Day in and day out, it converts an average of 17,000 tons of coal into electricity - and a yellow plume spiked with mercury.

While the power surges into homes and factories in the Midwest, the smoke billows from an 850-foot stack, joins the clouds and heads east. In widely scattered areas from Maine to Pennsylvania, mercury rains into lakes and ponds.

For years, the federal government has known that power plants produce mercury. It knows how technology could be used to reduce that pollution. But the Environmental Protection Agency's efforts to regulate the toxic metal have been slowed by industry lobbyists and their allies in Congress - including, until recently, Maine Sen. Olympia Snowe.

A key EPA study of major mercury pollution sources in the United States has been stifled since 1995 by critics in the utility and commercial fishing industries, who oppose submission of the report to Congress.

By preventing the report's official release, they have forestalled a national debate on the need for more regulations to cut mercury pollution.

The critics argue that the EPA still doesn't know enough about where mercury comes from, how it moves through the environment and whether relatively low doses are harmful to human and ecological health.

They want the agency to withhold the report, possibly for another year, until scientists who are now engaged in two major studies of the health effects of mercury finish their work.

At left:American Power Company's Conesville, Ohio coal-fired powerplant burned 3,000,000 tons of coal last year.

It is already a problem.

Thirty-five states, including Maine, have posted freshwater-fish consumption warnings to protect people from mercury poisoning. And wildlife biologists are finding troubling evidence that mercury may be harming the ability of some animals to reproduce and fight off disease.

As the broad scope of mercury contamination becomes clear, some question whether the EPA's critics are more concerned about profits in the electrical power industry than about protecting the environment and human health.

''This is not about philosophy or the public interest,'' said Doug Brogan of Clean Water Action in New Hampshire. ''This is about the bottom line.''

As a basic chemical element, mercury is naturally present in the air, land and water. But human activities like mining, manufacturing and coal burning have tripled or quadrupled atmospheric levels during the 20th century.

Winds can carry mercury great distances, where it eventually falls into lakes, accumulating in fish and the animals and people who eat them.

In 1990, the EPA proposed regulating mercury and other toxic air pollution from utility plants as part of the Clean Air Act amendments. It was ready, seven years ago, to crack down on mercury and other forms of air pollution.

But when the utility industry objected to the regulations, Congress watered down the proposal and directed the EPA instead to prepare a report about mercury pollution that would identify sources and solutions.

The agency did as it was told, and released a seven-volume, 1,700-page report in draft form in December 1995.

It identified coal-fired power plants and incinerators that burn household and medical waste as major industrial sources of mercury pollution.

Together, those sources release 171 tons of mercury into the environment a year, out of the 243 tons emitted by all industries.

At right: The control room at AEP's Conesville plant.

Mercury is a neurotoxin that, in large enough amounts, can cause problems with vision, a loss of muscular control in the extremities, sensory difficulties and, for developing infants, delays in walking and talking.

With present levels of contamination in the environment, it is generally agreed that most U.S. men are not at risk from mercury exposure, which usually occurs when people eat contaminated fish. But the EPA says up to 85,000 pregnant women who eat more than the average amount of freshwater fish could be endangering their unborn children, who are vulnerable to developmental damage from low levels of mercury.

Others at risk from the toxic metal include young children and groups with high freshwater fish-consumption rates, such as Indians and Asian-Americans.

The EPA has already proposed new standards aimed at reducing mercury emissions from municipal and medical waste incinerators by up to 95 percent. The use of mercury has also been phased out or greatly reduced in paint, batteries, electrical switches and other consumer goods.

But coal-fired power plants are still unregulated - and the utility industry, with help from commercial fishing interests and supporters in Congress, has been working furiously to keep things that way.

Since the EPA's draft report was completed in 1995, it has come under a broad, intense lobbying attack:

At left: Rob Reash, right, and Michael Robida of American Electric Power discuss mercury pollution in the Northeast. Reash, the company's senior biologist, says it hasn't been proven that the industry's mercury output contaminates fish.

