Tepco officials said detection of radioactive substances indicates the possibility that toxic water from the tank may have reached the groundwater, estimating that about 300 tons of highly toxic water had leaked from the tank, the Kyodo news agency reported Friday.
The power company said it collected the groundwater Wednesday at a well south of the tank from which the radioactive water leaked.
The level of radiation measured in the groundwater, which was lower than levels of the water in the holding tank, may have been diluted by rain, Tepco said.
The utility said although the majority of toxic water seeped into soil while some of it flowed into the sea via a drain ditch, Kyodo said. The amount of contaminated water escaping into the sea is unknown.
The Japanese government says it is looking into several options to help prevent contaminated water from the Fukushima plant from reaching the sea, including creating an underground "ice wall."
In the 2011 earthquake and tsunami that damaged the nuclear power plant, cooling systems to the reactors were knocked out and three reactor cores melted down, requiring cooling water to be pumped into them to keep them cool. Storing the resultant large quantities of radioactive water has proved a challenge for Tepco.