One of the state's largest taconite mines does not need a new environmental study to expand and keep operating for another 16 years, according to a Minnesota Court of Appeals ruling Monday.

A three-judge panel concluded that a lengthy environmental study is not needed for U.S. Steel to add 483 acres to its Minntac iron ore mine near Mountain Iron in St. Louis County.

The mine and its processing plant began operating in 1967 and have expanded twice before. The latest proposal would allow the mine to expand again and maintain its current rate of production until 2031 instead of closing in 2015. It employs 1,400 workers.

The Minnesota Department of Natural Resources, which regulates mining, prepared an environmental assessment for the expansion but said last April that a longer environmental impact statement, which could cost $1 million to $3 million, was not necessary.

The Minnesota Center for Environmental Advocacy took the matter to the Appeals Court and argued that a new and more comprehensive environmental study was needed to protect nearby waterways. One of the concerns is Minntac's 8,000-acre tailings basin that's used to dispose of mineral waste from the firm's crushing and concentrating plants. Heavy metals and sulfates that accumulate in the basin have leaked into the bordering Dark River and Sandy River watersheds, according to state studies.

"This is a facility that's been very damaging to wild rice harvests nearby and has violated state water quality standards in nearby lakes and rivers," said MCEA attorney Kathryn Hoffman. "The court seems unwilling to rectify that."

Hoffman said Minntac should be required to solve those problems before expanding its mine, not afterward.

The court noted that pollution from the tailings basin has been a problem since 2004, but cited evidence that Minntac has cooperated with state pollution officials and is making progress on reducing discharges, collecting and returning seepage water and monitoring groundwater quality.

The court also said the DNR can include additional requirements under an amended mining permit that Minntac needs to proceed with the expansion.

"This is not a situation in which [the DNR] failed to examine the potential environmental effects of the [mine] extension," the court said.

Steve Colvin, deputy director of DNR's ecological and water resources division, said the agency is pleased with the decision. The DNR "took a thorough and hard look at the complex mining operation, and provided a good direction for [future] permitting," he said.

In fact, the DNR sent an amended permit to U.S. Steel last week that will allow Minntac to begin the expansion, Colvin said, and the permit included "significant steps" to benefit the environment.

Minntac also needs permits from state pollution officials and the U.S. Army Corps of Engineers because the expansion will destroy some wetlands, Colvin said.

Hoffman said the environmental group may appeal to the state Supreme Court, but has not made that decision yet. She said years of promises have not produced cleaner water near Minntac's operations, so her trust level is low.