Radium-based estimates of cesium isotope transport and total direct ocean discharges from the Fukushima Nuclear Power Plant accident

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Biogeosciences Discuss., 9, 16139–16160, 2012
www.biogeosciences-discuss.net/9/16139/2012/
doi:10.5194/bgd-9-16139-2012
© Author(s) 2012. CC Attribution 3.0 License.

This discussion paper is/has been under review for the journal Biogeosciences (BG).
Please refer to the corresponding final paper in BG if available.

Radium-based estimates of cesium isotope transport and total direct ocean discharges from the Fukushima Nuclear Power Plant accident

M. A. Charette1, C. F. Breier1, P. B. Henderson1, S. M. Pike1, I. I. Rypina2,
S. R. Jayne2, and K. O. Buesseler1

1Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution,
Woods Hole, MA 02543, USA

2Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole,
MA 02543, USA

Received: 30 October 2012 – Accepted: 3 November 2012 – Published: 15 November 2012
Correspondence to: M. A. Charette (mcharette@whoi.edu)
Published by Copernicus Publications on behalf of the European Geosciences Union.

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 5 Fukushima Nuclear Power Plant (NPP). During a June 2011 cruise, the highest 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 NPP 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
10 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 = 27days). From these independent Tr values and the inventory
15 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 0.7m2 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
20 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
25 ocean directly from the Fukushima NPP.
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