134Cs and 137Cs activities in coastal seawater along Northern Sanriku and Tsugaru Strait, northeastern Japan, after Fukushima Dai-ichi Nuclear Power Plant accident

A total of 37 seawater samples were collected at 10 sites along the coastline of the Northern Sanriku and Tsugaru Strait, 250–450 km north of the Fukushima Dai-ichi Nuclear Power Plant in April–December 2009 and May–June 2011, and analyzed for 134Cs and 137Cs activities using low-background γ-spectrometry. The 134Cs and 137Cs activities measured in these samples in May 2011 were found to be 2–3 mBq/L and 2.5–4 mBq/L, respectively. By June, these values had decreased by 25–45%/month and 5–30%/month, respectively. These results can be plausibly explained by surface infusion of these isotopes into the sea by atmospheric transport from Fukushima and their subsequent reduction by water migration to off-shore and deeper regions.