BOISE, Idaho - A coalition of environmental groups is challenging in federal court a decision to allow the J.R. Simplot Co. to expand its phosphate mining operation across part of a wilderness area in southeastern Idaho.

The lawsuit filed in U.S. District Court on Friday seeks to stop the company from striking out in new territory near its Smoky Canyon mine in the Caribou-Targhee National Forest.

In court papers, the environmentalists argue the U.S. Forest Service acted arbitrarily when it approved the expansion plans in June.

They also point to past pollution problems, including the presence of toxic selenium caused by previous phosphate mining, that helped the area earn Superfund status in the 1990s. The groups also disagree with the agency's decision to clear the way for mining across more than 1,100 acres of roadless forest.

"We're very concerned about the fact that this is going to turn substantial acreage of roadless land into an open pit phosphate mine," said Tim Preso, an attorney for Earthjustice in Bozeman, Mont. "We doubt this whole project is going to work in the way they claim it will to prevent another Superfund site in southeast Idaho.

The lawsuit names as defendants only federal agencies and top administrators, including Lawrence Timchak, supervisor of the Caribou-Targhee National Forest, Bureau of Land Management Director Jim Caswell and Interior Secretary Dirk Kempthorne.

Lynn Ballard, spokesman for the Caribou-Targhee National Forest, declined comment, citing agency policy not to discuss matters in litigation.

Last week, Deputy Regional Forester Cathy Beaty said Timchak complied with applicable laws and regulations in his decision to approve the expansion.

Simplot, along with Monsanto Corp., have mined areas in southeast Idaho for decades to feed their fertilizer factories. Simplot officials say expansion of Smoky Canyon, along the Webster Range 10 miles from the Wyoming border, is critical to keeping its plant near Pocatello running through 2025. Monsanto's proposed new Blackfoot Bridge mine, near Soda Springs, is also under federal review

But Smoky Canyon and 17 other former mines scattered along the edge of the Greater Yellowstone Ecosystem continue to pose an environmental threat to clean water, fish and wildlife. All are designated under Superfund status and Smoky Canyon has existing problems linked to selenium pollution leaking into streams and groundwater.

In large amounts, selenium can cause liver disease and death and can quickly move up the biological food chain. Biologists have already detected toxic amounts of it in fish in streams near the Smoky Canyon Mine, and dead salamanders and deformed bird eggs with elevated levels of selenium have been found in the region, according to the lawsuit.

In December 1996, five horses grazing on private land downstream from one of southeastern Idaho's more than 30 phosphate mine sites were poisoned with selenium and had to be destroyed. A year later, more horses and hundreds of sheep also died not far from another phosphate mine near Soda Springs.

In their lawsuit, the environmentalists contend that because so little has been done to fix pollution from the past, the Forest Service should be stopped from allowing companies to do more.

The suit says the agency's environmental analysis and approval is based on unfounded scientific assumptions, analytical shortcuts and irrational conclusions, "all of which set the stage for another toxic pollution site."

The groups also raise the question of broadening Smoky Canyon into undeveloped, roadless forests.

Other plaintiffs include the Greater Yellowstone Coalition, the Natural Resources Defense Council, Sierra Club and Defenders of Wildlife.

Simplot, which started removing phosphate from Smoky Canyon's open pits in 1984, announced five years ago its hopes of extending operations to the south.

But the expansion would push into federal wilderness lands once deemed too sensitive by agency officials for mining.

For the past two years, Idaho has been working with the Forest Service on a rule to manage the state's 9.3 million acres of roadless land. The most recent version of that plan removes the Smoky Canyon expansion acreage from protected roadless status, a conflict the groups are also challenging in the lawsuit.