Distribution and transportation of Fukushima-derived radiocesiums in the seawater of the Northwest Pacific ocean in May 2013

Abstract

The Fukushima Daiichi Nuclear Power Plant (FDNPP) has generated quantities of polluted water since the accident in 2011 triggered by the massive earthquake. In order to understand the FDNPP accident comprehensively and to provide a basic reference for predicting the transport of the treated nuclear contaminated water in the Northwest Pacific further, the distributions of 137Cs and 134Cs in the seawater as deep as 2000 m were determined in the subtropical region in May 2013. The results suggested that the radiocesium from FDNPP still existed in May 2013. But no FDNPP-derived radiocesium was found below 1000 m layer. The FDNPP accident contributed 0.46 PBq of 137Cs to the upper 500 m of water column, which was ∼1.6 times of the background amount of 137Cs (0.28 PBq). The maximum activities of 137Cs and 134Cs were 7.88 Bq/m3 and 3.40 Bq/m3, respectively. It is mainly because of the Subtropical Mode Water (STMW) that carried 137Cs and 134Cs to the subtropical region along the subsurface isopycnals (25.0–25.6 δθ). As time went on, more FDNPP-derived radiocesiums were transported to the subtropical region and to the subsurface layer by the STMW than ever. The cyclonic mesoscale eddy further promoted more radiocesiums downward transport and deeper penetration on the basis of the subduction of STMW. However, the formation of the vertical stratification and the presence of the low salinity water mass (at the depth of ∼500–∼700 m) restrained the penetration of the radiocesium into deeper and interior ocean and thus the FDNPP-derived 137Cs and 134Cs in the subtropical area mainly distributed in the upper 500 m layer.