Prompted by a string of accidental radioactive discharges, federal monitors said Wednesday that they have formed a task force to investigate the spills at several power plants across the country, including the one at the Palo Verde Nuclear Generating Station in Wintersburg.

"It does appear that it's bang, bang, bang, one right after the other," Steve Klementowicz, a Nuclear Regulatory Commission senior health physicist, said of discharges of radioactive tritium-laced water at nuclear plants in Arizona, Illinois and New York.

Tritium, a byproduct of nuclear power generation, is a relatively weak source of radiation. But long-term exposure can increase the risks of cancer, miscarriages and birth defects. It can be ingested or absorbed in human tissue.

Klementowicz and other NRC officials said at a hearing here that the task force of experts will evaluate the health effects of what has happened at at least five plants since December and possibly earlier incidents. But they emphasized that the latest reports from all the sites, including Palo Verde, do not indicate any immediate public hazards.

At the Palo Verde plant about 50 miles west of downtown Phoenix, the largest nuclear generating site in the country, an NRC health inspector has been working during the past week with officials from Arizona Public Service and the Arizona Department of Environmental Quality to pinpoint the source and amount of the contamination.

APS, which operates the plant on behalf of itself and six other owners, first notified the state on March 2 that it found tritium in a maze of underground pipes. Water samples taken a day before had turned up levels 3½ times those considered acceptable by the Environmental Protection Agency for drinking water.

State, federal and APS officials said Wednesday that, so far, there is no evidence Palo Verde-generated tritium has migrated beyond the boundary of the plant or seeped into aquifers about 70 feet to 200 feet underground that supply water for the area.

NRC officials said the task force is to be made up of 11 experts from the commission around the country and one nuclear safety official from Illinois. The group will review the effects on public health, how well incidents of such discharges are communicated to the public and authorities, gauge the nuclear industry's remediation efforts and evaluate their own agency's oversight of the issue.

A written report summarizing the findings is due by Aug. 31.

In only one case so far, at the Braidwood Nuclear Power Station near Braceville close to Chicago, has contaminated water been found to have seeped outside the plant's property.

But there are questions about how diligently some plant operators have been reporting such discharges, as required by federal law.

Last week in Illinois, state and local officials filed suit against Braidwood's operators alleging they failed to report earlier discharges before announcing another leak in December. The operators did so only after state officials became aware of already existing groundwater damage and contamination of at least one nearby private drinking-water well. One such spill in 1998 is believed to have dumped about 3 million gallons of water that remains in the ground.

"Companies are suddenly deciding to report these discharges more openly now because they've got their covers pulled off; spills have gotten into people's yards," said Paul Gunter, a member of the Takoma Park, Md.-based Nuclear Information and Resource Service, a watchdog group.

Among groups that have been calling for an NRC investigation of the leaks is the Nuclear Safety Project for the Union of Concerned Scientists.

In Arizona, although APS has not pinpointed the source of the tritium contamination in water found at Palo Verde, company officials said more and more evidence suggests that rainfall, rather than a cracked or leaking pipe, could be a source.

Adding to this "washout" theory, they said, is that recent rainfall samples collected from a roof vent found tritium levels similar to the samples found in the contaminated water.

"This is what we believe is going on," said Craig Seaman, Palo Verde's general manager of regulatory affairs. "We're certainly not willing to hang our hat on this yet and say this is the absolute answer."

Palo Verde vents tritium into the air as a normal byproduct of nuclear power generation. Other  nuclear power plants typically dispose of the chemical in streams or lakes where it quickly dissipates, Seaman said.

Seaman said APS officials believe rainfall captured the tritium released from the plant and washed it into the soil there.

He said APS believes it is a "localized phenomenon" restricted to Palo Verde, so it is unlikely rainfall outside the plant would carry heavier tritium samples.

State environmental officials who also are working with APS to determine the source of the tritium said rainfall would be more problematic than a leaking pipe.

"If that is their conclusion, that tritium is being released into the air and coming down to earth with the rain, that raises a heck of a lot more questions in my mind than it answers," said Steve Owens, director of the DEQ.

Residents who live near Palo Verde say the federal government's effort to step up oversight of contaminated water at nuclear power plants is a good move.

"I think it's important," said Charlotte Brafford, a Tonopah resident who lives near Palo Verde. "It is not a normal element or chemical that we hear about. So it's a concern."

Brafford splits her time between her Tonapah home and a second home by the Perry nuclear power plant near Cleveland, so she is concerned about the safety and environmental impacts of nuclear plants on surrounding communities.

"When the news broke about Palo Verde, we weren't told much, so it was a question of whether we were being kept in the dark," Brafford said.

Yet Brafford and other residents seem satisfied that APS and state officials have done a sufficient job of keeping nearby residents informed.

Within a week of discovering the tritium, APS and state environmental officials notified a Palo Verde community advisory panel of its findings. The contamination at Palo Verde also was the main topic discussed Tuesday at the Tonopah Valley Community Council.

"They gave us a very thorough briefing on this," said Judith Shaw, a Tonopah resident active in two community groups. "From what I can gather, they are right on top of it."

Owens said Arizona's strict aquifer protection laws require an immediate report of such releases. "APS must disclose these releases," Owens said. "It is a pretty stark contrast to situations in other states, where releases have been occurring."

Because of the lax reporting standards at other nuclear power plants, Owens said, the review by the NRC "is long overdue."