The density of radioactive tritium in samples of seawater from near the Fukushima No. 1 nuclear plant doubled over 10 days to hit a record 1,100 becquerels per liter, possibly indicating contaminated groundwater is seeping into the Pacific, Tokyo Electric Power Co. said.

The latest sample was taken June 21 from the sea near a water intake point east of the reactor 1 turbine building.

The legally permitted level of tritium is 60,000 becquerels per liter. Water taken from the same place June 10 had a reading of 500 becquerels per liter.

Tepco said late Monday it was still analyzing the water for strontium-90, which would pose a greater danger than tritium to human health if absorbed via the food chain. The level of cesium did not show any significant change between the two sample dates, according to the embattled utility.

On June 19, Tepco revealed that a groundwater sample taken from a nearby monitoring well was contaminated with both tritium and strontium-90.

At that time, seawater samples did not show any significant changes in the level of radioactive materials, and Tepco denied the dirty water was seeping into the sea.

But during a news conference Monday in Tokyo, Masayuki Ono, a Tepco executive and spokesman, this time did not deny the possibility of leakage into the sea, while he said Tepco is still trying to determine the cause of the spike.