Abstract
Abstract. Radium has four naturally occurring isotopes that have proven useful in constraining water mass source, age, and mixing rates in the coastal and open ocean. In this study, we used radium isotopes to determine the fate and flux of runoff-derived cesium from the Fukushima Dai-ichi Nuclear Power Plant (FNPP). During a June 2011 cruise, the highest cesium (Cs) concentrations were found along the eastern shelf of northern Japan, from Fukushima south, to the edge of the Kuroshio Current, and in an eddy ~ 130 km from the FNPP site. Locations with the highest cesium also had some of the highest radium activities, suggesting much of the direct ocean discharges of Cs remained in the coastal zone 2–3 months after the accident. We used a short-lived Ra isotope (223Ra, t1/2 = 11.4 d) to derive an average water mass age (Tr) in the coastal zone of 32 days. To ground-truth the Ra age model, we conducted a direct, station-by-station comparison of water mass ages with a numerical oceanographic model and found them to be in excellent agreement (model avg. Tr = 27 days). From these independent Tr values and the inventory of Cs within the water column at the time of our cruise, we were able to calculate an offshore 134Cs flux of 3.9–4.6 × 1013 Bq d−1. Radium-228 (t1/2 = 5.75 yr) was used to derive a vertical eddy diffusivity (Kz) of 0.7 m2 d−1 (0.1 cm2 s−1); from this Kz and 134Cs inventory, we estimated a 134Cs flux across the pycnocline of 1.8 × 104 Bq d−1 for the same time period. On average, our results show that horizontal mixing loss of Cs from the coastal zone was ~ 109 greater than vertical exchange below the surface mixed layer. Finally, a mixing/dilution model that utilized our Ra-based and oceanographic model water mass ages produced a direct ocean discharge of 134Cs from the FNPP of 11–16 PBq at the time of the peak release in early April 2011. Our results can be used to calculate discharge of other water-soluble radionuclides that were released to the ocean directly from the Fukushima NPP.
Highlights
Hydrology and Earth System On and1re1suMltainrcghts2u0n1a1m,ialnedotfofssheovreSeramcl hiaeygdnnriotcugdeeens9ex.0pleoasritohnqsuaankde reactor meltdowns at the Fukushima Dai-ichi Nuclear PowerPlant (FNPP) in Japan
The one exception was at stations associated with the eddy, which was pulling in contaminated coastal water from the vicinity of the Fukushima Dai-ichi Nuclear PowerPlant (FNPP) and wrapping it cyclonically around the outside of itself
In order for the model to be strictly an estimate of the ocean discharge-derived Cs, we must assume that no atmospherically sourced Cs was present within our study area at the time of the cruise. This generally agrees with the results reported in Rypina et al (2013) and is supported by the excellent correlation between Cs isotopes and 90Sr, with the latter being a non-volatile fission product and could only have been sourced from direct discharge and not atmospheric fallout (Casacuberta et al, 2013)
Summary
Hydrology and Earth System On and1re1suMltainrcghts2u0n1a1m,ialnedotfofssheovreSeramcl hiaeygdnnriotcugdeeens9ex.0pleoasritohnqsuaankde reactor meltdowns at the Fukushima Dai-ichi Nuclear PowerPlant (FNPP) in Japan. The accident resulted in a large-scale release of radioactivity to the environment through atmospheric fallout over lanOd canedasena, Sascwieelnl acsedirect discharge to the ocean from water used to cool the reactors (Buesseler et al, 2011; Chino et al, 2011). The largest ocean-related impact was from the release of two cesium isotopes (137Cs, t1/2 = 30.2 yr; 134Cs, t1/2 = 2.06 yr), highly soluble relatively volatile fission products in seawater Concentrations in the ocean peaked on 6 April (activities of uespt,toan6d0a0s00of00th0isBwq mrTit−ihn3ge), cwCohnretinynuodeisretpocthmdeaiskrceehathrgeeFwukausshhiigmhaevent the largest accidental release of 137Cs to the ocean (Buesseler et al, 2012). In June of 2011, an international group of scientists set out to measure the oceanic distribution of radionuclides released from the FNPP along the coast and offshore of
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