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 ¹³⁴ Cs and ¹³⁷ Cs in seawater from 2011 to 2015 were measured. The highest activities of ¹³⁴ Cs and ¹³⁷ Cs in seawater were 0.73 Bq/m ³ and 3.34 Bq/m ³ , 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 ¹³⁷ 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.
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