Transport and fate of Fukushima-derived Cs and Cs in the seawater of the Northwest Pacific in 2015.

To understand the influence of the Fukushima accident on the Northwest Pacific, the distributions and transportations of Cs and Cs in the seawater in the Northwest Pacific in May and September 2015 were studied. The data showed that the Fukushima-derived Cs and Cs at some stations can still be distinguished from background level ~ 4 years later. On the whole, the activities of Cs and Cs in seawater were decreasing from May to Sep 2015. But the increased inventories and the surface activities of Cs imply that there has ever been an extra Cs from offshore water transported to this study area (from 31° N to 27° N, 145° E to 152.5° E) in May 2015. The average activities of Cs in subtropical gyre area in south of KE were the highest and the least were to the east of Luzon Strait in 2015. In vertical direction, Cs in subtropical gyre area were mainly distributed at 100 ~ 500 m layer and Cs only at 500 m layer in this area showed an increasing trend from May to Sep 2015 which reflects more Cs were still penetrating to deeper layer of 500 m from upper water. But they were almost not found below 1000 m layer. It was associated with the subsurface transport of radiocesiums by Northwest Pacific Mode Water (NPMW) and the diffusion of mesoscale eddy. Different distribution characteristics of Cs existed between north of KE and south of KE. The low-temperature-low-salinity water mass likely to be the first Oyashio Intrusion was the main factor that resulted in higher Cs appearing at the upper 100 m layers in north of KE.