Although the land near the Fukushima nuclear reactors was heavily contaminated by the aerial release of radioisotopes, the majority of the radioactive releases drifted out over the Pacific. There, they were joined by substantial amounts of water that were discharged from the reactors directly into the ocean. A new study, based on data from a NOAA research vessel, takes a look at radioactivity levels near Japan a few months after the disaster. The data suggests that the highest estimates of radioactive discharges are likely to be accurate, but the rapid dilution of the water has kept the levels from Fukushima's isotopes below those of the naturally occurring radioactivity.

Although the peak of discharge into the ocean occurred in early April, NOAA didn't manage to get a vessel in place until June. For the first half of the month, the Ka'imikai-o-Kanaloa (Hawaiian for "Heavenly Searcher of the Sea") sampled the waters and oceanic life off Japan (between 30km and 600km), all while releasing floats that helped researchers identify the predominate currents in the region. Most of the radioactivity was released in the form of cesium isotopes that have half-lives of over two years, so the time needed to get a vessel in place did not allow for a significant decay of the discharged material.

By then, however, some of the radioactivity had drifted a considerable distance. 134Cs was present at levels that produced 325 becquerel (325 decay events per second) per cubic meter as far as 600km offshore. A small eddy in the prevailing currents, located closer to shore, had activities of 3,900Bq per cubic meter. In general, however, there had not been sufficient time for the radioisotopes to mix vertically; the vast majority of the hot cesium was present within 100m of the surface. Sampling the sea life, including plankton and fish, also revealed the presence of radioactive cesium, although not consistently, and at levels substantially lower than seen in the seawater. Radioactive cesium isn't normally present in seawater, so these numbers are above a baseline that's essentially zero.

In addition to the cesium isotopes, which are volatile enough to have spread through the air, the authors discovered a radioactive isotope of silver, 110mAg, that hadn't been detected previously. So the direct discharge into the ocean had put a somewhat different collection of elements into the ocean than the aerial release had.

(The "m" in the isotope name of silver isn't a typo. Some isotopes of the element exist in both regular and "metastable" states, in which the configuration of its nucleus is in a higher energy than the ground state. In the case of silver, many of these metastable isotopes decay more slowly than their ground-state peers.)

The authors do a good job of putting this all in perspective. The raw numbers look enormous: levels of cesium 1,000 times higher than background across an area of about 150,000 square kilometers. But cesium levels are normally extremely low, and the ocean naturally contains several radioactive isotopes, such as 40K, which is also a source of background radiation on land. In the seafood samples, the additional radioactivity from the cesium and silver was less than the naturally occurring sources, typically only about a third. The net result is that the 137 in fish was about 150 times lower than the legal limits in Japan. Even if all the isotopes were considered, the fish would be safe to eat.

The authors also estimate the risk of going for a swim in the waters as pretty minimal, as it would result in an exposure of only 0.3 percent of the natural background a typical Japanese citizen experiences.

The authors use a computer model of the local currents to figure out how much radioactivity must have been discharged to produce the pattern they see. The answer turns out to be on the high side of estimates (22 PetaBecquerels), indicating that the direct discharge was a significant route for contamination, accounting for about two-thirds of the total radioactivity release.

Nevertheless, the isotopes that landed on the ground have stayed there, creating a serious contamination problem that may take years to resolve. Although the seas in the immediate vicinity of Fukushima probably experienced a very high dose of radioactivity during the months immediately after the disaster, as long as none of the isotopes accumulate in any organisms, the effects are unlikely to be long-lasting.

PNAS, 2012. DOI: 10.1073/pnas.1120794109 (About DOIs).