The purpose of this post is to compare the concentrations of Sr-90 and Cs-137 in the North Pacific Ocean over the last 50 years to the concentrations predicted to arrive on the west coast associated with waters affected by release of radionuclides from the Fukushima-Daiichi Nuclear Power Plant. Given present levels that are being measured in the eastern Pacific and barring release rates that significantly exceed past rates in March-April 2011, when release rates were 10,000-100,000 times greater than ongoing releases at the plant, the impact on marine organisms and the marine environment is likely to be less significant than impacts owing to radioactivity in the 20th century. What follows is a comparison of the concentrations measured and predicted over much of the Pacific owing to Fukushima to the concentrations that were present in the mid-1960s from the fallout of atmospheric weapons testing that is free from any discussion of safe doses or models of radiation exposure to organisms.
Let us consider Cs-137 and Sr-90 both because they are potentially dangerous to marine organisms through bioaccumulation, they have similar half-lives and persistence in the environment, and because their history in the North Pacific and release from Fukushima are relatively well understood (Povinec and others (2013) Biogeosciences, Casacuberta and others (2013), Povinec and others (2012) ES&T). Of course there is a whole suite of radionuclides that were released by weapons tests and from Fukushima but we can use Cs and Sr to trace the distribution and impact I think.
A total of about 950 PBq (PBq = 10^15 Bq) Cs-137 and 600 PBq Sr-90 were released through weapons testing with about 600 PBq Cs-137 and 380 PBq Sr-90 deposited to the oceans. This resulted in maximum concentrations of Cs-137 of 80 mBq/L and similar concentrations of Sr-90. These concentrations decreased up until the Fukushima disaster (with a perceptible bump from Chernobyl in 1986) through decay, mixing and sinking of isotopes associated with particulate matter. Cs-137 had an effective half-life in the surface of 13 yr and Sr-90 had a half-life of 14 yr.
Fukushima inputs are much smaller in magnitude and despite ongoing release unlikely to exceed weapons fallout. Initial Cs-137 release to atmosphere and ocean from Fukushima is ~80 PBq and the relative Sr-90 associated with the initial pulse is much, much less than weapons fallout being only about 3% of Cs release. Models with realistic ocean mixing parameterizations of the likely concentrations (1-30 Bq/m^3)(Behrens et al. 2012, Rossi et al. 2013 DSR) and measurements of Cs-137 in the leading edge of the plume in the North Pacific (Smith et al. 2013 PICES) do not suggest that Cs-137 will exceed 1960’s maximums from bomb testing. So Sr-90 will certainly not approach 1960s levels. The concentration of the radioisotopes Sr-90 and Cs-137 determine the degree to which they are bioaccumulated and control the tissue concentrations, exposure and health impacts in marine organisms. Based on current data any environmental effects and negative impacts expected from Fukushima should be less than any impacts on organisms in the 1960’s and 1970’s.
The power of this argument is that non-anthropogenic background radiation was the same in the mid-20th century as it is today with roughly similar fission product radiation concentrations in the Pacific. Because concentration determines biological uptake and therefore body load and exposure the impact on organisms should be similar then and now. The weakness of this approach is that there are other pressures (ocean acidification, warming, oxygen depletion) on the marine environment that one could qualitatively say might make the ecosystem more vulnerable to these very small increases in radiation.