LACEY TOWNSHIP, N.J. -- Radioactive water that leaked from the nation's oldest nuclear power plant has reached an aquifer that supplies drinking water to much of southern New Jersey, the state's environmental chief said Friday.

The state Department of Environmental Protection has ordered the 39.814771,-74.206191 6 Oyster Creek Nuclear Generating Station Oyster Creek Nuclear Generating Station to halt the spread of contaminated water underground, even as it said there was no imminent threat to drinking-water supplies.

The department launched a new investigation Friday into the April 9, 2009, spill and said the actions of plant owner Exelon Corp. have not been sufficient to contain water contaminated with tritium.

Tritium is found naturally in tiny amounts and is a product of nuclear fission. It has been linked to cancer if ingested, inhaled or absorbed through the skin in large amounts.

"There is a problem here," said environmental Commissioner Bob Martin. "I am worried about the continuing spread of the tritium into the groundwater and its gradual moving toward wells in the area. This is not something that can wait. That would be unacceptable."

Department spokesman Lawrence Ragonese said there are several possible ways to address the contamination, including pumping tainted water out of the ground, or injecting a different part of the aquifer with water to force the plume of contaminated water backward and away from wells.

No option has been decided upon.

The radioactive water leaks were found just days after the plant got a new 20-year license in 2009 that environmentalists had fought bitterly for four years. Those problems followed corrosion that left the reactor's crucial safety liner rusted and thinned.

The Nuclear Regulatory Commission and Exelon insist Oyster Creek can operate safely until it is 60 years old. But environmental groups disagree.

"The bad news is Exelon's Oyster Creek plant ... has now become a major threat to South Jersey's drinking water," said David Pringle of the New Jersey Environmental Federation. "The good news is NJDEP Commissioner Martin is taking aggressive action to safeguard our water and hold Exelon accountable for this leaky 40-year-old plant."

Julia LaMense of the Eastern Environmental Law Clinic hailed the state's action and condemned the NRC "for letting it come to this."