Abstract
Abstract. Closed and semi-closed water bodies, such as lakes and ponds, are important water resources in Fukushima area and they are the most sensitive environments to radioactive contamination after the Fukushima Dai-ichi nuclear power plant accident. Wakiyama et al. (2017) investigated 137Cs activity in water and bottom sediment in four ponds; Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ), and Kashiramori (KM), within 10 km zone from the FDNPP during 2015–2016. This study follows up their observation to address longer time trends of 137Cs activity concentration in pond waters and to show speciation of 137Cs in soil and bottom sediment. Mean total 137Cs activity concentration in water ranged from 2.5 to 29 Bq L−1. There was not found steady trend in the activity concentration of total and particulate 137Cs for four ponds. The concentration of dissolved 137Cs was usually low in winter during the entire observation period in all four ponds. A tendency to a decrease in the 137Cs activity concentration in suspended sediments was found for four ponds and the decreasing rate constants, including radiological decay, on SU, FS, IZ and KM were 0.33, 0.53, 0.29 and 0.25 yr−1, respectively. The results of sequential extractions of soil and bottom sediment samples showed higher proportion of bioavailable 137Cs, i.e., exchangeable and organic bound 137Cs, in bottom sediment than in the soil.
Highlights
Closed and semi-closed water bodies, such as lakes and ponds, are the most sensitive environments to radionuclide contamination
More than 3700 individual irrigation ponds of different sizes are located in Fukushima Prefecture, which are radiologically contaminated after the Fukushima Daiichi nuclear power plant (FDNPP) accident
These values were higher than those observed in the Chernobyl area by at least one order of magnitude, as previous authors suggested (e.g., Tsukada and Ohse, 2016)
Summary
Closed and semi-closed water bodies, such as lakes and ponds, are the most sensitive environments to radionuclide contamination. The Tohoku Regional Agricultural Administration Office of the Ministry of Agriculture, Forestry and Fishery (MAFF) and the Fukushima Prefectural government (MAFF and Fukushima prefecture, 2015) investigated 2679 ponds in Fukushima Prefecture and reported their water and sediment contamination levels. The long-term behaviour of radiocesium in a closed and semi-enclosed water system affected by the Chernobyl accident can serve as a basis for long-term prediction of changes in environmental radioactive contamination in Fukushima. Putyrskaya et al (2009) presented the results of more than ten years of observation on the behaviour of 137Cs in several lakes affected by the Chernobyl accident and elaborated a model calculation of temporal variations in the concentration of 137Cs activity concentration of the lake water. The meteorological and geographical conditions in these areas are different from those in the Fukushima area, and the applicability of these results requires verification (Konoplev et al, 2016)
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