A radioactive substance recently found in groundwater monitoring wells at a Vermont nuclear plant has turned up again at levels more than nine times those previously reported and more than 37 times higher than a federal safe drinking water limit, officials said Thursday.

Officials at the Vermont Yankee nuclear plant, state Health Department and federal Nuclear Regulatory Commission said a newly dug monitoring well at the Vernon reactor turned up a reading of nearly 775,000 picocuries per liter. It was by far the highest reading reported yet for tritium, which has been linked to cancer when ingested in large amounts.

Despite the much higher reading, an NRC spokeswoman said Thursday there was nothing to fear.

"There's not currently, nor is there likely to be, an impact on public health or safety or the environment," the NRC's Diane Screnci said in an interview. She had maintained previously that the Environmental Protection Agency drinking water safety limit of 20,000 picocuries per liter had an abundance of caution built into it.

California's state limit is 50 times lower than the EPA's, 400 picocuries per liter. The National Academy of Sciences said in 2005 that any exposure to ionizing radiation from an isotope like tritium elevates the risk of cancer, though it also said with small exposures, the risk would be low.

News of the new reading came nearly a month after the Jan. 7 announcement that tritium had been discovered in a monitoring well at the Vernon reactor. No source has yet been found.

Plant spokesman Robert Williams said in an e-mail Thursday that the new reading is "good news" because it could indicate plant technicians searching for the leak are zeroing in on a source.

"The good news is that one newly installed well, located just to the east of the plant's condensate water storage tank and some underground piping, appears to be closer to the source because its concentration is 774,825 picocuries per liter," Williams wrote in the e-mail.

But he also said test results can fluctuate with the flow of groundwater through the area being tested. He noted that the well in which contamination was first reported was up over 36,000 picocuries per liter, from 17,000 when first reported. A second well, which showed a reading above about 85,000 picocuries per liter on Tuesday, had dropped below 70,000.

Gov. Jim Douglas, meanwhile, told reporters he did not know whether to be satisfied with a management shake-up at Vermont Yankee. He had called for such changes last week, three weeks after it was revealed that officials with Vermont Yankee and it's owner, New Orleans-based Entergy Corp., had misled state regulators and lawmakers by saying the nuclear plant did not have the sort of underground piping that could carry tritium.

Entergy has vowed to investigate the statements and this week announced it had replaced a top official in Vermont and was sending in a team made up partly of people from its corporate headquarters to try to mend relations with the state.

"I think it's too early to tell," if the changes would restore public confidence in Entergy and Vermont Yankee, the governor said Thursday. "I think we ought to wait until we see what the results of those changes bring as these investigations wrap up."

Also Thursday, a group of Democratic legislators from Windham County, where the plant is located, said consideration should be given to shutting down Vermont Yankee until the source of the leak can be found and stopped.

The officials released a statement signed by the county's two state senators, Peter Shumlin and Jeanette White, and 10 of 12 House members.