134Cs and 137Cs in the North Pacific Ocean derived from the March 2011 TEPCO Fukushima Dai-ichi Nuclear Power Plant accident, Japan. Part One - Surface pathway and vertical distributions

We collected 2 - 10 litre surface seawater samples at more than 300 stations in the North Pacific Ocean and two coastal stations at Tomioka in Fukushima Prefecture and Hasaki in Ibaraki Prefecture. In addition to our own data, we compiled already published data including monitoring data by TEPCO. After July 2012, the activities of 137Cs in surface water at near FNPP1 site were still kept around 1000 Bq m-3 which corresponds about 10 GBq day-1. A zonal speed of FNPP1 derived radiocaesium in surface water at mid latitude in the North Pacific Ocean was 7 km day-1, 8 cm s-1 until March 2012 , however it after March 2012 till August 2014 was ca. 3 km day-1, 3.5 cm s-1. In June 2012 at 29 deg. N, 165 deg. E, clear increase of 134Cs due to STMW formation was observed and maximum 134Cs activity at 151 meters depth, 25.3 sigma theta, was 6.12 +- 0.50 Bq m-3. This increase might be southward transport of Fukushima derived radiocaesium due to STMW formation which was also observed along 149 deg. E. In June 2012 at two stations at 34 deg. N and 39 deg. N along 165 deg. E, 134Cs activity showed maximums at a density around 26.3 which corresponds CMW. 134Cs activity in CMW was highest among the 134Cs activity of surrounding waters including STMW. These observations indicate that CMW formation was most effective pathway to introduce Fukushima derived radiocaesium into ocean interior about one year time scale.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard