Water near the Fukushima Daiichi plant's reactor 1 contained radioactive iodine at 3,355 times the legal limit, Japan's nuclear safety agency said.
However, an official said the iodine would have deteriorated considerably by the time it reached people.
Meanwhile, the president of Fukushima operator Tepco has been hospitalised.
Masataka Shimizu is being treated for high blood pressure and dizziness, a Tepco spokesperson said.
Mr Shimizu has barely been seen in public since the earthquake and tsunami on 11 March which damaged the Fukushima plant.
Tepco officials have announced a press conference for 1500 local time (0600 GMT).
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