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 test 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.
Temporal variations of 137Cs concentrations in surface water of the western North Pacific: (A) subtropical gyre (25–36° N); (B) mixed region (36–45° N); black circles, open ocean waters; empty squares, coastal water off Tokai; blue squares, coastal water off the Fukushima Dai-ichi NPP.
Temporal variations of 90Sr concentrations in surface water of the western North Pacific: (A) subtropical gyre (25–36° N); (B) mixed region (36–45° N); black circles, open ocean waters; empty squares, coastal water off Tokai; blue squares, coastal water off the Fukushima Dai-ichi NPP.
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. 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. But again the increases in exposure are orders of magnitude lower than organisms are presently exposed to by the uptake of naturally occurring Po-210.
Comments are closed on this story.