Transport and fate of Fukushima-derived 137Cs and 134Cs 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 ¹³⁴Cs and ¹³⁷Cs in the seawater in the Northwest Pacific in May and September 2015 were studied. The data showed that the Fukushima-derived ¹³⁴Cs and ¹³⁷Cs at some stations can still be distinguished from background level ~ 4 years later. On the whole, the activities of ¹³⁷Cs and ¹³⁴Cs in seawater were decreasing from May to Sep 2015. But the increased inventories and the surface activities of ¹³⁷Cs imply that there has ever been an extra ¹³⁷Cs 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 ¹³⁷Cs 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, ¹³⁷Cs in subtropical gyre area were mainly distributed at 100 ~ 500 m layer and ¹³⁷Cs only at 500 m layer in this area showed an increasing trend from May to Sep 2015 which reflects more ¹³⁷Cs 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 ¹³⁷Cs 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 ¹³⁷Cs appearing at the upper 100 m layers in north of KE. [ABSTRACT FROM AUTHOR]