BILLINGS, Mont. (AP) — Environmentalists filed notice Wednesday that they plan to sue the six companies that co-own eastern Montana's Colstrip power plant over alleged pollution violations.

The Sierra Club and Montana Environmental Information Center say the plant's owners failed to upgrade pollution control equipment as required under the Clean Air Act for older power generation facilities that undergo significant changes.

Colstrip is the second largest coal-fired power plant west of the Mississippi River, burning over 10 million tons of the fuel a year to generate about 2,200 megawatts of electricity.

That power is distributed on high-voltage transmission lines to customers in Montana, Oregon and Washington state.

Over a two decade period beginning in 1992, attorneys for the environmental groups claim at least eight "major modifications" were made to the Colstrip Steam Electric Station.

Those changes "would result in emission increases of sulfur dioxide, nitrogen oxide and particulate matter," the groups stated in a Wednesday letter to the plants owners. Those pollutants can cause health problems in people and are linked to environmental problems including acid rain, haze and surface water degradation.

"We're pretty confident that over the long haul and with the amount of money invested in those plants, they much more qualify as upgrades rather than maintenance," said Ken Toole, a former Montana Public Service Commissioner and member of the Montana Environmental Information Center.

The 60-day notice of the environmental groups' intent to sue in federal court was sent to the plant's six co-owners — PPL Montana, NorthWestern Energy, Puget Sound Energy, Portland General Electric, Avista and PacifiCorp.

A spokesman for PPL Montana, which operates and owns a one-quarter stake in the plant, said changes made to the facility were part of a regular maintenance program. Spokesman David Hoffman said that falls outside the government's criteria for a "new source" of pollution that would require additional review.

Hoffman added that the threatened lawsuit appears to stem from a coordinated campaign against companies that mine, ship or burn coal.

Other lawsuits have targeted mining of the fuel in Montana and a proposed new railroad to service coal mines along the Tongue River. Environmental groups also have mounted campaigns against proposals to increase shipments of the fuel by rail to the West Coast for export overseas.

"I see this as another effort to continue this attack on coal," Hoffman said of Wednesday's notice. "We take plants offline on a regular basis for maintenance, just like you might have oil changed in your car."

The first two units of the power plant began operating in the mid-1970s and two more units came online in 1984 and 1986.

Because the plant was grandfathered in under the Clean Air Act, it was not required to comply with tougher pollution control standards unless the plant's operator made upgrades modernizing it.

The industry argued at the time that plants such as Colstrip would eventually be retired, but many have instead been upgraded and expanded without the same pollution controls as newer plants, according to the Sierra Club.

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