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 ⁹⁰Sr in seawater in the coast off Japan has received limited attention. However, ⁹⁰Sr 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 ⁹⁰Sr, 124 ± 3 Bq·m^–3 for ¹³⁷Cs, and 54 ± 1 Bq·m^–3 for ¹³⁴Cs, appear to be influenced by ongoing releases from the FDNPP, with a characteristic ¹³⁷Cs/⁹⁰Sr activity ratio of 3.5 ± 0.2. Beach surface water and groundwater collected in Sendai Bay had ¹³⁷Cs concentrations of up to 43 ± 1 Bq·m^–3, while ⁹⁰Sr was close to pre-Fukushima levels (1–2 Bq·m^–3). These samples appear to be influenced by freshwater inputs carrying a ¹³⁷Cs/⁹⁰Sr activity ratio closer to that of the FDNPP fallout deposited on land in the spring of 2011. Ongoing inputs of ⁹⁰Sr 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 surveillance of artificial radionuclides in the Pacific Ocean is still required