Arrival of the Fukushima radioactivity plume in North American continental waters

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-02, Vol.112 (5), p.1310-1315
• Main Authors: Smith, John N., Brown, Robin M., Williams, William J., Robert, Marie, Nelson, Richard, Moran, S. Bradley
• Format: Article
• Language: English
• Subjects: Continental shelves; human health; Knowledge; North America; Nuclear accidents & safety; Nuclear power plants; Nuclear reactors; Nuclear weapons; Ocean circulation; Oceans; Physical Sciences; Radioactivity; Seawater; time series analysis; tracer techniques; Water circulation; Water column
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1412814112

The large discharge of radioactivity into the northwest Pacific Ocean from the 2011 Fukushima Dai-ichi nuclear reactor accident has generated considerable concern about the spread of this material across the ocean to North America. We report here the first systematic study to our knowledge of the transport of the Fukushima marine radioactivity signal to the eastern North Pacific. Time series measurements of ¹³⁴Cs and ¹³⁷Cs in seawater revealed the initial arrival of the Fukushima signal by ocean current transport at a location 1,500 km west of British Columbia, Canada, in June 2012, about 1.3 y after the accident. By June 2013, the Fukushima signal had spread onto the Canadian continental shelf, and by February 2014, it had increased to a value of 2 Bq/m ³ throughout the upper 150 m of the water column, resulting in an overall doubling of the fallout background from atmospheric nuclear weapons tests. Ocean circulation model estimates that are in reasonable agreement with our measured values indicate that future total levels of ¹³⁷Cs (Fukushima-derived plus fallout ¹³⁷Cs) off the North American coast will likely attain maximum values in the 3–5 Bq/m ³ range by 2015–2016 before declining to levels closer to the fallout background of about 1 Bq/m ³ by 2021. The increase in ¹³⁷Cs levels in the eastern North Pacific from Fukushima inputs will probably return eastern North Pacific concentrations to the fallout levels that prevailed during the 1980s but does not represent a threat to human health or the environment. Significance The radionuclide results in this report represent the first systematic study, to our knowledge, of the arrival of the Fukushima radioactivity signal in continental waters off North America. The present time series results are critical to an understanding of the circulation of Fukushima tracers in the eastern North Pacific and to the tuning and validation of ocean circulation models that are being used to predict the future evolution of this signal. They are also important for informing the public of the magnitude of the Fukushima radioactivity signal in North American continental waters and enabling a science-based assessment of the significance of its potential effects on human health and the environment.