3:30 PM - 3:45 PM
[MAG34-06] Distribution and dynamics of dissolved organic radiocarbon in the Fukushima coast
Keywords:Fukushima Daiichi NPP, Radiocarbon, Dissolved organic carbon, ALPS treated water, Accelerator Mass Spectrometry, Western North Pacific
Since August 2023, Fukushima Daiichi Nuclear Power Plant (F1NPP) has been releasing "treated water" into the ocean after purifying contaminated water generated in the facility using Advanced Liquid Processing System (ALPS). Carbon-14 (14C) is one of the target nuclides for pre-discharge monitoring, and it has been reported that its concentration is kept sufficiently low.1) However, the monitoring of 14C only targets carbonate (inorganic) species, and the concentration of 14C in dissolved organic carbon (DOC), has not been evaluated. Understanding the distribution of DO14C in the waters around Fukushima is expected to help us to more accurately understand the level of changes in 14C concentration associated with the release of treated water, as well as to be useful for long-term prediction of 14C dynamics.
On the other hand, information on DO14C in the western North Pacific, including the waters around Fukushima, is limited, and its dynamics remain largely unknown. In this study, we provide an overview of the distribution of 14C isotope ratios (Δ14C) of DOC in surface seawater in the northwestern North Pacific Ocean and its marginal areas, and report the distribution of DO14C concentrations around the F1NPP before and after the release of treated water.
Figure 1 shows the sampling points in this study. Seawater samples in the waters around Fukushima were collected at five stations located 2 to 18 km from the F1NPP from October 2021 to April 2024. In addition to the data previously reported by Otosaka et al.2), data from the broad area of the Northwest Pacific were obtained from January 2021 to August 2023. Seawater samples were collected using a Niskin water sampler, and salinity and water temperature were measured using a CTD at the time of collection. DOC concentration and DOC-Δ14C were analyzed by freezing seawater in glass containers and bringing it back to the laboratory on land. DOC-Δ14C in seawater was analyzed using the method of Otosaka et al. 3). 14C measurements were performed using the single-stage accelerator-assisted mass spectrometer (YS-AMS) at the Atmosphere and Ocean Research Institute, the University of Tokyo.
Figure 2 shows the relationship between Δ14C of DOC in surface seawater and the inverse of DOC concentration at 27 stations in the Northwest Pacific and its marginal seas. The straight line in the figure is a schematic representation of the results obtained by Otosaka et al. 2) at upper (<800 m depth) at a station in the subarctic Northwest Pacific (47° N, 160° E). Surface seawater obtained from many stations in the Northwest Pacific, except some marginal stations, was plotted near the straight line in the figure, and the DOC in these seawaters could be explained as a mixture of two components: fresh DOC produced in the surface layer and old DOC derived from the deep layer.
The Δ14C values of DOC in the Fukushima coast were −248 to −170‰. The DOC-Δ14C values in October 2022, before the release of treated water, were up to 50‰ higher than the mixing line shown in Fig. 2, indicating the possibility of an increase in values due to anthropogenic 14C input, but the associated increase in DO14C concentration was estimated to 0.01mBq/kg. The DOC-Δ14C values in March 2024, after the release of ALPS treated water, were lower than in 2022, and no remarkable deviation from the mixing line was observed. The observations in 2024 were made 7 to 8 days after the third ALPS-treated water release period, and it was estimated that, at least at that point, the increase in DO14C concentration associated with the release was extremely small.
References
1) TEPCO HD
https://www.tepco.co.jp/en/hd/decommission/data/analysis/pdf/2024/measurement_confirmation_241015-e.pdf
2) Otosaka, S. et al. (2024) Otosaka S, et al., J Coastal Res. 116, 181-185.
3) Otosaka, S. et al. (2022) Nucl. Inst. Methods Phys. Res. B 527, 1-6.
Acknowledgements
This research was supported by Grants-in-Aid for Scientific Research (KAKENHI 23H24815, 19H04260, 20H04315) and by the Environmental Radioactivity Research Network Center (P-24-12).
On the other hand, information on DO14C in the western North Pacific, including the waters around Fukushima, is limited, and its dynamics remain largely unknown. In this study, we provide an overview of the distribution of 14C isotope ratios (Δ14C) of DOC in surface seawater in the northwestern North Pacific Ocean and its marginal areas, and report the distribution of DO14C concentrations around the F1NPP before and after the release of treated water.
Figure 1 shows the sampling points in this study. Seawater samples in the waters around Fukushima were collected at five stations located 2 to 18 km from the F1NPP from October 2021 to April 2024. In addition to the data previously reported by Otosaka et al.2), data from the broad area of the Northwest Pacific were obtained from January 2021 to August 2023. Seawater samples were collected using a Niskin water sampler, and salinity and water temperature were measured using a CTD at the time of collection. DOC concentration and DOC-Δ14C were analyzed by freezing seawater in glass containers and bringing it back to the laboratory on land. DOC-Δ14C in seawater was analyzed using the method of Otosaka et al. 3). 14C measurements were performed using the single-stage accelerator-assisted mass spectrometer (YS-AMS) at the Atmosphere and Ocean Research Institute, the University of Tokyo.
Figure 2 shows the relationship between Δ14C of DOC in surface seawater and the inverse of DOC concentration at 27 stations in the Northwest Pacific and its marginal seas. The straight line in the figure is a schematic representation of the results obtained by Otosaka et al. 2) at upper (<800 m depth) at a station in the subarctic Northwest Pacific (47° N, 160° E). Surface seawater obtained from many stations in the Northwest Pacific, except some marginal stations, was plotted near the straight line in the figure, and the DOC in these seawaters could be explained as a mixture of two components: fresh DOC produced in the surface layer and old DOC derived from the deep layer.
The Δ14C values of DOC in the Fukushima coast were −248 to −170‰. The DOC-Δ14C values in October 2022, before the release of treated water, were up to 50‰ higher than the mixing line shown in Fig. 2, indicating the possibility of an increase in values due to anthropogenic 14C input, but the associated increase in DO14C concentration was estimated to 0.01mBq/kg. The DOC-Δ14C values in March 2024, after the release of ALPS treated water, were lower than in 2022, and no remarkable deviation from the mixing line was observed. The observations in 2024 were made 7 to 8 days after the third ALPS-treated water release period, and it was estimated that, at least at that point, the increase in DO14C concentration associated with the release was extremely small.
References
1) TEPCO HD
https://www.tepco.co.jp/en/hd/decommission/data/analysis/pdf/2024/measurement_confirmation_241015-e.pdf
2) Otosaka, S. et al. (2024) Otosaka S, et al., J Coastal Res. 116, 181-185.
3) Otosaka, S. et al. (2022) Nucl. Inst. Methods Phys. Res. B 527, 1-6.
Acknowledgements
This research was supported by Grants-in-Aid for Scientific Research (KAKENHI 23H24815, 19H04260, 20H04315) and by the Environmental Radioactivity Research Network Center (P-24-12).