Radiocaesium derived from the TEPCO Fukushima accident in the North Pacific Ocean: Surface transport processes until 2017

Publication date: September 2018
Source:Journal of Environmental Radioactivity, Volume 189
Author(s): Michio Aoyama, Yasunori Hamajima, Yayoi Inomata, Yuichiro Kumamoto, Eitarou Oka, Takaki Tsubono, Daisuke Tsumune
We report temporal variations of ¹³⁷Cs activity concentrations in surface waters of six regions of the western and central North Pacific Ocean during 2011–2017 using a combination of 1264 previously published data and 42 new data. In the western and central North Pacific Ocean at latitudes of 30–42°N and longitudes of 140°E to 160°W, eastward transport of radiocaesium was clearly apparent. ¹³⁷Cs activity concentrations in surface water decreased rapidly to ∼2–3 Bq m⁻³ in 2015/2016, still a bit higher than ¹³⁷Cs activity concentrations before the FNPP1 accident (1.5–2 Bq m⁻³). ¹³⁴Cs/¹³⁷Cs activity ratios decay-corrected to 11 March 2011 were ∼0.5–0.8. To the south of 30°N and between 130°E and 160°W in the western and central Pacific Ocean, ¹³⁷Cs activity concentrations were around 1–7 Bq m⁻³ in 2011/2012 but then stabilized at a few Bq m⁻³ up to 2017.¹³⁴Cs activity concentrations were detected at levels of 0.1–0.9 Bq m⁻³, and ¹³⁴Cs/¹³⁷Cs activity ratios decay-corrected to 11 March 2011 were ∼0.3–0.5. Temporal variations of model-simulated ¹³⁷Cs activity concentrations in surface water in the region of interest showed good agreement with observations, except in the southwestern North Pacific Ocean.



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