When Elizabeth Chimento and Poul Hertel began their inquiry into the chalky dust that seemed to cover everything in their north Alexandria neighborhood, they had only three questions: What is it, where does it come from and what are the health effects?

But they never asked what turned out to be the hardest question: How do we get rid of the visible and invisible pollution coming from the hulking power plant perched on the edge of the Potomac River?

It took years of self-education, money out of their own pockets, and battles with corporate and government officials before they would get an answer.

The accidental activists are now close to seeing their decade’s worth of work redeemed. Officials from Alexandria and GenOn, the current owner of the 62-year-old, 485-megawatt coal-fired electrical station, announced Tuesday that they have reached an agreement to shut the facility next year.

Still, Chimento, 69, and Hertel, 52, are not ready to declare victory.

“I do fear there’s wiggle room” in the agreement, said Hertel, a ruddy, robust, white-haired economist. “Anything other than a shutdown by October 2012, or within a year, is going to be jeopardizing public health.”

Chimento, a volunteer teacher with a passion for research, wants first to examine the agreement, which neither she nor Hertel has seen, and then monitor the shutdown, if and when it happens, to be sure no more pollution is visible from her front step.

“It’s been an uphill struggle all the way,” she said.

In spring 2001, a few years after Pepco sold its power plant to Mirant, residents began to notice the residue, Chimento and Hertel said. Many suspected Mirant.

Armed with a hand-drawn diagram from her father that explained how power plants work, Chimento and her friend, Hertel, went first to the plant manager. He dismissed their concerns but said the company had hired a consultant who would investigate the residue.

Samples were taken from Chimento’s townhouse and from the townhouse next door. The material, Mirant’s consultant said, was common dirt that looked different from the plant’s fly ash. But when Chimento and Hertel, looking at the samples through an electron microscope, had the neighborhood samples magnified, it suddenly looked like the plant’s fly ash. Under questioning, the Mirant staff discovered that the samples were chemically identical.

But by then, the consultant had already sent a letter to City Hall reporting the finding of dirt. Most council members looked at it and shrugged.

Neither Chimento nor Hertel had training in chemistry, engineering or epidemiology, so when they were invited to the World Coal Congress in 2001, they were lost until they found a Pennsylvania State University professor willing to test their samples. Months later, the results were in: The dust was from the Mirant plant.

Seeking to confirm the university’s results, Hertel and Chimento turned to Richmond. First, the state Department of Environmental Quality sent a junior staff member to talk to them. Later, seeking production records from Mirant, the DEQ put them in “a room with 10,000 boxes” and told them they could look but not photocopy anything. Like in a scene from the movie “Erin Brockovich,” they found what they needed in the first box.

The smokestacks at Mirant were notably short, a result of the fact that it lies in the flight path of Reagan National Airport. Chimento wondered if the 214-foot Mirant stacks meant that the coal smoke that came from them was settling down, not blowing away.

They found a local meteorologist in the phone book and hired him for $1,000, the cheapest fee he would accept. His study showed that the plant’s fly ash settled most heavily in the neighborhood right around the plant.

“That was what forced the city to get involved. They had no choice,” Chimento said, smiling at the victory.

In 2003, the pair produced a half-inch-thick report, replete with scientific sources. They hand-delivered 3,000 fliers alerting residents to a meeting at which the report would be made public.

Their findings showed that the chalky soot was, in fact, residue and coal ash from Mirant. Worse than what could be seen were fine particulates, which can easily spread many miles downwind and have been found to be a public health hazard, triggering respiratory disease. They are also an important risk factor for cardiopulmonary and lung cancer.

Mirant officials had denied over the years that the plant’s emissions caused health problems and asserted that any fly ash that did escape its combustion was not harmful.

With the Alexandria City Council, led by Paul Smedberg (D), now on board, the city tried to shut Mirant down by revoking its 2004 zoning permit to operate. Courts later overturned that ruling, but Mirant did temporarily close in the summer of 2005. When it reopened, the residue was significantly reduced, Chimento said, and the plant didn’t seem to be operating as much. The pair felt sure that closing the plant was just months, perhaps a year, away.

But in 2006, the Federal Aviation Administration said the agency found it acceptable for the Mirant stacks to be raised. The D.C. Public Service Commission, worried about the reliability of the electrical grid, filed a petition to keep Mirant operating.

For years, Chimento and Hertel drove 100 miles to Richmond to sit in on hearings, research and agitate for state action. They found unexpected allies, such as when the federal Agency for Toxic Substances and Disease Registry warned that plant emissions could be harmful to nearby residents, especially those running and biking on the adjacent Mount Vernon Trail. The state DEQ also had a turnaround, and in May it penalized GenOn, the new owner of the power plant, $275,000 in civil penalties for emission violations and failing to submit required paperwork. Other activists, local and national, joined the effort and provided important pressure.

The prospect of increasingly expensive pollution controls, a looming deadline to commit to spending $32 million, a stagnating demand for energy because of the world’s economic doldrums and the possibility of more rigorous Environmental Protection Agency regulations were all factors in GenOn’s decision to sign the agreement, city officials said, and Mirant seemed to agree. “It was a good business decision,” a spokeswoman said.

“I am very happy for the citizens of Alexandria . . . and concerned for the employees who might lose their jobs,” Chimento said. “I’m very proud of how the city and the citizens have worked together throughout all these years. It never became political propaganda — it was always based on science. I’m so grateful for the city’s perseverance.”