Reassessment of 90Sr, 137Cs, and 134Cs in the Coast off Japan Derived from the Fukushima Dai-ichi Nuclear Accident

The years following the Fukushima Dai-ichi nuclear power plant (FDNPP) accident, the distribution of Sr-90 in seawater in the coast off Japan has received limited attention. However, Sr-90 is a major contaminant in waters accumulated within the nuclear facility and in the storage tanks. Seawater samples collected off the FDNPP in September 2013 showed radioactive levels significantly higher than pre-Fukushima levels within 6 km off the FDNPP. These samples, with up to 8.9 +/- 0.4 Bq.m(-3) for Sr-90, 124 +/- 3 Bq.m(-3) for Cs-137, and 54 +/- 1 Bq.m(-3) for Cs-134, appear to be influenced by ongoing releases from the FDNPP, with a characteristic Cs-137/Sr-90 activity ratio of 3.5 +/- 0.2. Beach surface water and groundwater collected in Sendai Bay had Cs-137 concentrations of up to 43 +/- 1 Bq.m(-3), while Sr-90 was close to pre-Fukushima levels (1-2 Bq.m(-3)). These samples appear to be influenced by freshwater inputs carrying a Cs-137/Sr-90 activity ratio closer to that of the FDNPP fallout deposited on land in the spring of 2011. Ongoing inputs of Sr-90 from FDNPP releases would be on the order of 2.3-8.5 GBq.d(-1) in September 2013, likely exceeding river inputs by 2-3 orders of magnitude. These results strongly suggest that a continuous radionuclides in the Pacific Ocean is still required.