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.