Lateral variation of 134Cs and 137Cs concentrations in surface seawater in and around the Japan Sea after the Fukushima Dai-ichi Nuclear Power Plant accident.

A total of 82 surface seawater samples was collected in the Japan Sea and the southwestern Okhotsk Sea before and after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Analysis of (134)Cs and (137)Cs concentrations using low-background γ-spectrometry revealed that the (137)Cs concentration of the samples collected in June 2011 was 1.5-2.8mBq/L, which is approximately 1-2 times higher than the pre-accident (137)Cs level, while the (134)Cs concentration was less than detectable to 1mBq/L. In addition to (134)Cs being clearly detected (∼1mBq/L), (137)Cs concentration in water samples from the northeastern Japan Sea (2-2.8mBq/L) was also higher than that from the coast in the southwestern Japan Sea (∼1.5mBq/L). These higher concentrations in the northeastern Japan Sea could be ascribed to the atmospheric transport of nuclides from the FDNPP as aerosols and subsequent transport and dilution after delivery to the sea surface.