ABSTRACT

A total of 150 surface seawater samples were collected from around the Japanese Archipelago between July 2009 and February 2012, with sample collection peaking in July and October 2011. Low-background measurements revealed that, except for coastal and offshore areas around western Japan, ¹³⁴Cs and ¹³⁷Cs concentrations in surface waters were significantly higher than those recorded before the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, although their concentration levels were generally very low (<~0.1 to 1 mBq/L for ¹³⁴Cs). In June 2011, concentration peaks of 1 mBq/L for ¹³⁴Cs and 2.5 mBq/L for ¹³⁷Cs were recorded in the Tsugaru Strait and off western Hokkaido, toward the Japan Sea approximately 150-300 km north of the high-deposition area of Akita in northern Honshu. In October 2011, ¹³⁴Cs was detected only in the surface waters off western Hokkaido at a concentration of 0.2 mBq/L, which is markedly lower than the values observed in June 2011. These findings indicate that the observed radionuclide distributions could primarily be attributed to the atmospheric deposition of radionuclides emitted from the FDNPP and transported by ocean currents.