Reassessment of 90Sr in the Pacific Ocean and the coast off Japan derived from the Fukushima Dai-ichi nuclear accident

On 11 March 2011 the Fukushima Dai-ichi nuclear power plant was badly damaged as a consequence of the Tohoku earthquake and subsequent tsunami, causing a large release of artificial radionuclides to the marine environment. Thereinafter, water was used to cool down the reactors, some of which directly entered the ocean via groundwater or leaks from the buildings. Large volumes of contaminated cooling water are generated each day, due to the influx of more than 300 tons of groundwater into the power plants. These contaminated waters are stored on site in large storage tanks, after having been treated using zeolite sorption for the removal of 137Cs and 134Cs, and therefore are enriched in 90Sr compared to 137Cs and 134Cs. Leakages from tanks led to 90Sr concentrations as high as 1000 Bq.L-1 in waters sampled near the leaking tanks. Contaminated water from the trenches and tanks may reach the ocean, leading to further releases of 90Sr to the Pacific Ocean, and adding to the initial ca. 102-103 TBq estimated until June 2011 (Casacuberta et al., 2013). Here we present an overview of the fate and distribution of Fukushima derived 90Sr in seawater in the Pacific Ocean from June 2011 to September 2013, resulting from samples collected during several research cruises covering transects between North America and Japan, as well as vertical profiles and groundwater near the coast of Fukushima (< 1 km from the power plants). We emphasize our study on the coastal region off Japan, where uptake in to the food web may pose a potential human health hazard due to the accumulation of 90Sr in seafood. Moreover, changes in the 90Sr/137Cs ratio will be also investigated, as an indicator that might be a useful tool for tracking Fukushima contaminated waters in the Pacific Ocean. Document available in abstract form only. (authors)