Contaminated water might have leaked from the damaged Fukushima nuclear reactors after erroneous settings on water gauges lowered the levels of groundwater nearby.

The plant's operator, Tokyo Electric Power Co., said Friday the settings on six of the dozens of wells around the reactors were 70 centimeters (3 feet) below the requirement. Groundwater at one well briefly sank below the contaminated water inside in May, possibly causing the radioactive water to leak into the soil.

TEPCO is investigating, but says samples have shown no abnormal increase in radioactivity and leaks to the outside are unlikely.

The problem in the six wells launched in April was caught this week during preparation at another well nearby.