The massive coal-powered Navajo generating station in Page, Ariz., spews tens of thousands of tons of nitrogen oxide a year into Western skies, spreading haze across the Grand Canyon and other national parks. By law, the 40-year-old plant is supposed to install the "best available retrofit technology" to scrub emissions from its smokestacks.

But two Arizona Republicans have called a congressional hearing for Tuesday in an effort to block the Environmental Protection Agency from requiring the retrofits, which they say would cost $1.1 billion and could force the plant, which employs 1,000 people at the power station and a nearby coal mine, to close.

In a letter to the chairmen of the Natural Resources Subcommittee on Water and Power and the Subcommittee on Indian and Alaska Native Affairs, Reps. Paul Gosar and Trent Franks said the electricity generated by the plant, which pumps water from the Colorado River to Tucson and Phoenix, "is essential to supplying water to 80% of the state’s population. We must carefully examine regulations that could threaten the state of Arizona’s water and power supply.”

The EPA is scheduled to decide this summer whether to require pollution controls for the plant, which is one of the biggest sources of nitrogen oxide emissions in the country. "Our job is to decide, 'Are the parks adequately protected?' " said Colleen McKaughan, associate director of the EPA's air division in San Francisco. "And if they're not, does the facility need additional pollution controls?"

Environmentalists say the plant, besides obscuring the views in parks, is also a health hazard, responsible for high levels of asthma and respiratory disease on the Navajo and Hopi Indian reservations. It is also a major source of toxic mercury emissions into the air and rivers. With Arizona in the midst of building several large solar-powered plants, conservationists say the coal generator could be replaced by clean, renewable energy.