A change in the concentrations of radioactive cesium in Tokyo-Bay's sediments released by the Fukushima Dai-Ichi nuclear power station accident

Abstract

The concentration of radioactive cesium in sediments of Tokyo-bay, released by the Fukushima Dai-ichi nuclear power station accident, was measured every half year from July '10 to February '13 in order to analyze the trend of concentration. The samples were collected at two artificial deeper sites in dredged trenches and one natural shallower site, which were located off Makuhari in Tokyo-bay, then they were brought into a Ge detector to measure the γ-rays. According to an analysis of the upper layer of the samples, both 134 Cs and 137 Cs had been detected since the samples of August '11, and they must have been released by the accident. Furthermore, from February '12 to February '13, the concentrations of 134 Cs and 137 Cs in upper layer of sediments had been higher at deeper sites than shallower site. The deeper sites look like pitfall traps for fine particles clinging to 134 Cs and 137 Cs, so we can call these sites "the hotspot in the sea". We also examined the depth profiles of 134 Cs and 137 Cs in samples taken on August '12 and February '13. As a result, 134 Cs and 137 Cs were found to have gone deeper in the sediment on February '13 than on August '12, and the inventory of them was also larger on February '13. In addition, this phenomenon was observed more clearly at deeper sites than shallower site. Though 134 Cs and 137 Cs had not increased very much in upper layer from August '12 to February '13, we clarified that they had been flowing into the Tokyo-bay. © 2013 The Japan Society for Analytical Chemistry.