This post is part of an ongoing effort to communicate the risks to people living on the west coast of North America resulting from the ongoing release of radionuclides from the Fukushima-Daiichi nuclear power plant after the Tohoku earthquake in March 2011.  The purpose of this post is to explain how the concentration of radionuclides in seawater impacts the amount of radioactive elements taken up by the marine biota.

The goal is to answer questions like:

How high can we expect radioactive element concentrations to get in marine organisms?

What might be the exposure of marine organisms and human consumers of these organisms to Fukushima sourced radionuclides?

What is a Concentration Factor (CF)?

Scientists normally report the amount of a radioactive element in an organism in units of concentration where the mass or activity of the radionuclide is given relative to the weight of the organism or its tissue.  The units of these measurements are, therefore, either kilogram (kg) or activity in Becquerel (Bq = disintegrations per second) divided by the mass of the organism or tissue (kg/kg or Bq/kg).  We want to understand how much radionuclide ends up in the organism relative to the isotopes concentration in seawater which can be reported in either kg per liter of seawater or Bq per liter of seawater (kg/L or Bq/L).  By determining the ratio of the concentration of a radionuclide in an organism to the concentration of the isotope in seawater we define the Concentration Factor (CF) which has units of L/kg:

So if the CF for an element in a given organism is a very high number then that radioisotope tends to bioaccumulate and is found at higher concentrations in the organism than in the surrounding marine environment.  Conversely, if the CF is low there is little risk of bioaccumulation in the organism.  Higher concentrations of an isotope in the environment will lead to higher concentrations in marine organisms.  The CFs derived for organisms rely on high quality measurements carried out in the natural environment as well as under controlled conditions in the laboratory. Details of this approach for modeling the uptake of radionuclides by the marine biota can be found in a technical report made freely available by the International Atomic Energy Agency (IAEA) link here

For the purpose of investigating the relative behavior of radionuclides with respect to their uptake by marine organisms lets consider two elements released after the Fukushima disaster to the ocean (cesium, Cs as Cs-134 and Cs-137 and strontium, Sr as Sr-90) and a naturally occurring isotope widely distributed in the ocean owing to the decay of uranium-238 (U-238), polonium-210 (Po-210).  We will consider the CFs for these elements in fish, molluscs (shellfish) and macroalgae (seaweed), but the CFs for other plants and animals are available in the linked report.  This is an empirical approach to the problem and relies on the collection of data for many organisms exposed to a range of isotope concentrations.

For fish, molluscs and macroalgae the CFs are highest for Po and decrease to a minimum for Sr as follows:

Fish   Po = 2000 > Cs = 100 > Sr = 3
Molluscs  Po = 20,000 > Cs = 60 > Sr = 10
Macroalgae  Po = 1000 > Cs = 50 > Sr = 10

So for these very different organisms we expect that Po-210 will bioaccumulate to levels roughly 100 to 2000 times greater than Sr-90 for a given concentration in seawater.

What are measured concentrations of natural Po-210 and Fukushima derived Cs-134 and Cs-137 in mussels and macroalgae collected along the Japanese coast?

In a study published in June 2013 in the open-access, peer reviewed journal Biogeosciences, Baumann et al. determined these isotopes in mussels and macroalgae collected in June 2011 after the peak release rates of isotopes from the damaged reactors. All of the mussels and algae were found to contain Cs-134 (51-393 Bq/kg) and Cs-137 (60-436 Bq/kg) post Fukushima, levels that were 100 to 1000 higher than background values before the disaster.  Using CFs from the linked IAEA report and assuming that the organisms were in equilibrium with ambient seawater (valid given the short biological half-life for Cs in most algae and mussels of 5-8 days) these concentrations in the organisms correspond to estimated seawater concentrations that were similar to or up to 4-fold lower (0.3 to 2 Bq/L) than measurements of Cs isotopes in seawater 30 km off the Fukushima coast during June 2011.

The concentration of naturally occurring Po-210 was found to be between 10.3 and 300 Bq/kg in the same organisms.  Even though its radioactivity is lower than the Cs isotopes in these samples collected along the Japanese shore doses from the naturally occurring Po-210 to the organisms or seafood consumers are more significant than the Fukushima Cs isotopes.  This is because Po-210 is an alpha particle emitter on decay which is 20-fold more damaging to cells than other types of emissions.

What can we expect on the west coast of North America?

Beginning in the new year we can expect seawater affected by the Fukushima disaster to arrive at our coast in the Pacific northwest. Peak concentrations in the heart of the plume of affected seawater are expected to be on the order of 0.001 to 0.020 Bq/L based on measurements and physical models of ocean circulation.  The much lower radionuclide concentrations are the result of mixing and the decay of shorter lived isotopes.  Given known CFs for marine organisms these seawater concentrations will result in much lower concentrations of radionuclides in organisms residing on the west coast compared to their Japanese cousins.  The radioactive dose to these organisms or consumers of these organisms will be dominated by the naturally occurring radionuclide Po-210.

It will be prudent and responsible to actively monitor the concentrations of radionuclides in our waters and in the marine biota to assess the risk to environmental and human health. This would be especially true in the event that conditions at the disaster site deteriorate and isotope release rates increase over time.

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