The Colville Tribes are celebrating after securing a huge victory in a long-running battle against the world's largest mining company.

In a unanimous decision, the 9th Circuit Court of Appeals on Friday backed the tribe's effort to hold Teck responsible for polluting its homelands in Washington state. The Canadian firm has been sending toxic mining waste down the Columbia River for nearly a century.

“The Colville Tribes is very pleased that the Ninth Circuit agrees that Teck has polluted our river for decades and that the company is responsible for the costs of cleanup,” Chairman Rodney Cawston said after the ruling. “This has been a long and very difficult road for our tribes, but we have never wavered in our efforts to force Teck to do the right thing.”

Doing the right thing includes admitting responsibility for polluting the Columbia, which has sustained the Colville peoples since "time immemorial," the 9th Circuit noted. Teck argued that it cannot be held liable because its mining operations are located across the border in Canada.

The court concluded otherwise. By "purposefully" dumping waste into the river -- and knowing it will cross the border into the United States -- Teck is subject to the federal Superfund law, Judge Ronald M. Gould wrote.

"It is inconceivable that Teck did not know that its waste was aimed at the state of Washington when Teck deposited it into the powerful Columbia River just miles upstream of the border," Gould wrote in the 55-page ruling

Additionally, since Teck falls under U.S. law, the court upheld an award of more than $8.25 million in "investigation costs" against the company. After filing the lawsuit in 2004 , the tribe has spent more than a decade proving that the toxic waste originated at the world’s largest lead and zinc smelter in Trail, British Columbia , the 9th Circuit concluded.

"Teck has maintained before and throughout this litigation that many other sources, including other smelters, are to blame for the Upper Columbia River’s pollution," Gould wrote. "The tribes commissioned a study investigating this claim, but the results show that the wastes match the Trail smelter’s isotopic and geochemical 'fingerprint.'"

9th Circuit Court of Appeals on YouTube: 9oseph Pakootas v. Teck Cominco Metals, Ltd.

Though the dispute is far from over, the victories mean the tribe is on solid ground in forcing Teck to pay for the future costs of cleaning up the mine waste. The state of Washington also intervened in order to hold the company responsible.

“This is a unique case, not only because a Canadian mining company has been found liable under U.S. law, but because an Indian tribe and a state have joined forces to protect a shared treasured resource—the Columbia River,” Cawston said on Friday. “Today’s ruling means that - the polluter, not U.S. taxpayers, would pay the cost of remedial action.”

According to the tribe and to the state, Teck has released 9.97 million tons of slag and effluent since late 1800s. The waste has affected the ability of Colville citizens to use the Columbia for fishing, recreation and other activities.

"The river is the natural resource and cultural lifeblood of the Colville Tribes and must be protected and restored," the tribe said after winning the award of $8,253,676 back in August 2016, an issue that was part of the appeal resolved by the 9th Circuit.

A close-up of a "black sand" beach along the Upper Columbia River in Washington. Photo from Environmental Protection Agency

In addition to polluting water and harming fish, Teck's mine waste has been deemed responsible for the so-called "black sand" beaches of the Upper Columbia. According to the tribe, around 150 miles of the river have been affected by slag, or the by-products of the lead and zinc operations in Canada.

Despite fighting the tribe in court, Teck has entered into settlements with the Environmental Protection Agency over the years to address the impacts of its operations. A 2015 agreement , for example, provided for the cleanup of 15 properties -- including a Colville allotment -- in Washington.

In a 2006 settlement , Teck agreed to study the "nature and the extent of contamination" along the Columbia. But the firm said it was only doing so on a voluntary basis.