, T. Misonou, T. Suzuki and T. Aono, Environ. Sci. Technol. 54, 21, 13778-13785 (2020)