Highly radioactive sludge is turning up in septic tanks at car washes in Fukushima Prefecture, and the readings are as much as seven times higher than the government's limit, auto industry officials say.

While the government-set limit is 8,000 becquerels per kilogram, some of the sludge is giving off 57,400 becquerels per kg, a document obtained by Kyodo News says.

The source of the radioactivity is believed to be ash and soot that stuck to vehicles shortly after the triple core meltdown at the Fukushima No. 1 nuclear power plant in March 2011, the officials said Saturday.