ABSTRACT

We present our April to December 2011 observations of ¹³⁴Cs and ¹³⁷Cs activities in surface water at Hasaki, a coastal station 180 km south of the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident site. We also investigate trends by using published data from several other coastal stations, including the accident site. The maximum in radiocaesium activity at Hasaki was observed in June 2011, representing a delay of two months from the corresponding maximum in April 2011 at FNPP1. Directly discharged ¹³⁴Cs and ¹³⁷Cs were transported dominantly southward along the coastline of northeastern Honshu, at least in May and June 2011. The reasons for the two-month delay at Hasaki are not yet clear, but clockwise current associated with a warm water eddy of which center located at 36.5 N, 141.4 E off Iwaki between Onahama and Hasaki in mid of May 2011 might prevent southward transport of ¹³⁴Cs and ¹³⁷Cs released from FNPP1 to Hasaki until the end of May 2011.