Article pubs.acs.org/est
Tracking the Fate of Particle Associated Fukushima Daiichi Cesium in the Ocean off Japan Ken O. Buesseler,*,† Christopher R. German,† Makio C. Honda,‡ Shigeyoshi Otosaka,§ Erin E. Black,† Hajime Kawakami,‡ Steven J. Manganini,† and Steven M. Pike† †
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 2543, United States Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa 237-0061, Japan § Japan Atomic Energy Agency, Tokai-mura Ibaraki 319-1112, Japan ‡
S Supporting Information *
ABSTRACT: A three year time-series of particle fluxes is presented from sediment traps deployed at 500 and 1000 m at a site 115 km southeast of Fukushima Daiichi Nuclear Power Plant (FDNPP). Results show a high fraction of lithogenic material and mass flux peaks that do not align between the trap depths, suggesting a lateral source of sediments. Fukushima cesium-137 and cesium-134 were enhanced in flux peaks that, given variations in trap 137Cs/210Pbex ratios, are characteristic of material derived from shelf and slope sediments found from 500 m. These lateral flux peaks are possibly triggered by passing typhoons. The Cs fluxes are an order of magnitude higher than were previously reported for the trap located 100 km due east of FDNPP. We attribute this large difference to the position of our trap under the southeasterly currents that carry contaminated waters and resuspended sediments away from FDNPP and into the Pacific. These higher Cs sedimentary fluxes offshore are still small relative to the inventory of Cs currently buried nearshore. Consequently, we do not expect them to effect any rapid decrease in Cs levels for the coastal sediments near FDNPP that have been linked to enhanced Cs in demersal fish.
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INTRODUCTION The triple disaster that began on March 11, 2011 in the Tohoku region of Japan started with an offshore magnitude 9.0 earthquake that triggered a devastating tsunami, leading to power losses, overheating, and subsequent release of unprecedented amounts of radioactive materials to the oceans from the Fukushima Daiichi Nuclear Power Plant (FDNPP). It has been estimated that >80% of the total radioactivity released entered the ocean,1,2 either as atmospheric fallout or the direct discharge of contaminated waters that continue to a lesser extent to this day.3,4 While there were hundreds of different radionuclides released, after the initial decay of those contaminants with half-lives less than days to weeks, much of the attention has remained focused on cesium-137 and -134, two isotopes with half-lives of 30.17 and 2.06 years, respectively, because they were released in larger activities relative to other radionuclides.5,6 The total activity of Cs released is still uncertain, ranging from 4 to 90 PBq, with most estimates of the combined releases in the 15 to 30 PBq range for each Cs isotope.7−10 As predicted by its geochemical properties, cesium is highly soluble in seawater, so its fate is determined largely by transport with ocean currents and mixing and dilution offshore.9,11−16 © 2015 American Chemical Society
Consistent with this soluble nature, 0.5 Bq g−1; cyan 0.3−0.5 Bq g−1; dark blue 0.1−0.3 Bq g−1; and gray 0.35 down to 0.02 but with the majority at values in excess of what the model would predict for sediments deeper than 500 m. This suggests that at times of high lithogenic input flux, the source component for detrital material entering the F1 trap at 1000 m is also influenced significantly by input of high Cs material from the shelf. Comparison to Other Sediment Trap Sites and Seafloor off Japan. Otosaka et al.26 report on a similar trap time-series at a site FS1 that is 115 km northeast of F1 at a similar depth and from an overlapping time period (Figure 1; 875 m trap; 992 m water depth; Aug. 2011−June 2013). Cs fluxes at that site were, on average, more than an order of magnitude smaller than the F1 trap at 1000 m (Figure 6, SI Table S3). They found evidence for a vertical supply of rapidly sinking Cs high in organic matter and biogenic minerals each spring. As discussed above, they can model this “moderate” flux period with vertical scavenging from a declining concentration of Cs in the waters that controls levels of Cs in biota and associated sinking biogenic materials. They also have a “turbulent” flux period in the winter that they attribute to a resuspended source of sediment, but it was also low in Cs relative to F1. If plotted on a ternary plot as done for F1 (Figure 5), then it is clear that the lithogenic component is much smaller at FS1 vs F1 as are the total Cs levels overall (SI Figure S3). We attribute the sharp difference between F1 and FS1 in terms of fluxes of Cs, but not of mass or 210Pb, as being due to differences in the offshore transport pathways for Fukushimaderived Cs and the increased influence of lateral processes at 9813
DOI: 10.1021/acs.est.5b02635 Environ. Sci. Technol. 2015, 49, 9807−9816
Article
Environmental Science & Technology represents 1% of the total 137Cs sedimentary inventory of 100 × 1012 Bq found along the
