Spatial and vertical distribution of radiocesium in seawater of the East China Sea

The ¹³⁷Cs activity in surface water of the East China Sea (ECS) was 0.66–1.36 Bq m⁻³ during May of 2011. The low activities were observed in the Changjiang Estuary and Zhejiang-Fujian coast and high activities were observed in the south offshore and Kuroshio Current pathway, suggesting that the influence from the current system in the ECS. The ¹³⁴Cs were undetectable (<0.03 Bq m⁻³) and the contribution of the Fukushima accident to ECS is estimated to be below 3%; hence it is negligible during the investigation period. Using the vertical profiles of ¹³⁷Cs in the ECS, the mass balance is obtained, which suggests that the oceanic input dominates the ¹³⁷Cs source in the ECS. ¹³⁷Cs is potentially useful to trace water mass movement in the ECS. Our study provides comprehensive baseline of ¹³⁷Cs in the ECS for evaluation of the possible influence of the nuclear power plants in the future.