Contaminated water might have leaked from the damaged Fukushima nuclear reactors after erroneous settings on water gauges lowered groundwater levels nearby, the plant operator said.
Tokyo Electric Power Co said the settings on six of the dozens of wells around the reactors were three feet below the requirement.
Groundwater at one well briefly sank below the contaminated water inside in May, possibly causing radioactive water to leak into the soil.
Samples have shown no abnormal increase in radioactivity and leaks to the outside are unlikely, Tepco spokesman Shinichi Nakakuki said.
Groundwater levels at wells dug closer to the reactors have stayed above minimum safety levels, suggesting that risks of a leak are unlikely, he said.
The problem in the six wells launched in April was caught this week during preparation at another well nearby.
The company said it is investigating.
The plant suffered triple meltdowns following the March 2011 earthquake and tsunami.
Cooling water leaks out of the damaged reactors and mixes with groundwater that seeps into the basements of the reactor building, increasing the amount of contaminated water.
The wells around the reactor and turbine buildings are designed to pump groundwater and reduce the amount around the reactor area.
Some of them were at the plant from before the accident to manage groundwater there, but more have since been added.
Six and a half years after the accident, the plant still struggles with managing the contaminated water.
Details about the melted fuel inside the reactors remain unknown.
Decommissioning the damaged reactors is an uncertain process that is expected to take 30 to 40 years.