The prefectural government in January surveyed the density of radioactive materials in soil and other sediment that has accumulated on the bottoms and banks of 72 rivers in the prefecture, public broadcaster NHK reported.
The study came in response to the 2011 nuclear accident at the Fukushima Daiichi power plant.
The researchers found up to 54,500 becquerels per kg of radioactive substances in the Maeda river in Futaba town, where the plant is situated, and 39,600 becquerels in the Hiru river in Fukushima city. They also detected more than 10,000 becquerels at five other locations in four municipalities.
The prefectural government plans to study restricting access to rivers with high concentrations of radioactive materials.
It also plans to urge the central government to remove contaminated soil and other sediment.(IANS)