Tokyo Electric Power Co. has detected radioactive cesium in groundwater samples taken from the premises of the crippled Fukushima No. 1 nuclear complex, reversing an earlier announcement that any contamination was negligible.

The announcement came Monday as Tepco was trying to secure the consent of local fishermen for dumping groundwater that has been pumped out of wells at the site into the Pacific, claiming it has confirmed that concentrations of radioactive substances are sufficiently low.

Tepco had said radioactive cesium in the groundwater was at a level that could not be detected by an instrument at the Fukushima No. 1 complex. But the same sample was found to contain 0.22 becquerel of cesium-134 and 0.39 becquerel of cesium-137 per liter when checked at the nearby Fukushima No. 2 plant, where radiation levels are lower.

According to the utility, there was a problem in accounting for background radiation.

The revised amount of cesium-137 is still below the level that Tepco views as the upper limit for releasing groundwater, which is less than 1 becquerel.

Currently, about 400 tons of groundwater seeps into the crippled reactor buildings every day, where it becomes contaminated with radioactive substances. This means the total volume of toxic water is increasing by the same amount daily.

To slow the rate of accumulation, Tepco has created a system to direct part of the groundwater into the ocean by pumping it out before it flows into the reactor buildings. The groundwater is stored in tanks before it is discharged.

But the utility has not been able to fully operate the system amid concern from fishermen that dumping groundwater will contaminate the marine environment.

The latest revelation could undermine the credibility of related data presented by Tepco, possibly making it hard for the utility to get the nod to discharge the groundwater and standing in the way of the overall plan to address the massive amounts of radioactive water at the plant.