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The purpose of this diary is to provide new information reported by a Canadian led effort to determine the timing of the arrival and the concentrations of Fukushima sourced radionuclides in ocean waters off North America.  Since 2011 measurements of Cesium(Cs)-137 (half-life ~30.1 yr) and Cs-134 (half-life ~2 yr) have been made in the north Pacific and Arctic Oceans.  Most recent measurements show that Fukushima derived Cs has reached the west coast as of June 2013 by ocean transport but that concentrations of Cs continue to be well below levels thought to pose environmental or public health threats.

There have been a number of popular press articles that have summarized the results of this program as reported in a presentation at the PICES – North Pacific Marine Sciences Consortium meeting held in Nanaimo, BC Canada in October 2013.  Most of these report the timing of the arrival of the radionuclides but offer no perspective on the actual levels and the associated risk to residents of the west coast (e.g. link).  The presentation is available online here and represents the work of Department of Fisheries and Oceans scientists working in the North Pacific and the Arctic Oceans. The raw data summarized here can be found in the presentation.

It is a useful exercise to first familiarize ourselves with some of the units that scientists use when discussing radioactivity. A commonly seen unit is the Becquerel (abbreviated as Bq), which represents an amount of radioactive material where one atom decays per second and has units of inverse time (per second). Another unit in common use is disintegrations per minute (dpm) where the number of atoms undergoing radioactive decay in one minute are counted (so 1 Bq = 60 dpm). When we talk about the radioactivity measured in seawater the measurements are reported normalized per litre of seawater (Bq/L).

In previous diaries I have presented a primer on the major contributors to radionuclide concentrations in average seawater and discussed other aspects of radionuclide behavior in the ocean.  About 93% of radioactivity in seawater results from the presence of primordial, naturally occurring potassium-40 (K-40) and rubidium-87 (Rb-87). The remaining 7% are radioactive elements deposited to the ocean from past atmospheric nuclear testing. The sum of these activities is about 14 Bq/L on average though there are regional differences that scale with ocean salinity.

Ongoing time series measurements are being carried out by the Department of Fisheries and Oceans to determine when and how much radioactivity from the Fukushima disaster will arrive along the west coast of North America.

Results of the Line P Time Series Program thus far can be summarized as follows:
1) Cs levels in June 2011 were consistent with pre-Fukushima levels present from atmospheric nuclear test fallout
2) In June 2012 surface waters ~1500 km offshore had detectable Cs-134 from Fukushima and associated Cs-137 of 0.0003 Bq/L or roughly 0.002% of naturally occurring background radiation
3) Fukushima derived Cs was detected all the way to the coast in June 2013 with the highest levels of Cs-137 farthest offshore (0.0009 Bq/L or roughly 0.006% of background radiation) and lower levels of 0.0003 Bq/L toward the coast.
4) The timing of the arrival of the plume agrees with the modeling study of Rossi et al. (2013) published in the peer-reviewed journal Deep-Sea Research (link) but the concentrations are lower than predicted

Ongoing monitoring will constrain the likely environmental and health risks posed by ocean transport of Fukushima derived radionuclides.  

Originally posted to MarineChemist on Sat Jan 04, 2014 at 02:14 PM PST.

Also republished by Koscadia and SciTech.

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