The radiation level measured about 19 inches above the water was about 100 millisieverts per hour, Tepco said.
Tepco said about 31 gallons of the toxic water has so far leaked from one of the storage tanks, but said it hadn't flowed into the Pacific Ocean.
The Nuclear Regulation Authority ordered an investigation to discover if the radioactive water had escaped into the ocean through nearby drains, Kyodo News reported Monday.
Meanwhile, Tepco estimated about 300 tons of highly radioactive water have leaked from one of the storage tanks at the Fukushima Daiichi power plant, but also denied toxic water was flowing into the Pacific Ocean.
The plant said it hasn't yet identified from where the water is escaping.
"We think the leakage is continuing," Tepco spokesman Masayuki Ono said.
The Fukushima nuclear power plant was damaged during Japan's March 2011 earthquake and subsequent tsunami.