Hot tuna

As an undergraduate student at Ohio State (I studied Geology) I was intrigued to find that hydrogeologists were indirect fans of the era of nuclear weapons testing. By “indirect” I mean that they didn’t really approve of the testing itself, but they loved the fact that the radioactivity put into the environment could be quite useful to them in tracking groundwater flows. Consider, for example, a well sunk into an aquifer deep underground – perhaps in a location in which the nearest groundwater “recharge” location (where rainwater or water from rivers or lakes first enters the ground to become groundwater). Now think of a hydrogeologist sampling the well who notices that concentrations of tritium (radioactive hydrogen) start to increase, level off after a few years, and then drop again, and trying to figure out what it all means and – more to the point – how to make use of this new information.

One thing that our hydrogeologist will find out is that tritium, although produced naturally in the atmosphere, is also produced in copious quantities by nuclear weapons testing. So it’s fairly easy to assume that the excess tritium showing up in the aquifer was from the era of atmospheric nuclear weapons testing – an era that peaked in the late 1950s and had largely ended in the early 1960s. So our hydrogeologist can make the assumption that tritium in the groundwater had to have entered the water in the 1950s or 1960s – the fact that it just showed up in his well means that it took a half-century or so to reach him. And with that, he can calculate how quickly the groundwater flows through the aquifer and he can start to make some good guesses as to the properties of the aquifer and whether or not it is being over-pumped.

Another of my professors used radioactive carbon-14 in a similar manner, except that he was using it to date layers of ice in Antarctic glaciers. Again, looking for the peak levels of C-14 helped to nail down the age of the ice in one location, and that could be used to date the layers around it. In both of these cases, the radionuclides could be used as tracers – to help trace the path of the radioactivity and of the medium it was in – and as “time markers” in the natural world. And in both cases the amount of radioactivity present was far too low to have any health impact at all.

A paper that was published yesterday in the Proceedings of the National Academy of Sciences is the first to suggest that radioactive cesium (the nuclides Cs-134 and Cs-137) can be used the same way, although this isn’t the angle that is being played up in the media. Here’s a little about it.

The backstory (as it were) is that bluefin tuna travel the Pacific – as do a number of other species – and it’s reasonable to wonder if they might be exposed to radionuclides in one location and to transport them to another. That’s what the authors of this study, Daniel Madigan, Zofia Baumann, and Nicholas Fisher set out to study. So Madigan took samples of some tuna caught off the coast of Southern California and sent the samples to Fisher and Baumann, who found traces of radioactive cesium in the flesh.

Before going further it’s important to note that all radioactive cesium on Earth is artificial. But that doesn’t mean that all of the cesium found is from Fukushima – there is cesium in the soil from the atmospheric nuclear weapons testing as well as from the Chernobyl accident. In fact, it is fairly easy to find Cs-137 just about anywhere in the northern hemisphere – dig up the soil in pretty much any location and you’re bound to find Cs-137 within a foot or so of the surface, and many trees whose roots penetrate these soil layers will have traces of Cs-137 in their wood. So it’s reasonable to wonder if the radioactivity found in the tuna came from Fukushima or from the background sources.

There are two primary nuclides of radioactive cesium – Cs-134 and Cs-137. The latter is long-lived with a half-life of about 30 years; we still have over a quarter of the Cs-137 from nuclear weapons testing and over half of what Chernobyl put into the environment. Cs-134, on the other hand, has a half-life of only about 2 years. The Chernobyl accident took place 25 years ago and we’re down to less than one tenth of a percent of Chernobyl-produced Cs-134, and far less from weapons testing. The bottom line is that the presence of Cs-134 would indicate unequivocally that the cesium in the tuna came from Fukushima.

And in fact, that’s exactly what the authors found – the presence of both Cs-134 and Cs-137, indicating that the tuna picked up radioactive cesium off the shores of Japan and carried it across the Pacific to the US tuna-fishing grounds. So the next question, I guess, is whether or not we can still eat sushi, or if the tuna is too “hot” for human consumption. After all, we are really good at detecting trivially low levels of radioactivity – we can detect radioactivity at levels far too low to be a problem. And, incidentally, I should also point out that there are other, more pressing concerns about tuna – chief among them mercury, to the point that the Food and Drug Administration has advised limiting uptake of some species to a few servings weekly. By comparison, the cesium is a trivial concern.

In this case, we would normally expect to find about a Becquerel (Bq) or so of Cs-137 in every kilogram of tuna caught in the ocean – that’s the background level. What Fisher, Madigan, and Baumann found was a little over 6 Bq per kg of Cs-137 and about 4 Bq/kg of Cs-134 – both being clearly higher than expected. On the other hand, they also found 30 times as much natural potassium-40 in the fish – up to 367 Bq/kg – from the potassium that every living organism relies on as part of its fundamental biochemistry. In other words, we can detect the Cs, but at such small concentrations that it poses no health risk at all. So would I eat sushi from the tuna that Madigan tested? Gladly (and voraciously). And if you have any that you don’t want, let me know and I’ll take if off your hands!

OK – so the lab work showed some radioactivity from Fukushima wound up in their fish, but they did NOT find a health hazard – what they found was a useful tracer. Just as tritium can be used to trace (and to time) the flow of groundwater and carbon-14 can be used for the same purpose with glaciers, Cs-137 and Cs-134 can be used to trace the migration of animals throughout the Pacific, and possibly further. And this could end up being a very positive side effect of the Fukushima accident – a better understanding of the Pacific ecology – call it the scientific and ecological fallout.

