Fukushima’s impact on the ocean environment revealed

Many environmentalists oppose nuclear power presumably because they think it is bad for the environment. Concerns about waste, radioactive releases and so on are often cited as evidence for this. Having released a very large quantity of artificial radioactive isotopes into the marine environment, Fukushima gives us a very good opportunity to test this concern in a real-world setting. Some fascinating results are now in, courtesy of Ken Buesseler and colleagues, in a paper just published in the highly-respected journal PNAS.

The numbers are impressive, which is not particularly surprising given that perhaps a tenth as much radiation as was released by Chernobyl came from Fukushima (still a very large amount). For example, levels of the two caesium isotopes Cs-134 and Cs-137 in seawater were measured by the researchers at concentrations up to 1000 times higher than before the 11 March tsunami disaster, and were found in an area of the Pacific Ocean covering 150,000 square kilometres off Japan.

This sounds scary, but bear in mind that only 30 km offshore levels drop down by 50 times, to about 600-800 becquerels per cubic metre on average, well below Japanese regulatory limits for the ocean of 90,000 Bq/m3, and also 15 times lower than the radiation released naturally by the most common natural radionuclide in the ocean, potassium-40 (which on average causes activity of 12,000 Bq/m3).

So what are the impacts on marine biota? Once again, the numbers are important – many who worry about the impact of artificial radionuclides forget that natural radionuclides are ever-present in the environment, and that living organisms are well-accustomed through billions of years’ of evolution to dealing with their impact on DNA. Accordingly, radiation doses to marine organisms further out than 30km from Fukushima are still dominated by the naturally-occurring radionuclides polonium-210 and potassium-40. The authors point out that just to reach the levels of radiation emitted by the natural polonium, caesium-137 levels in fish would have to be 1-3 orders of magnitude (10-1000x) higher than they observed in the waters off Japan.

What if you were to swim in this slightly-more-radioactive sea? The authors calculate an external dose of less than 0.01 microsieverts per day (equalling 0.003 mSv/yr) for anyone fully immersed in the ocean there, which they point out is less than 0.3% of the average daily Japanese dose from all radiation sources which adds up to about 1.4 mSv/yr. In terms of the marine biodiversity impacts, the authors conclude that

these levels are several orders of magnitude lower than those used in one study that assumed exposure to the most heavily impacted water discharged from the Fukushima NPPs to predict marked reproductive effects and possible mortality in marine biota 

The “one study” in question is referenced as ‘Fukushima wildlife dose reconstruction signals ecological consequences’, published last year in the journal Environmental Science and Technology (early PDF here). That study bore the much more worrying conclusion that

marked reproductive effects, and even mortality for the most radiosensitive taxa are predicted for all marine wildlife groups whose life history characteristics confine them to the near-field, contaminant release area

a statement based on early assessments of doses which appear now to have been massive over-estimates – albeit with the caveat that doses can be assumed to be much higher closer than 30km in to shore, an area apparently not covered in this PNAS study.

The upshot is that Fukushima-derived radiation doses experienced by marine organisms throughout the food chain are well below levels which could cause harm to ocean biodiversity, and even in the most part below levels experienced due to naturally-occurring radionuclides.

Unfortunately, perhaps, given that fish caught offshore from Fukushima are entirely safe for human consumption (measured levels in fish are 150-fold less than the legal limit on average), there is little prospect of a no-fishing zone which might provide great marine wildlife benefit as the Chernobyl exclusion zone currently does for land-based rare species.

Context is also key – I end this piece with a shot of the likely extent of tsunami debris distributed throughout the Pacific Ocean after having been washed out of coastal areas of Japan:

Given that much of this will be non-biodegradeable plastics, the effects can be expected to be vastly more significant to marine life than the relatively trivial effects of radionuclides released from Fukushima. This is very similar to the obvious but often neglected fact that the human effects of the tsunami – 19,000 deaths – vastly overwhelm any human health effects from Fukushima-originated radiation (no deaths), something never clear from most of the overblown media coverage on 11 March 2012 one-year anniversary, which tended to focus on the nuclear accident to the exclusion of all else.

Opponents of nuclear power often make out that the consequences of a serious accident are some variant of Armageddon (sterilising large areas for generations, etc). Fukushima shows that this is absolutely not the case. The environmental effects of an accident are transient, and virtually non-existent compared to other human impacts on biodiversity.

© Mark Lynas
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