Multi-decadal projections of surface and interior pathways of the Fukushima Cesium-137 radioactive plume

Following the March 2011 Fukushima disaster, large amounts of water contaminated with radionuclides, including Cesium-137, were released into the Pacific Ocean. With a half-life of 30.1 years, Cs-137 has the potential to travel large distances within the ocean. Using an ensemble of regional eddy-resolving simulations, this study investigates the long-term ventilation pathways of the leaked Cs-137 in the North Pacific Ocean. The simulations suggest that the contaminated plume would have been rapidly diluted below 10,000Bq/m³ by the energetic Kuroshio Current and Kurushio Extension by July 2011. Based on our source function of 22 Bq/m³, which sits at the upper range of the published estimates, waters with Cs-137 concentrations >10Bq/m³ are projected to reach the northwestern American coast and the Hawaiian archipelago by early 2014. Driven by quasi-zonal oceanic jets, shelf waters north of 45°N experience Cs-137 levels of 10-30Bq/m³ between 2014 and 2020, while the Californian coast is projected to see lower concentrations (10-20Bq/m³) slightly later (2016-2025). This late but prolonged exposure is related to subsurface pathways of mode waters, where Cs-137 is subducted toward the subtropics before being upwelled from deeper sources along the southern Californian coast. The model suggests that Fukushima-derived Cs-137 will penetrate the interior ocean and spread to other oceanic basins over the next two decades and beyond. The sensitivity of our results to uncertainties in the source function and to inter-annual to multi-decadal variability is discussed.