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.