Distribution of ¹³⁷ Cs in the Bohai Sea, Yellow Sea and East China Sea: Sources, budgets and environmental implications

The transport processes and fate of ¹³⁷ Cs in eastern Chinese seas (ECSs) that consists of the Bohai, Yellow, and East China Seas, have not been well established. In this study, we measured the concentrations of ¹³⁷ Cs in the ECSs water and surface sediments during 2013–2014. Combined with a number of published ¹³⁷ Cs inventory data from drainage basins and sediment accumulation rates in the ECSs, the distribution, sources and budgets of ¹³⁷ Cs in the ECSs were investigated. The ¹³⁷ Cs activity in the water column and surface sediments ranged from 0.03 to 1.92 Bq/m ³ and from 0.30 to 5.22 Bq/kg, respectively. No ¹³⁴ Cs signal was observed, suggesting that the Fukushima accident had limited impact on the ECSs during the investigation period. Mass balance of ¹³⁷ Cs suggests that at least 7.4 × 10 ¹² Bq/y of ¹³⁷ Cs imported into the ECSs from the Northwestern Pacific that accounts for 0.7% of the ¹³⁷ Cs transported by the Kuroshio Current, and this value is 5.2 times higher than the sum of atmospheric fallout and total riverine input. The apparent half-lives of ¹³⁷ Cs are estimated to be 15.1 y for the ECS and 7.7 y for the YS. The vertical profiles in the continental shelf edge and the Yangtze River Estuary reveal that the upwelling of the Kuroshio Subsurface Water is the main mechanism of ¹³⁷ Cs import into the ECSs. The high level of ¹³⁷ Cs in oceanic water masses and the low level of ¹³⁷ Cs in riverine and coastal waters make ¹³⁷ Cs a good indicator for tracing water mass movement and interaction. In addition, good correlation between ¹³⁷ Cs activity and mean grain size (φ) indicates that ¹³⁷ Cs can serve as an effective tracer to track dispersal pathways of fine sediments in river-dominated marginal seas.