The operator of the damaged Fukushima Daiichi plant says water samples taken from a newly-dug well contained the highest levels of radioactive cesium detected so far in groundwater at the site.
Tokyo Electric Power Company says the record levels suggest that the leakage point could be near the well.
The utility on Thursday said it had detected 54,000 becquerels per liter of cesium 137 and 22,000 becquerels per liter of cesium 134 in water samples.
The samples were taken on Wednesday from a new observation well located 50 meters from the ocean near the Number 2 reactor.
The level of cesium 137 is 600 times the government standard for radioactive wastewater that can be released into the sea.
It is more than 30,000 times the level of cesium 137 found in water samples taken from another observation well to the north last week.
TEPCO officials believe radioactive water is leaking from an underground tunnel that extends from the reactor buildings towards the ocean. They have been taking measures to prevent the tainted water from reaching the sea, but have yet to determine where the leak originates.
TEPCO suspects the leakage point is near the new well because radioactive cesium is easily absorbed into soil and is unlikely to be carried over a wide area in groundwater.
Feb. 13, 2014 - Updated 06:12 UTC