Between them, the two senators have collected $107,300 in campaign contributions since 1990 from mining groups, utilities and other industries that could be affected by mercury regulations, according to the Federal Election Commission.

''Our position is that the study should use the best available science,'' said Linda Schoumacher, the institute's spokeswoman.

Snowe said in a written statement last week that she wanted to postpone only one part of the EPA's findings, a section that relates to mercury contamination in seafood and its potential health effects.

That section implied that eating seafood with low levels of mercury contamination could be a serious problem, ''even though there was no credible scientific evidence supporting such a conclusion,'' Snowe said.

Caution is important when dealing with public health issues, she added, but the report would have given the public misleading information - undermining the credibility of future efforts to protect public health.

The EPA has since rewritten the section that troubled Snowe, and the entire report has undergone another independent scientific review.

''I (now) support EPA's prompt issuance of the report,'' she said.

''The report . . . could be read as providing a rationale for a set of very expensive end-of-pipe control measures to reduce risks whose magnitudes are not yet well-understood,'' said a December 1995 council memo on the report.

Public health and money

The first involves public health, and the assumptions the EPA makes in its report about how dangerous low doses of mercury are to humans.

The EPA devised its estimates of how toxic mercury is to unborn children by studying the health problems of children in Iraq whose mothers ate seed grain that had been treated with a preservative containing mercury.

Critics argue that those findings shouldn't be used in judging dangerous fish-consumption levels - because the Iraqis ate grain. They also want the EPA to wait for the results of two ongoing studies of fish consumption and mercury.

Those studies, which focus on the residents of islands in the Atlantic and Indian oceans, could lead the EPA to reassess its assumptions about health risks.

The other issue surrounding the EPA report has nothing to with health. It's about money.

For utilities, the battle over mercury is in large part a fight to contain the enormous costs of meeting new environmental regulations.

At right: A plow pushes coal toward American Electric Power's Conesville, Ohio, plant, which burned 3 million tons of coal last year. Coal-fired plants are among the major industrial sources of mercury pollution.

Among its 39 power plants, spread through the industrial heartland of the Midwest, 20 are fired by coal. They account for 88 percent of the company's generating capacity - and most of its pollution-control costs.

American Electric Power spent $293 million over the last two years reducing air pollution, burying sludge and meeting other environmental laws. That amounted to less than 3 percent of the company's operating revenues in those years, which topped $11.5 billion.

It expects to spend another $200 million soon on equipment to meet the next phase of federal sulfur-dioxide emissions standards.

At the Conesville plant, tractors inch through cornfields in the shadows of the soaring smokestacks. Transmission lines, crackling with high voltage, slice a path toward the big cities of the Midwest.

The company spends $6 million a year here just burying the sludge produced by the sulfur-dioxide scrubbers on one smokestack.

The sludge landfill, which receives 35,000 truckloads of material a year, takes up 250 acres on the 13,000-acre plant site, a collection of old surface coal mines now overgrown with trees and brush.

To see so much money and effort go into a landfill, which doesn't generate any kilowatt hours, frustrates company officials.

''This is where the tail begins to wag the dog,'' said Jeffrey C. Lytle, a senior environmental specialist for American Electric Power. ''We're power producers, and we've had to stick a chemical-production process on the end of our operation.''

Big money is also at stake for the fishing industry.

In the Great Lakes alone, commercial fishermen land freshwater species worth more than $20 million a year. The state with the most landings, Michigan, collects $21 million annually in fishing-related income taxes.

If concerns about mercury contamination depress the public appetite for fish, the state economies supported by fishing could suffer.

That explains why congressmen like Rep. John D. Dingell, D-Mich., the ranking Democrat on the House Commerce Committee, have assailed the EPA report.

In an April 3, 1996 letter to Browner, the EPA administrator, Dingell accused the agency of moving too fast on the mercury issue.

