Transport of radioactive materials from terrestrial to marine environments in Fukushima over the past decade

While 20 % of radionuclides released from the Fukushima Daiichi Nuclear Power Plant accident had been deposited in the terrestrial environment, rivers remain the long-term source for Cs-137, primarily through particulate transfer, from terrestrial to marine ecosystems. In this study, we estimated the suspended Cs-137 flux to the ocean at 11 sites in the coastal area between October 2012 and December 2020 to be 17 TBq, based on our long-term monitoring data of concentrations of suspended Cs-137 and suspended solids. The cumulative loss of suspended Cs-137 from each site to the ocean ranged from 0.1 % to 1.7 % of initial deposition throughout the observation period, depending on the effect of dam lakes and normalized river discharge. The higher loss is also thought to be the large outflow of runoff during the typhoon. The current level of the suspended Cs-137 concentrations was lowered to 1/10-1/100 of those immediately after the accident. The average value of the decreasing trend for each site was approximated using the equation: y = a1 e -k (1) t + a2 e -k (2)t. The rate constant k (1) is higher in areas with intensive land use, such as pastures, bare land, and water surfaces, and k (2) is lower in urban areas.