Temporal trends of ~(137)Cs activity concentration in pond waters in the vicinity of Fukushima Dai-ichi nuclear power plant

摘要

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 ~(137)Cs activity in water and bottom sediment in four ponds; Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ), and Kashiramori (KM), within 10km zone from the FDNPP during 2015-2016. This study follows up their observation to address longer time trends of ~(137)Cs activity concentration in pond waters and to show speciation of ~(137)Cs in soil and bottom sediment. Mean total ~(137)Cs 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 ~(137)Cs for four ponds. The concentration of dissolved ~(137)Cs was usually low in winter during the entire observation period in all four ponds. A tendency to a decrease in the ~(137)Cs 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 ~(137)Cs, i.e., exchangeable and organic bound ~(137)Cs, in bottom sediment than in the soil.