A fraction of the radioactive cesium released during Japan’s Fukushima Daiichi nuclear plant disaster in 2011 was deposited on the nearby seafloor and will likely persist for decades, says a study by scientists from Woods Hole Oceanographic Institution (WHOI) in Massachusetts.

Less than 1% of the cesium released ended up on the ocean floor, but it accounts for the majority of marine radioactive contamination near the plant site, the study showed (Environ. Sci. Technol., 2015, DOI: 10.1021/acs.est.5b02635). This is the likely source of high levels of cesium found in the area’s bottom-feeding fish.

An estimated 99% of cesium released in the disaster entered the Pacific Ocean and was carried away by currents. Lead author Ken Buesseler, a chemist at WHOI, and colleagues examined the fraction of cesium that clung to sediments. To do that, they placed an ocean trap southeast of the nuclear plant where they found 137Cs, which has a half-life of 30 years, and 134Cs, which has a half-life of two years.

The sediments didn’t come from directly above the trap but moved from the ocean floor near the Fukushima site. They likely were stirred up by typhoons, Buesseler says. The amount of contaminated sediments moving each year is small, he adds. “It will take decades for all of the Fukushima cesium to be gone.”

James Day of Scripps Institution of Oceanography, who was not involved in the study, says the study shows that “the thing that is going to reduce the amount of cesium is really going to be the radioactive decay.”