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8 Responses to “Hot tuna”

  1. Jennifer McGuire May 31, 2012 at 5:01 PM #

    Hello – Thanks for this in-depth explanation of the latest hot topic, radioactive cesium in bluefin tuna. You touched on another tuna topic many people are also confused about, mercury. As an RD with the National Fisheries Institute, I’d like to clear the waters about this. Here’s the official seafood advice from the Food and Drug Administration and the 2010 Dietary Guidelines for Americans:

    -For the general population: Eat a variety of fish twice a week, and there are no fish to limit or avoid because of mercury.

    -For women who are or may become pregnant, nursing moms, and young kids: The nutrients in fish are especially important for you, so eat a variety of fish no less than twice a week. All types of tuna are safe, and up to half (6 ounces) of the fish you eat every week can be white albacore tuna. There are four fish to avoid that you probably aren’t eating anyway: shark, tilefish, king mackerel, and swordfish.

    The more we learn about the good things eating fish does for your body, the more doctors and dietitians are focusing on the health risks of not eating enough fish. The average American eats less than half the seafood recommendation, and low-seafood consumption is currently the second-biggest dietary contributor to preventable deaths in the U.S.

    For a look at what plenty of seafood looks like in the real-life diet of a registered dietitian (me!) visit my BlogAboutSeafood.

    Jennifer McGuire, MS, RD

    • Dr. Y May 31, 2012 at 8:29 PM #

      These are some good points – and I guess it means that I’m guilty of the same thing I mentioned here, except that in my case I’m less familiar with mercury so that worries me more than the radiation. Thanks!

  2. James Greenidge June 2, 2012 at 5:48 AM #

    Your feature title does your audience an injustice. There’s nothing “hot” about these animals. They simply found bare traces of Cesium with equipment so sensitive that it would’ve otherwise been missed in the background grass of even more active elements. Misleading the public is not the way to advance your agenda.

  3. Dr Y June 4, 2012 at 3:35 PM #

    I’ve got to apologize, James – I was going for “ironic” with the title but didn’t do it as well as I’d thought. I completely agree with you with regards to the health risk here, and I hope that it came out in the body of the posting. And I guess I need to be a little more careful with the titles – I wasn’t attempting to mislead or manipulate and I’m sorry it came out that way.

  4. Kit P June 7, 2012 at 12:05 PM #

    Zero is the number of American mothers and children that have a body burden of mercury above the threshold of harm from eating fish. Looking at the research I would conclude it is impossible to exceed the threshold of harm from eating fish.

    Basically the issues with environmental mercury were solved long ago.

    Anti-coal and anti-nukes like to direct their fear mongering at young mothers.

    In general, making electricity in the US does not hurt our customers. Many would disagree but no one can provide a reference. We have the ability to measure stuff down to the tracer level, you would think it would be easy to find a smoking gun.

  5. P.F. McCracken, III June 8, 2012 at 3:36 PM #

    The article and title were good. It was the title that attracted my attention. Having forgotten these ideas myself even after growing up around Three Mile Island, Pennsylvania. I have my own knowledge and experience with radiation. As for mercury, do not forget that young children and pregnant women are more sensitive to chemicals in the environment. So the small possible amounts of mercury would affect them more than the others.
    As for the power industry, the by-products when not controlled do influence the environment and customers. The EPA lists the amount of known polutants released released by power plants. Many power companies also publish the details of tons of CO2, Carbon, Sulfur Dioxide and such released per year by the sources of electricity producing facilities.

  6. Sanity prevails down under June 10, 2012 at 9:22 AM #

    This is fantastic news means we can keep on polluting our planet, the oceans, and all its living creatures.

    The uranium/nuclear behemoth will be tickled “pink” or should that be an iridescent shade of blue? Of course no worries about the decay chain of U-238 which has two radioactive leads (Pb) and one stable (Pb-206) which the peer-reviewed literature says can send you troppo.

    Oh and the International Conference on Mercury as a Global Pollutant (ICMGP) is simply a forum for charlatans chasing the filthy lucre? All that stuff about mercury being a potent neurotoxin is load of old cobblers eh?:

    Every thought is a seed. When you plant crab apples, don’t expect to harvest golden delicious.

  7. Dr. Y June 11, 2012 at 9:16 AM #

    Do I sense a bit of sarcasm? There are a few things here – first is that the scientific paper addressed cesium (not uranium) that was detected at levels far below natural radionuclides. Not saying that it’s OK to dump into the ocean – just that in this particular case what was found is not dangerous.

    Second – you are factually correct that lead is present in the U-238 and U-235 decay series, but in levels far too low to be chemically toxic. But uranium is naturally present in the environment AND there was no release of uranium by the Fukushima accident AND the halflife of uranium is far too long to decay to the lead isotopes – for all of these reasons the comment about uranium is irrelevant to this particular post.

    Finally, the comment about mercury has already been addressed by others commenting here.

    As Paracelsus (a 15th-16th century alchemist) pointed out “Poison is in everything, and no thing is without poison. The dosage makes it either a poison or a remedy.”

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