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

We report temporal variations of 137 Cs 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. 137 Cs activity concentrations in surface water decreased rapidly to ∼2-3 Bq m-3 in 2015/2016, still a bit higher than 137 Cs activity concentrations before the FNPP1 accident (1.5-2 Bq m-3 ). 134 Cs/137 Cs 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, 137 Cs activity concentrations were around 1-7 Bq m-3 in 2011/2012 but then stabilized at a few Bq m-3 up to 2017.134 Cs activity concentrations were detected at levels of 0.1-0.9 Bq m-3 , and 134 Cs/137 Cs activity ratios decay-corrected to 11 March 2011 were ∼0.3-0.5. Temporal variations of model-simulated 137 Cs activity concentrations in surface water in the region of interest showed good agreement with observations, except in the southwestern North Pacific Ocean.