''I am asking that you give your personal attention to this matter,'' he wrote, ''in hopes of avoiding embarrassment to the EPA, and in particular to your agency's troubled reputation for publishing research that serves a particular ideological agenda but lacks a sound scientific basis.''

Waiting for more science

Howard Fox, a lawyer at the Earthjustice Legal Defense Fund, a Washington environmental group, said the level of scientific certainty critics are demanding for mercury is unprecedented.

At left: Jeffrey Lytle, a senior environmental specialist for American Electric Power, explains his company's cooling system for treating water used in the power production process. The company closely monitors its release of heated water, but has focused less attention of the release of mercury. American Electric Power accuses the EPA of pushing for mercury regulations without the scientific evidence to support them.

Fox's organization, formerly known as the Sierra Club Legal Defense Fund, has sued the EPA to force it to turn the mercury report over to Congress.

He said formal submission of the report is a crucial political step because it signals the EPA's confidence in the findings and legitimizes public debate over regulating utility and other emissions.

''When you have something that's stamped 'draft' it's very easy for someone to dismiss it,'' he said. ''There are measures that might be taken by individual citizens, or by state governments, that will not be taken as long as this is a draft.''

Congressmen from states with contaminated waters, most notably Sen. Patrick Leahy, D-Vt., have tried to shake the report loose by sponsoring resolutions on Capitol Hill and lobbying President Clinton.

Clinton has made no public comment on the issue.

In Maine's congressional delegation, Reps. John Baldacci and Thomas Allen support the release of the report. So does Sen. Snowe, as of last week. Sen. Susan Collins has not taken a position, but is studying the issue.

The EPA now says it will transmit the report to Congress by the end of the year. Whether Dingell or other critics will try to delay the report further is unknown.

If and when the report is finally submitted, it will include several proposed methods for reducing coal-plant mercury emissions.

The options include switching to other fuels, washing coal extensively before it is burned and installing carbon beds or injecting activated carbon into smokestacks. Some of those technologies are used already at power plants in Europe.

The EPA's estimate of the cost: $2.9 billion a year for all U.S. utilities.

Executives cast a nervous eye at those numbers in places like downtown Columbus, Ohio, where American Electric Power runs its corporate holdings from a 30-story building overlooking the Scioto River.

The company's top environmental managers are among the 1,780 employees who work out of the pink stone and plate glass building, one of the tallest in Ohio's capital city.

Rob J. Reash, American Electric's senior biologist, questioned the need for controls on coal plants because no one has proven the utility industry's mercury pollution is what really contaminates fish.

The corporation's manager of air quality, Michael R. Robida, said tough limits on mercury could force utilities to switch from coal to more expensive natural gas, driving up the cost of electricity.

He questioned whether forcing coal-burning plants to spend millions on new pollution controls would significantly reduce mercury contamination in water and in fish.

A regional dispute

For some utility executives, that's what this political issue is really all about: a regional dispute.

Within the industry, some top executives say Northeastern U.S. competitors with more expensive power plants - which produce little or no mercury - want tough regulations to drive up the cost of Midwestern power.

''I'm afraid they may have more to do with some high-cost utilities in the Northeast hiding behind a kind of 'green smokescreen,' '' said William J. Lhota, an American Electric Power vice president, in a 1996 speech.

As the political debate goes on, mercury keeps filtering from the clouds, and scientists scurry to assess the environmental impact.

In Maine, researchers set up equipment on a site overlooking Highland Lake in Bridgton last summer to test rain and snowfall for mercury.

Maine also changed its fish-consumption advisory several months ago. The recommended limits on cold-water species like trout and salmon were loosened, after a re-examination of data showed those fish were less contaminated than warm-water species - bass, perch and pickerel.

But new testing of fish also forced the state to include rivers and streams with the lakes and ponds where advisories were first posted in 1994.

The warnings now apply to every body of fresh water in the state. And still, mercury levels in Maine are on the rise.