Tokyo Electric Power Co. said Sunday it has detected radioactive tritium in groundwater collected from a newly built observation well by the sea at the crippled Fukushima No. 1 nuclear power plant.

The groundwater sample collected Saturday from the well contained 34,000 becquerels of tritium per liter, up from 23,000 becquerels detected in a sample collected Thursday, a day after the well was installed some 4 meters away from the sea, the plant operator said.

The observation well was set up Wednesday about 4 meters north of the water intake for the No. 1 reactor.

Radioactive water is increasing at the Fukushima complex daily because groundwater is contaminated as it passes through the plant's premises, where three reactors experienced meltdowns following the March 2011 earthquake and tsunami disaster.

The utility suspects that groundwater has been contaminated at an underground trench by the sea that is connected to the No. 2 reactor building. The newly built well is located some 160 meters north of the trench.

Around 300 tons of groundwater containing radioactive substances could be flowing into the Pacific Ocean daily from the damaged Fukushima No. 1 nuclear complex, according to the government. Tepco recently admitted that toxic groundwater is leaking into the ocean.

The utility has been trying to prevent the further outflow of toxic water into the Pacific by pumping up contaminated groundwater and building a water-shielding wall on the sea bank.