The study by the Japan Atomic Energy Agency's agency's Horonobe Underground Research Center in Hokkaido discussed the effects of materials released from the quake-crippled Fukushima-Daiichi nuclear plant sinking into the ground.
The plant was struck March 11, 2011, by a 9-magnitude earthquake and a tsunami, causing a catastrophic radiation-leaking nuclear disaster. Efforts are still under way to bring its damaged reactors to what is called a permanent "cold shutdown."
The latest report said radioactive materials released into atmosphere from the plant were found 5 centimeters (nearly 2 inches) underground three months after the disaster struck but are now believed to have gone 10 to 30 centimeters (4 inches to about a foot) deep, Kyodo News reported.
The Japan Atomic Energy Agency says the materials must have been brought to ground by rain.
"Further delay in decontamination works will make the radioactive materials sink into the ground deeper, and it will impose more burdens on those involved in the decontamination," said Haruo Sato at the agency's research center.
Kyodo reported the center's researchers examined the penetration of four radioactive materials, including cesium 137, at 11 areas in the Fukushima Prefecture within a radius of as much as 60 kilometers (37 miles) from the plant.
Most of the materials were found to be within about 2 inches from ground surface in June, but more recent estimates indicated they had gone deeper.