Earthworms collected in Kawauchi, a village near the Fukushima No. 1 nuclear plant, have cesium levels averaging some 20,000 becquerels per kilogram, government researchers said.
The finding indicates the radioactive substance "may accumulate in other animals through the food chain," Motohiro Hasegawa, senior researcher at the Forestry and Forest Products Research Institute, said Monday, noting earthworms are eaten by birds, boars and other wild animals.
Last August and September, Hasegawa and other researchers collected earthworms in Kawauchi, 20 km from the crippled power plant; Otama, 60 km from the plant; and Tadami, 150 km away.
The researchers analyzed average cesium levels in five earthworms gathered in each of the three locations. Worms from Otama were found to have 1,000 becquerels of cesium and those from Tadami had 300 becquerels, compared with the government's allowable limit of 500 becquerels for human exposure.
Fallen leaves exposed to fallout decompose in the soil before being consumed by earthworms, the researchers said, explaining how the cesium contamination spreads.
Cesium levels in the soil where earthworms were collected stood at some 1.38 million becquerels per square meter in Kawauchi, 80,000 to 120,000 becquerels in Otama and 20,000 becquerels in Tadami, they said.