High levels of radioactive material were detected in the groundwater at a damaged Japanese nuclear power plant on Wednesday, according to Tokyo Electric Power Co Inc (TEPCO).
A survey showed the level of strontium-90 at about 30 times the legal limit, and the tritium level at some eight times the limit, said the operator of the Fukushima Daiichi Nuclear Power Station, 230 km north-east of Tokyo.
The levels of strontium-90 in groundwater at an observation well had increased by 116 per cent and those of tritium by a factor of 17 since December, according to TEPCO.
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The groundwater at the well, located 27 metres from the sea, may not have affected nearby seawater, said Takeo Iwamoto, a TEPCO spokesman.
The utility has yet to find the cause of such high levels of contamination, Mr. Iwamoto said.
The contaminated groundwater could be from highly radioactive water, which was found flowing into the Pacific Ocean in April 2011, soon after the nuclear emergency, he added.
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The plant suffered meltdowns at three of its six reactors after it was hit by an earthquake and tsunami in March 2011.