Impact of fluvial discharge on 137Cs in the ocean following the Fukushima Daiichi Nuclear Power Station accident

After the Fukushima Daiichi Nuclear Power Station (F1NPS) accident, Cs-137 activity concentrations have not yet decreased to pre-accident levels because of direct release from the site and fluvial discharges of Cs-137 deposited on land. It is necessary to consider dispersion processes in the coastal area to understand the impact of multiple river discharges and direct release. To achieve this goal, we carried out oceanic dispersion simulations that considered direct release and fluvial discharges and compared the results with the annual averages of observed data. We assumed that particulate Cs-137 discharged from rivers to the ocean quickly resuspended and re-leached after coagulation and precipitation, and that all of the Cs-137 was dispersed. The reproducibility of results was improved by considering fluvial discharges of particulate Cs-137 at all sites between 2013 and 2016, except near the F1NPS. In other words, particulate Cs-137 discharged from rivers was found to influence the results of ocean surface activity concentrations within a relatively short period of time. The impact of direct release was dominant for the observed Cs-137 activity concentrations adjacent to the F1NPS, which was used to estimate direct releases.