NOAA Three months after the tsunami-stricken Fukushima Daiichi plant began leaking unprecedented amounts of radioactive isotopes, an interdisciplinary group of scientists will begin measuring the radioactivity in the ocean east of Japan on 4 June. Led by Ken Buesseler of Woods Hole Oceanographic Institution (WHOI), the cruise will carry 17 researchers from several countries on a 15-day expedition. Although the greatest leaks have been capped and overall radiation levels are decreasing due to isotope decay and dilution in the ocean, measurements of the water near the plant suggest that the ocean is still picking up new radioactivity, although it's unclear whether this is coming directly from the plant, carried by rainfall, or seeping in with groundwater. Meanwhile, the water that has been pumped into the reactors to cool them is threatening to overflow its tanks in the next few days and carry radioactivity into the sea. "We need to trace where the radionuclides are going, and how much is making it offshore," says Steven Jayne, a physical oceanographer from WHOI. The research vessel, Kaimikai‑O‑Kanoloa, will monitor radiation levels over 400 square km, crisscrossing the strong Kuroshio Current that runs northward along the east coast of Japan. The radiation spill, Jayne says, also provides a unique way of tracing the Japanese currents. "It's like pouring dye into the ocean," he says. The Kuroshio Current, similar to the Gulf Stream of the Atlantic, is a crucial factor in fish migration patterns and can slow or speed international shipping in the Pacific Ocean. The WHOI group will monitor water 300 km offshore, an area with large fisheries for tuna and other seafood. By the time the water reaches this area and is taken up into seafood, radioactivity is probably well diluted below what is probably dangerous for human consumption, but marine biologist Nicholas Fisher of Stony Brook University in New York says that the study will be a useful baseline to understand how radiation is dispersed in the specific ocean patterns and sea life of the Pacific. Fisher is leading the effort to study how marine life takes up radionuclides into its tissues. Because 3 months have passed and most isotopes, particularly the short-lived iodine-131 with an 8-day half-life, have decayed considerably, he doesn't expect to see any toxicity. However, there will still be detectable levels in organisms such as brown seaweed, which can store iodine at 10,000 times the concentration in the water. Such a measure might help researchers understand how the isotopes move through the food chain, even up to seafood-eating humans. Meanwhile, Japanese researchers continue to keep a close eye on the water close to the plant. Calculations by the French Institute for Radiological Protection and Nuclear Safety (IRSN) in Cherbourg have calculated that, close to the Fukushima plant, there is a risk of harm to marine life, particularly to animals born in the spring. Fisher hopes that by directly field-testing the water in the area, more precise measurements can be added to their calculations. *Update, 3 June: In an interview last night with ScienceInsider, expedition lead investigator Buesseler explained that in addition to the well-known isotopes iodine-131 and cesium-137, the cruise will measure the spread and bioaccumulation of rarer isotopes such as plutonium, strontium, and tritium, about which little is known. The extensive data set he expects to gather could take up to a year to analyze.