Distribution of radionuclides in surface seawater obtained by an aerial radiological survey

We investigated the distribution in seawater of anthropogenic radionuclides from the Fukushima Daiichi Nuclear Power Plant (FNPP1) as preliminary attempt using a rapid aerial radiological survey performed by the U. S. Department of Energy National Nuclear Security Administration on 18 April 2011. We found strong correlations between in-situ activities of I-131, Cs-134, and Cs-137 measured in surface seawater samples and gamma-ray peak count rates determined by the aerial survey (correlation coefficients were 0.89 for I-131, 0.96 for(134)Cs, and 0.92 for(137)Cs). The offshore area of high radionuclide activity extended south and southeast from the FNPP1. The maximum activities of I-131, Cs-134, and Cs-137 were 329, 650, and 599 Bq L-1, respectively. The I-131/Cs-137 ratio in surface water of the high-activity area ranged from 0.6 to 0.7. Considering the radioactive decay of I-131 (half-life 8.02 d), we determined that the radionuclides in this area were directly released from FNPP1 to the ocean. We confirm that aerial radiological surveys can be effective for investigating the surface distribution of anthropogenic radionuclides in seawater. Our model reproduced the distribution pattern of radionuclides derived from the FNPP1, although results simulated by a regional ocean model were underestimated.