Long Term Temporal Changes of 90Sr and 137Cs in Seawater, Bottom Sediment and Marine Organism Samples - from the Chernobyl Accident to Immediately after the Fukushima Accident -

A radioactivity survey program was launched in 1983 to deteimine the background levels of artificial radionuclides, such as Sr-90 and Cs-137 in the marine environment off commercial nuclear power stations. In this paper, we report on the long-term temporal changes of Sr-90 and Cs-137 in seawater, bottom sediment and marine organism samples. Both Sr-90 and Cs-137 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 Cs-137 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 Cs-137 in the bottom sediments widely varied, unlike that in seawater from one sampling site to another. The highest Cs-137 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 Cs-137 concentration gradually decreased thereafter during the 1990s. The most recent results before the Fukushima accident suggested that the Cs-137 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 Sr-90 and Cs-137 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.