Intrusion of Fukushima-derived radiocesium into the East China sea and the Northeast South China Sea in 2011-2015.

To investigate the potential long-term impact of the Fukushima Dai-ichi Nuclear Power Plant Accident (FDNPPA) on the South China Sea (SCS) and the East China Sea (ECS), radiocesium isotopes Cs and Cs in seawater from 2011 to 2015 were measured. The highest activities of Cs and Cs in seawater were 0.73 Bq/m and 3.34 Bq/m, respectively. The results demonstrated that FDNPPA-derived radiocesium intruded into the Northeast SCS and the ECS in 2013 and reached a maximum in 2014. The intrusion occurred within the upper 100 m and contributed ≤72.5% of the total Cs in the seawater of the SCS and ECS. The formation, subduction, and transport of subtropical mode water (STMW)/central mode water (CMW) trapped FDNPPA-derived radiocesium in the ocean interior and transported it southwestward from the high-latitude open ocean to the low-latitude western boundary area. Then, the FDNPPA-derived radiocesium entered the Northeast SCS and the ECS by the intrusion of the subsurface high-salinity water of the Kuroshio Current into the Northeast SCS and the ECS.