Presentation The Distribution and behavior of radiocaesium in marine environment around the Fukushima NPS in 2012-2014

青野, 辰雄  ,  福田, 美保  ,  山崎, 慎之介  ,  吉田, 聡  ,  早乙女, 忠弘  ,  水野, 拓治  ,  山田, 学  ,  伊藤, 友加里  ,  神田, 穣太  ,  石丸, 隆

An accident of the TEPCO’s Fukushima Daiichi Nuclear Power Station (FDNPS) has been passed about four and half years in July 2015 since March 2011. It is important to clarify behaviors of the atmospheric and oceanic releases and the dispersion of various artificial radionuclides in the marine environment immediately after and since the March 2011 accident at the FDNPS. The pre-accident levels of 137Cs activities in seawater and sediments were observed 1–2 mBq/L and 1 Bq/kg-dry, respectively. Direct release of contaminated water to the ocean had caused the high levels of radiocaesium (134Cs+137Cs) in seawater, sediments and biota in the coastal area of Fukushima during the end of March to the early April. After the accident, the 137Cs activities in seawater had exponentially decreased more than 10 times compared to the activities before the accident. However, the activities in sediment have decreased more slowly than those in seawater and had large fluctuations. The sedimentary environment depends on several factors such as the seabed topography and freshwater and terrestrial material input via rivers. The aims of the present study were to examine the temporal and spatial variation in the 137Cs activities and to investigate the distribution and behavior in the marine environment immediately after and since this accident.Samples collected in the marine environment included seawater and sediments and also marine biota such as plankton, benthos, fish and shellfish. They were collected with a plankton net, dredge sampler, and trawl net during cruises of some research vessels and by fishing ships. The 131I activities could not be measured as 131I in biological sample had been volatilized by the treatment. Fukushima-derived radionuclides were observed with the gamma-ray spectrometry, were 134Cs, 137Cs and 110mAg in sediments and marine biota around off Fukushima. 90Sr were not detected in the bony parts of fish and 239+240Pu activities in visceral parts were the same levels as before the accident. The 137Cs activities in seawater decreased ten-fold during 6 months, and consequently activity in the marine biota also dropped. It is thought the decreases of 137Cs and 110mAg activities in marine biota depend on differences in the mechanisms of accumulation and metabolism.The concentration ratio (CR) [= activity in biota (Bq/kg (wet wt))/activity in seawater (Bq/kg or Bq/L)] is usually represented in terms of the concentration in biota relative to that of seawater for modeling purposes. This value of cesium in marine biota was calculated with the activity of cesium in seawater of same location. The estimated CR of cesium (CR-Cs) in marine organisms ranged from 26 in the muscle part of squid to 10,000 in clam viscera. Large differences in CR-Cs in different parts of marine biota were not observed. On the other hand, Sediment-seawater distribution coefficients Kd (L/kg) were observed to be from 1,900 to 25,000 in the same area in 2013-2014. For CR-Cs values, it is necessary to consider the influence by the sediment and particles other than the marine biota, as biota samples contain not only tissue but also particulate matter and sediment in the coastal area.
13th International Conference on the Biogeochemistry of Trace Elements

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