HARRISBURG - Amid concerns of fallout from Japan's damaged nuclear plants reaching the United States, tests performed on public drinking-water supplies throughout Pennsylvania found no elevated levels of radioactivity, Gov. Corbett said Monday.

But low concentrations of iodine 131 - a radioactive byproduct of nuclear fission - have been detected in rainwater at sites throughout the state, he said.

"The bottom line is the drinking water is safe," said Corbett, whose announcement in the Capitol, 10 miles from Three Mile Island, came on the 32d anniversary of the nation's worst nuclear accident.

Radiation-protection specialist Kathryn Higley said that while monitors might detect radiation, it did not represent a public-health threat. She said the radiation that had turned up would likely soon disappear.

"The point I keep making to people is that we have really sensitive detection systems," said Higley, a professor of nuclear engineering and radiation health physics at Oregon State University. "It's not a health threat. Winds circulate around the globe and what escaped is diluting and decaying every day."

Rainwater tests have shown trace amounts of radiation in samples taken in a number of Western states and as far east as Massachusetts, according to the Environmental Protection Agency.

Samples of drinking water were tested at sites in six regions of Pennsylvania during the weekend after tests Friday at the state's nuclear power plants - including Three Mile Island and Limerick, in Montgomery County - registered very low concentrations of radiation.

Speaking at a news conference, Corbett said the level measured in rainwater, including in Norristown, was still 25 times below what would be of concern.

That news did not ease the concerns of Eric Epstein, whose Harrisburg group monitors radiation levels at Three Mile Island and other nuclear plants in Pennsylvania.

"It is not normal to detect radioactivity from Japan in Pennsylvania waterways," said Epstein, chairman of Three Mile Island Alert Inc. He urged Corbett to step up the monitoring of radiation levels statewide.

Corbett said Monday that he believed nuclear power remained a "safe industry" and that state agencies would continue to take readings to ensure drinking water was safe.

He said that those residents whose drinking water came from wells or springs were unaffected and that pets and livestock were not at risk.

Corbett advised the public that taking potassium iodide pills - designed to counter the harmful effects of radiation to the thyroid - was "unnecessary under the circumstances" and could cause harmful side effects.

Contact staff writer Amy Worden at 717-783-2584 or aworden@phillynews.com.