Fukushima Radiation Tracked Across Pacific Ocean

/ Source: LiveScience





Drifting radiation




Naturally, the oceans hold about 1-2 becquerels (Bq) of radioactivity per cubic meter of water, where a becquerel is one decay per second. The researchers found hundreds to thousands of times more, with up to 3,900 Bq per cubic meter in areas closer to the shore, and 325 Bq in sites as far as 372 miles (600 km) away.

Currents and eddies

Ocean phenomena, big and small, also affected the radiation spread. For instance, the team found that the Kuroshio Current, which runs roughly east-northeast from the south of Japan toward the Aleutians, acts as a kind of boundary for the spread of radioactive material, even as it also pushes a lot of it away from the coast. In addition, eddy currents that arise at the edge of the Kuroshio caused the cesium and other radioactive pollutants to reach higher concentrations in some places closer to the coast, carrying some of the drifters toward populated areas south of Fukushima.

"It's [an] interesting thing to think about, as the concentrations vary by a factor of 3,000," Buesseler said. "With what we knew about transport prior to this work, you wouldn't know why it is so different."


Even so, Buesseler said, the radioactivity levels are still below what is allowed in food in Japan, which is 500 Bq per kilogram of "wet" weight. And while cesium was present in the fish, it doesn't accumulate up the food chain the way polychlorinated biphenyls (PCBs) or mercury do. Mercury and PCBs tend to stay in an animal's tissues for long periods, so when a tuna eats smaller fish, it takes in all the chemicals those small fish have eaten. Cesium tends to be excreted from animals much faster.

The WHOI expedition calculated that some 1.9 petabecquerels — or 1.9 million billion becquerels total — were in the stretch of ocean studied. The total released by the Fukushima accident was much greater, but a lot of the radionuclides were dispersed by the time of the sampling in June.

The researchers also found silver-110, but it wasn't clear that was from the Fukushima plant. Another set of experiments measured strontium-90 levels, but that work hasn't been published yet.

Kara Lavender Law, an oceanographer at the Sea Education Association, noted this kind of work is important because the picture of how ocean currents affect environmental pollutants isn't always clear. "From an ocean-current standpoint we know what large-scale circulation is like, but when you get into where contaminant spills will end up, sometimes the picture is a whole lot different when you look at smaller areas," Law told LiveScience.