Tokyo Electric Power Company says it detected high levels of radioactive substances in water collected on Wednesday at a monitoring well about 10 meters from the tank.
It says the water contained 650 becquerels per liter of beta-ray emitting substances, including strontium. The water was taken from about 7 meters deep, where groundwater is flowing.
Last month, TEPCO discovered more than 300 tons of contaminated water had leaked from the tank. Company officials said some of the water may have reached the sea.
Sep. 5, 2013 - Updated 13:23