Migration of the FDNPP-derived 134Cs and 137Cs along with 226Ra and 228Ra concentrations across the northwestern North Pacific Ocean

We examined lateral distributions of 134Cs, 137Cs, 226Ra, and 228Ra in the surface seawaters around the Kuril Islands and the Kamchatka Peninsula in the northwestern North Pacific Ocean during June 2014. The sampling area included three water current areas, the Oyashio Current, the current from the Okhotsk Sea, and the coastal current along the east Kamchatka Peninsula. 226Ra and 228Ra distributions differed along the three currents. Low levels of 134Cs were detected in the surface waters of the Oyashio Current (0.09-0.35 mBq/L), but it was <∼0.1 mBq/L at the surface along the other two currents. This indicates that the distribution of Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived radiocesium in surface waters off the Kamchatka and along the Kuril Islands is predominantly governed by the Oyashio current system.