Distribution of 137Cs and 134Cs in the North Pacific Ocean: impacts of the TEPCO Fukushima-Daiichi NPP accident

Impact of the TEPCO Fukushima-Daiichi NPP accident, FNPP1, to the North Pacific Ocean occurred through two pathways, namely direct release and atmospheric deposition to wide ocean surface. We collected more than 100 seawater samples in the North Pacific Ocean in April and May 2011 by seven commercial ships as VOS. Since the sample volume was 2 l each, we measured radiocaesium activity at Ogoya Underground Facility to obtain reliable activity. Cs-137 was detected at all stations and Cs-134 was detected at most of the stations in the North Pacific Ocean. The Cs-137 activity ranged from around 1 to 1,000 Bq m(-3) with activity ratios of Cs-134/Cs-137 close to 1 which is a signature of radiocaesium originated from the FNPP1 accident. At east of the International Date Line north of 40A degrees N in the Pacific Ocean in April 2011, the Cs-134 activity ranged from 2 to 12 Bq m(-3).