POLLUTION : Studies Clouding Issue of Haze in the Grand Canyon : Power plant is blamed for obscuring scenic vistas. Federal regulators may order environmental controls.

The murk is thickening in the debate over dirty Grand Canyon air only weeks before federal regulators must determine whether to order expensive pollution controls at an Arizona power plant partly owned by the city of Los Angeles.

The National Park Service says its tests show that sulfur dioxide emissions from the coal-fired plant, the 2,250-megawatt Navajo Generating Station north of the canyon, are the main cause of a dense haze which often obscures scenic vistas.

But plant operators claimed recently that a $12-million study they commissioned cleared Navajo as a major haze source. And that proves, they argued, that nothing would be accomplished by forcing the utility to spend $1 billion or more on a cleanup--a cost that would be covered by jacking up consumer electric rates.

“The study pretty well reaffirms our belief that haze in the Grand Canyon is primarily a natural phenomenon,” explained Mike Rappoport, a spokesman for the plant. “Most of that which is attributable to man-made sources comes from the Los Angeles area.”

The scientific cross claims only complicate the task of the Environmental Protection Agency, which is under court order to decide by Feb. 1 if Navajo should be required to add sophisticated exhaust “scrubbers” to its stacks.

The plant is run by the Salt River Project, a Tempe, Ariz.-based utility. It is owned by a six-member consortium that includes Salt River and the Bureau of Reclamation, like the Park Service an arm of the U.S. Department of the Interior. Another partner is the Los Angeles Department of Water and Power, which draws about 13% of its electricity needs from Navajo.

Both environmentalists and industry representatives are closely watching the Grand Canyon question because it has the potential to break new and costly ground in the war against pollution. Nobody’s claiming that Navajo emissions pose any health or safety threat. Any cleanup would be for purely aesthetic reasons.

In a six-week 1987 study called WHITEX, the Park Service tracked a chemical tracer from the plant to a sensor in the heart of the canyon. As a result, the agency concluded that Navajo could be to blame for up to 70% of a dense wintertime haze that periodically descends on the canyon between November and March.

Experts from the National Academy of Sciences reviewed WHITEX findings and agreed that Navajo emissions “contributed significantly” to canyon haze, but stopped short of blaming the lion’s share of the visibility problem on the plant.

Salt River’s study, conducted last winter, was undoubtedly more exhaustive than WHITEX. It cost six times as much, covered a period nearly twice as long, followed the paths of two chemical tracers instead of one and collected data from 26 sensors in the canyon area as opposed to one.

The utility’s conclusion: Navajo emissions were a significant contributor to between only 4% and 7% of the visibility problem.

Bill Malm, a Park Service research physicist who worked on the WHITEX study, said Salt River officials are “emphasizing things that make them look good and de-emphasizing others.”

But Jerry Shapiro, an engineer who is Salt River’s chief consultant on Navajo, said: “I think it would be a shame if we put scrubbers in because politically it seemed like a good idea and then they had no impact.”