ON SOME DAYS, visitors at the Grand Canyon can barely see its opposite rim. In part that's because the canyon acts as a gathering point for pollution-laden winds from sites as distant as Southern California and northern Mexico. During much of the year, this makes it difficult to determine the amount of pollution that comes from particular locations. But in winter, the winds come from the northeast. It's then that one northern Arizona power plant accounts for significant amounts of the haze, according to the Environmental Protection Agency and the National Park Service.

In what would be the first use of federal law to protect the scenic vista of a national park, EPA Administrator William K. Reilly has proposed that the Navajo Generating Station should reduce sulfur dioxide emissions by 70 percent, at a cost of as much as $2.3 billion. Over the years, however, the EPA had dragged its feet. In 1982 the Environmental Defense Fund sued to force the EPA to uphold provisions of the 1977 Clean Air Act that enforced "visibility standards" in national parks. The Defense Fund won in 1984, and as part of a court order, the EPA undertook a pollution study at the Grand Canyon.

Other federal officials are also on both sides of this fence. The National Park Service supports the EPA's call for a forced cleanup of emissions at the plant. The federal Bureau of Reclamation actually owns a 24 percent share in the generating plant, with the rest owned by a consortium of public and private utilities. Those owners say that the pollution studies are flawed and that the plant is not a primary factor. But the Navajo plant is one of the largest coal-fired plants in the West and one of the biggest producers of sulfur dioxide, releasing 12 to 13 tons of that pollutant every hour.

The EPA's proposal is also supported by two scientific studies. After the plant's owners questioned a 1987 analysis of canyon pollution, a second analysis was performed by the National Research Council, an affiliate of the National Academy of Sciences. That study said the Navajo plant contributes significantly to the haze that affects Grand Canyon views.

If the EPA proposal stands, the costs of the cleanup would be ultimately borne by taxpayers and the customers of the Navajo Generating Station. In the past, such costs have generally been justified in terms of health benefits and prevention of damage to the economy. This would mark the first time that such standards would be applied for purely aesthetic purposes. If there is one place in the United States where an investment in cleaner air is justified, it's the Grand Canyon.