Typhoons that hit Japan each year are helping spread radioactive material from the Fukushima nuclear disaster into the country's waterways, researchers say.
A joint study by France's Climate and Environmental Science laboratory (LSCE) and Tsukuba University in Japan found contaminated soil is being washed away by the high winds and rain and deposited in streams and rivers.
A tsunami sparked by an earthquake slammed into the Fukushima plant in March 2011, sending reactors into meltdown and causing the worst atomic accident in a generation.
A large number of radioactive particles were spewed into the atmosphere, dispersing cesium particles which cling to soils and sediment.
Studies have shown that soil erosion moves the radioactive varieties of cesium-134 and 137 from the northern mountains near Fukushima into rivers, and then out into the Pacific Ocean.
"There is a definite dispersal towards the ocean," LSCE researcher Olivier Evrard said. The typhoons "strongly contribute" to soil dispersal, though it can be months later, after the winter snow melts, that contamination passes into rivers.
Local populations who escaped the initial fallout in November 2011 could now find their food or water contaminated by the cesium particles as they penetrate agricultural land and coastal plains, said researchers.
Last year, the radioactive content of Japan's rivers dropped because of fairly moderate typhoons. But more frequent and fierce storms this year have brought a new flood of cesium particles.
This is, said Evrard, "proof that the source of the radioactivity has not diminished upstream".
Tsukuba University has done several studies on Fukushima since the disaster. Scientists "concentrated mostly on the direct fallout from Fukushima yet this is another source of radioactive deposits" that must be taken into account, Evrard warned.
Coastal areas that are home to fishermen or where people bathe faced a particular risk.
Tens of thousands were evacuated around the Fukushima plant after the disaster and nearby villages and towns are largely empty as residents fear the radiation. Decommissioning the site is expected to take decades.