The reducing rate of radiocesium 137 Cs in the North Pacific surface water after the TEPCO fukushima Dai-ichi nuclear power plant accident

We started monitoring the radiocesium released from Fukushima in the North Pacific surface water from May 2011 after the accident soon to June 2016, using the cruise of the Oshoro-maru of Hokkaido University. We found that the reducing rate from the ocean surface of 137 Cs regardless of the distance from Fukushima was almost constant at 0.0033 ± 0.0005 day −1 . This finding indicated that 96% of 137 Cs in the surface water over the North Pacific were removed by 1000 days after the accident from the surface water to the ocean interior excluding the decrease by radioactive disintegration.