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.