One year tracking of 134 Cs and 137Cs in the North Pacific Ocean released from Fukushima Dai-ichi NPP accident

On 11 March 2011, an extraordinary earthquake of magnitude 9.0 centered about 130 km off the Pacific coast of Japan's main island, at 38.3ºN, 142.4ºE, was followed by a huge tsunami with waves reaching up to 40 m height in Iwate region and about 10 m in Fukushima region. The station blackout developed into a disaster that left three of the six FNPP1 reactors heavily damaged and caused radionuclides to be discharged into the air and ocean. Estimates of the total amount of ¹³⁷Cs released by the FNPP1 accident range from 7 to 35 PBq, thus differing by a factor of 5. This large range is due to the large uncertainties of the transport and inversion processes and lack of observations at offshore side over the Pacific Ocean in the atmospheric models used for the estimations. We also do not have direct measurements of released radiocesium from the reactors. Therefore we compared the model-simulated and observed radiocesium inventories in the open ocean to correct the estimated total atmospheric deposition of ¹³⁴Cs and ¹³⁷Cs in the North Pacific from the FNPP1 accident based on the first guess of release scenario of radiocesium. We have been tracking ¹³⁴Cs and ¹³⁷Cs in the North Pacific Ocean released from Fukushima Dai-ichi NPP, FNPP1, accident since 1st April 2011 onward to reveal ¹³⁴Cs and ¹³⁷Cs distribution in the North Pacific Ocean. We found that ¹³⁴Cs and ¹³⁷Cs released from the FNPP1 to the atmosphere were transported over the wide area in the North Pacific Ocean and deposited mainly on sea surface close to the FNPP1 site.  Also ¹³⁴Cs and ¹³⁷Cs directly released from the FNPP1 to the coastal water was transported together at the ocean surface by ocean current at the beginning of the advection. We observed clear eastward movement of ¹³⁴Cs and ¹³⁷Cs along the Kuroshio and the Kuroshio extention and found that they reached the International Dateline about several months later after the accident by surface transport. More important finding is that radiocaesium released from the FNPP1 was observed in ocean interior at least the fall in 2011 these were effectively transported by lateral mixing but not vertical mixing.

We integrated our observational results to estimate total amount of radiocaesium in the North Pacific Ocean except close area of the FNPP1 and compared with atmospheric/ocean transport models results, then we obtained a reliable total amount of radiocaesium in the North Pacific Ocean as 15-18 PBq of ¹³⁴Cs. This estimation includes an estimation of 3.5 ± 0.7 PBq as direct discharge. We also integrate aerial monitoring results within Fukushima Pref. and obtained 2.0 PBq and deposition data at 46 stations in a mainland Japan as 0.5 PBq to obtained reliable total amount of radiocaesium deposited in mainland of Japan. Finally we obtain 2.5 PBq as total deposition of ¹³⁴Cs on land in Japan. Before the FNPP1 accident, distribution and inventory of ¹³⁷Cs which originated from atmospheric weapons tests had been studied in the Pacific Ocean since the late 1950s and the ¹³⁷Cs inventory in the North Pacific Ocean was 290 ± 30 PBq in January 1970 based on 10º x 10º mesh data of the ¹³⁷Cs deposition by Aoyama et al., 2006 and it decreased to 69 PBq in 2011 because of decay and inter-basin transport from the North Pacific Ocean to the Indian Ocean and the South Pacific Ocean. Therefore impact of the FNPP1 accident to the Pacific Ocean in terms of inventory was estimated to be around 22 – 27 % increase.