Spatial variations of low levels of 134Cs and 137Cs in seawaters within the Sea of Japan after the Fukushima Dai-ichi Nuclear Power Plant accident

• Low background γ-spectrometry detected radiocesium concentration down to 2 mBq.
• 134Cs and 137Cs concentrations in seawaters within the Sea of Japan were determined.
• In May 2011, 134Cs concentration in surface waters was <0.1 to 1 mBq/L.
• 134Cs detected was due to deposition from the Fukushima Dai-ichi Nuclear Power Plant.

Our method based on low background γ-spectrometry enabled the measurement of low radiocesium concentrations in only 20 L of seawater. In May 2011 after deposition of radiocesium, 134Cs concentration in surface water within the Sea of Japan was confirmed to be significantly small (<0.1–1 mBq/L) by the method. The concentration was not detected (<0.1 mBq/L) below 50 m depth. The Fukushima-derived radiocesium migrated from the surface water of the Sea of Japan without advection to below the thermocline.