Presentation Radiocaesium activity concentrations in overlying water collected off the coast of Fukushima Prefecture in 2014-2016 (福島県沖の堆積物と直上水中の放射性セシウム濃度分布)

Fukuda, Miho  ,  Shinnnosuke, Yamazaki  ,  Tatsuo, Aono  ,  Takashi, Ishimaru  ,  Jota, Kanda  ,  Toru, Hirawake

After the accident at the Fukushima Dai-ichi Nuclear Power Station happened in March 2011, large amounts of radionuclides were dispersed by hydrogen explosions, and radionuclides also leaked from the FDNPS into the terrestrial and marine environments. In the seawater, Cs mainly exists in a dissolved state and parts of particulate radiocaesium have settled to seafloor. Radiocaesium accumulated in sediment have partly re-suspended as particulate form and re-eluted as dissolved form due to several factors such as bottom current and deformation. In this study conducted using dissolved radiocaesium activities in trapped water (~30 cm over seafloor) on sediment at thirteen stations collected using Multiple Corer, which is considered as overlying water, collected off the coast of Fukushima Prefecture in 2014-2017. Collected overlying waters were filtered through 0.45 μm and 0.20 μm pore sizes filter and these filtrate water was regarded as F1 and F2 fraction, respectively. Radiocaesium in the filtered overlying water was concentrated by the ammmonium phosphomolybdate (AMP) adsorption method (Aoyama and Hirose, 2008) and were measured using high-purity Ge-detector. Based on corrected sampling date 137Cs activity concentration in overlying water, we firstly performed F-test to confirm the equal variance as follow two topics: i) between F1 and F2 fractions at each station ii) between within 10 km and outside 10 km from the east coast of Japan. 137Cs activity concentrations in samples showing equal variance were also calculated by the t-test and we confirmed which of these activity concentrations had significant differences or not. Samples that not showed equal variance were verified by the Wilcoxon signed-rank test to confirm whether or not there was a significant difference (P <0.05 level). In overlying water, the dissolved 137Cs activity concentrations about F1 and F2 fractions were 2.3-27 mBq/L and 2.4-31 mBq/L, respectively. In 2014-2017, 137Cs activity between F1 and F2 fractions had not significant differences in all sampling periods. Thus, the dissolved 137Cs in overlying water generally exist as less than 0.2μm grain size. In both F1 and F2 fractions, the 137Cs activity concentrations between within 10 km and outside 10 km from the east coast of Japan had not significant differences.
2018 Oean Science meeting

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