Fukushima-derived radionuclides in the ocean and biota off Japan

Abstract

The Tōhoku earthquake and tsunami of March 11, 2011, resulted in unprecedented radioactivity releases from the Fukushima Dai-ichi nuclear power plants to the Northwest Pacific Ocean. Results are presented here from an international study of radionuclide contaminants in surface and subsurface waters, as well as in zooplankton and fish, off Japan in June 2011. A major finding is detection of Fukushima-derived (134)Cs and (137)Cs throughout waters 30-600 km offshore, with the highest activities associated with near-shore eddies and the Kuroshio Current acting as a southern boundary for transport. Fukushima-derived Cs isotopes were also detected in zooplankton and mesopelagic fish, and unique to this study we also find (110 m)Ag in zooplankton. Vertical profiles are used to calculate a total inventory of ~2 PBq (137)Cs in an ocean area of 150,000 km(2). Our results can only be understood in the context of our drifter data and an oceanographic model that shows rapid advection of contaminants further out in the Pacific. Importantly, our data are consistent with higher estimates of the magnitude of Fukushima fallout and direct releases [Stohl et al. (2011) Atmos Chem Phys Discuss 11:28319-28394; Bailly du Bois et al. (2011) J Environ Radioact, 10.1016/j.jenvrad.2011.11.015]. We address risks to public health and marine biota by showing that though Cs isotopes are elevated 10-1,000× over prior levels in waters off Japan, radiation risks due to these radionuclides are below those generally considered harmful to marine animals and human consumers, and even below those from naturally occurring radionuclides.

Fig. 1.

(A) Concentrations of ¹³⁴Cs in surface water (Bq⋅m⁻³), (B) ¹³⁴Cs in biological samples (Bq⋅kg⁻¹ dry weight), and (C) ^110mAg in biological samples (Bq⋅kg⁻¹ dry weight). Biological samples were separated into mixed zooplankton, crustaceans, and fish as indicated by legend (SI Discussion). Red star is location of Fukushima NPPs. Gray shaded area in A shows approximate position of Kuroshio Current during the cruise (Fig. S1). Dashed lines in A are areas used to calculate near-shore and offshore Cs inventories.

Fig. 2.

(A) Tracks of surface drifters released at the sampling stations, from time of release through October 26, 2011. Trajectories are color coded: blue drifters traveled north of the Kuroshio, red were advected east in the Kuroshio, green went south of the Kuroshio, and magenta stayed close to the coast. (B) Expanded view of coastal region showing more clearly the near-shore eddies and coastal transport.

Fig. 3.

Profiles of ¹³⁴Cs (Bq⋅m⁻³) vs. depth for offshore (gray solid lines; mean, gray circles) and near-shore stations (black dashed lines; mean, black circles).