[MAG41-P10] Estimating impacts of direct release and riverine discharge on oceanic 137Cs derived from the Fukushima Dai-ichi Nuclear Power Plant accident by an regional ocean model
Keywords:Fukushima Dai-ichi Nuclear Power Plant accident, Cs-137, Regional Ocean Model, Direct release, River input
Simulated 137Cs activity attributable to direct release were in good agreement with measured data in the coast zone adjacent to the 1F NPP, because the effect of direct release was dominant from 2013 to 2016. On the other hands, simulated results attributable to inflow from boundary sections were slightly underestimated to the measured data offshore area. This suggests that recirculation of subducted 137Cs to the surface layer was underestimated in the North Pacific model. Apparent half-life of direct released and river discharged 137Cs activity were estimated to be about 1 year and 2 years, respectively. And apparent half-life of inflow of 137Cs activity was much longer due to time scale of dilution process in the North Pacific. Apparent half-life of each source should be similar to the measured one attributable to each source. Apparent half-life of measured 137Cs activity adjacent to the 1F NPP was about 1 year, on the other hand, the ones increased with increasing distance from the 1F NPP. Apparent half-life of measured data was about 2 years in front of the Uda river mouth where is far from the 1F NPP. Although 137Cs activity in this area was mainly affected by the river input, simulated 137Cs activities with river input were one fifth of observations. There is a brackish lagoon, Matsukawa-ura in front of Uda river mouth. The observed 137Cs activities in the Matsukawa-ura were 3-5 times larger than the one in the Uda river. This suggests the removal process from particle 137Cs to dissolved form in the brackish lagoon may be important.