Reassessment of 90Sr, 137Cs, and 134Cs in the coast off Japan derived from the Fukushima Dai-ichi nuclear accident
American Chemical Society
School of Natural Sciences / Centre for Marine Ecosystems Research
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.