Studies have shown that soil erosion can move 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 Wednesday.
The typhoons "strongly contribute" to soil dispersal, said Evrard, though it can be months later, after the winter snow melts, that contamination actually passes into rivers.
Local populations who escaped the initial fallout two-and-a-half years ago could now find their food or water contaminated by the cesium particles as they penetrate agricultural land and coastal plains, researchers warned.
Last year, the radioactive content of Japan's rivers dropped due to fairly moderate typhoons. But more frequent and fierce storms in 2013 have brought a new flood of cesium particles.