2013 Volume 62 Issue 6 Pages 455-474
A radioactivity survey program was launched in 1983 to determine the background levels of artificial radionuclides, such as 90Sr and 137Cs in the marine environment off commercial nuclear power stations. In this paper, we report on the long-term temporal changes of 90Sr and 137Cs in seawater, bottom sediment and marine organism samples. Both 90Sr and 137Cs have been detected since the beginning of the program in the seawater samples. Their concentrations decreased slowly over time, except for a prompt rise of the 137Cs concentration caused by the Chernobyl nuclear accident in 1986, and reached the level corresponding to 1-2 mBq L−1 for both radionuclides just before the Fukushima accident. The concentration of 137Cs in the bottom sediments widely varied, unlike that in seawater from one sampling site to another. The highest 137Cs concentration was observed in marine organisms in 1986, when the Chernobyl nuclear accident occurred, and was followed by relatively high concentrations for some years. The 137Cs concentration gradually decreased thereafter during the 1990s. The most recent results before the Fukushima accident suggested that the 137Cs concentration would not be more than 1-2 mBq L−1, ND (below the detection limit) to 8 Bq kg−1-dry and ND to 0.24 Bq kg−1-wet, respectively, for seawater, bottom sediment and marine organism samples. A post-accident monitoring after the Fukushima accident revealed a heavy burden of artificial radionuclides in the marine environment adjacent to Fukushima Prefecture. This paper also summarizes the 90Sr and 137Cs monitoring data in the seawaters, bottom sediments and marine organisms immediately after the accident. A comparison of data was made between the precedent situation before the accident and the post-accident situation in order to assess the impacts of the Fukushima accident on the adjacent marine environment.
