Abstract
Abstract. Quantifying the amount of cesium incorporated into marine sediments as a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident has proven challenging due to the limited multi-core sampling from within the 30 km zone around the facility; the inherent spatial heterogeneities in ocean sediments; and the potential for inventory fluctuations due to physical, biological, and chemical processes. Using 210Pb, 234Th, 137Cs, and 134Cs profiles from 20 sediment cores, coastal sediment inventories were reevaluated. A 137Cs sediment inventory of 100 ± 50 TBq was found for an area of 55 000 km2 using cores from this study and a total of 130 ± 60 TBq using an additional 181 samples. These inventories represent less than 1% of the estimated 15–30 PBq of cesium released during the FDNPP disaster. The time needed for surface sediment activities (0 to 3 cm) at the 20 locations to be reduced by 50% via sediment mixing was estimated to range from 0.4 to 26 yr. Due to the observed variability in mixing rates, grain size, and inventories, additional cores are needed to improve these estimates and capture the full extent of cesium penetration into the shallow coastal sediments, which was deeper than 14 cm for all cores retrieved from water depths less than 150 m.
Highlights
The Tohoku earthquake and tsunami of 11 March 2011 led to multiple system failures at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP)
While later reports indicate additional releases to the ocean (TEPCO, 2014; Kanda, 2013), the initial 137Cs activities measured in the FDNPP discharge channel from March to May of 2011 represent the most significant oceanic contribution to date, peaking at 68 million Bq m−3 on 6 April 2011 (Buesseler et al, 2011). 137Cs emerged as the major oceanic contaminant, warranting long-term study due to its ∼ 30 yr half-life and large release, generally estimated to be around 15–30 PBq (Buesseler, 2014; Charette et al, 2013; Povinec et al, 2013; Chino et al, 2011; Stohl et al, 2011). 137Cs and 134Cs (t1/2 ∼ 2 years) have been found with comparable activities near the FDNPP and were released with a ratio of ∼ 1 (Buesseler et al, 2011)
The isotopes are highly soluble in the coastal ocean (Buesseler et al, 2012), a small percentage of the release will have been incorporated into the marine sediments and may remain associated with the seafloor, even after currents transport much of the dissolved phase away from the coast
Summary
The Tohoku earthquake and tsunami of 11 March 2011 led to multiple system failures at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). Oceanic inputs included direct cooling discharge, runoff, riverine flow, and an estimated 70 to 80 % of the total atmospheric radionuclide release (Aoyama et al, 2012). The isotopes are highly soluble in the coastal ocean (Buesseler et al, 2012), a small percentage of the release will have been incorporated into the marine sediments and may remain associated with the seafloor, even after currents transport much of the dissolved phase away from the coast. Initial studies of the sediments near the FDNPP have shown the widespread incorporation of 137Cs well above the relatively small remaining 1960s fallout signal of 1–2 Bq kg−1 (dry) (NRA, 2014b; Kusakabe et al, 2013; Otosaka and Kato, 2014; Otosaka and Kobayashi, 2013)
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