Raised levels of radioactive tritium in groundwater sample collected from a newly built observation well by the Pacific Ocean near the crippled Fukushima No. 1 nuclear power plant were found by Tokyo Electric Power Co. (Tepco).
The levels of tritium in the groundwater sample collected from the well have risen to 34,000 becquerels per liter in last four days, the Japan times reports.
Earlier, 23,000 becquerels of tritium were detected in the sample collected on Thursday, after the well was installed for the reactor no. 1 some 4 meters away from the sea.
The levels of radioactive water has been found to be increasing at the Fukushima reactor on a daily basis as groundwater gets contaminated when it passes through the plant's premises.
The contamination of underground water has reportedly started since three reactors of the plant suffered serious meltdowns after March 2011 earthquake and tsunami disaster.
Tepco has said that groundwater might have been contaminated at an underground trench that is connected to the No. 2 reactor building, the report added.
The newly built well is located some 160 meters north of the trench.