Impacts of direct release and river discharge on oceanic Cs derived from the Fukushima Dai-ichi Nuclear Power Plant accident.

A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) following the Great East Japan Earthquake and tsunami of 11 March 2011 resulted in the release of radioactive materials to the ocean. We used the Regional Ocean Model System (ROMS) to simulate the Cs activity in the oceanic area off Fukushima, with the sources of radioactivity being direct release, atmospheric deposition, river discharge, and inflow across the domain boundary. The direct release rate of Cs after the accident until the end of 2016 was estimated by comparing simulated results with measured Cs activities adjacent to the 1F NPP. River discharge rates of Cs were estimated by multiplying simulated river flow rates by the dissolved Cs activities, which were estimated by an empirical function. Inflow of Cs across the domain boundary was set according to the results of a North Pacific Ocean model. Because the spatiotemporal variability of Cs activity was large, the simulated results were compared with the annual averaged observed Cs activity distribution. Normalized annual averaged Cs activity distributions in the regional ocean were similar for each year from 2013 to 2016. This result suggests that the annual averaged distribution is predictable. Simulated Cs activity attributable to direct release was in good agreement with measurement data from the coastal zone adjacent to the 1F NPP. Comparison of the simulated results with measured activity in the offshore area indicated that the simulation slightly underestimated the activity attributable to inflow across the domain boundary. This result suggests that recirculation of subducted Cs to the surface layer was underestimated by the North Pacific model. During the study period, the effect of river discharge on oceanic Cs activity was small compared to the effect of directly released Cs.