Tritium and radiocarbon in Pacific Ocean after Fukushima accident

Published in Journal of Environmental Radioactivity 218 (2020) 106238

On 11 March 2011, large tsunami waves, caused by the Tōhoku earthquake with the epicenter located approximately 130 km offshore of the eastern coast of Japan, stroke the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), resulting in a release of large amounts of radioactive material to the atmosphere and seawater, including tritium and radiocarbon. The radionuclides emitted to the atmosphere were mostly transported over the North Pacific Ocean where about 80% of the released radioactivity was deposited by processes of dry and wet deposition.
We compared temporal changes in ³H and ¹⁴C concentrations in surface and vertical water profiles, measured along the 149°E line in the western North Pacific Ocean between 1993 and 2005, and also calculated water column inventories of ³H and bomb-produced ¹⁴C in the respective years. The impact of the FDNPP accident disturbed equilibrated ³H levels, hindering any possible investigation of tritium remaining from nuclear weapons testing. Because ten months passed between the FDNPP accident and 2012 expedition, tritium penetrated in the water column inventories, which were higher by a factor of 2–7 in comparison to the year 1993. The total amount of ³H released from the Fukushima damaged power plant to the western North Pacific Ocean were estimated to be 0.7 ± 0.3 PBq.
In the case of radiocarbon, seasonality of the Oyashio Current was also noticeable in the datasets. The acceleration of the surface Δ¹⁴C values decrease was probably caused by downwelling of bomb-produced radiocarbon into deeper layers of the water column and its transport along isopycnal layers. Bomb-produced radiocarbon inventories decreased as well in the northern part of the transect, with highest decline (24%) at the station at around 34.9°N, while in the southern part they remained almost the same or were slightly increased. The ¹⁴C data did not show a clear impact of the Fukushima accident on the radiocarbon levels.

Distribution of surface tritium and radiocarbon in the western North Pacific Ocean