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The Kensington tailings - pulverized rock left over from the planned mine's gold-extraction process - are just as clean as natural sediment or the sand at Juneau's Sandy Beach, company officials claimed in numerous ads and articles.

"None of the data suggest that Kensington tailings may be contaminated," Coeur consultant Ed Kline stated in a memo to Coeur last January. He ran the toxicity tests in Juneau.

But the slurry of water piped along with the tailings as they are dumped into Lower Slate Lake will exceed various water quality standards. The slurry includes a few chemicals from the ore processing mill. The tailings themselves exceed the criteria for chromium, a heavy metal, according to the project supplemental environmental impact statement.

"If you look at the concentrations of dissolved metals and the (acidity), they are all well in excess of water quality standards for water organisms," said David Chambers, an independent scientist in Bozeman, Mont., who analyzed the environmental impact studies for the mine.

Kline advised Coeur that it should not have to routinely test the tailings for toxicity during Kensington Mine operations. One reason is that such testing will not be relevant until the mine's final period of operations, Kline stated in the Jan. 10, 2005 memo. The lake is not required to meet water quality standards until then.

In the end, Coeur must restore the aquatic life in Lower Slate Lake or forfeit its reclamation bond.

The question of the tailings' toxicity erupted in Juneau this fall and winter after environmental groups sued federal regulators who approved a permit allowing Coeur to fill the subalpine lake with tailings.

A letter-writing campaign ensued, with Coeur, mineral specialists and environmentalists weighing in on the tailings' chemical composition and its effects on the lake, both short term and long term.

Toxicity isn't a critical issue in the lawsuit, said Buck Lindekugel, an attorney for the Southeast Alaska Conservation Council, a lead plaintiff in the case.

It has become an issue in Juneau because Coeur keeps claiming it will create a "bigger, better lake ... and there isn't going to be any impact," Lindekugel said.

"Are the (tailings) toxic? It's clear that they are," Lindekugel said.

Coeur Alaska officials declined to comment in detail for this story, citing the sensitivity of continuing litigation over their environmental permits.

Coeur tests indicated that the many metals in the tailings are lower in concentration than what exists in Lower Slate Lake sediment. But the acidity and dissolved sediment of the tailings slurry does exceed natural conditions, according to federal reports.

Regulators said the acidic slurry - about 210,000 gallons per day - would dilute in the lake. Any fish remaining there would move away from the acidic material, they added.

The company is allowed, under its permits, to exceed water quality standards in the lake. Regulators will require Coeur to scrub the pollutants out of the water, using reverse osmosis, before the Lower Slate Lake water empties into a stream.

A second point raised by those who call the tailings toxic: Aquatic and plant life in the lake is not expected to survive the filling of the lake with 4.5 million tons of tailings. Coeur plans to remove Dolly Varden char from the lake before the dumping begins.

And that brings up the last point: Can the company really restore the lake when it ends operations?

Kline and state regulators contend that restoration of the lake is a no-brainer, noting their successful tests of the tailings in ocean water, and other analyses of the tailings' composition.

"You can't take that as an absolute," Chambers responded. "If you look at the literature, you keep running into these aberrations at mines. ... It's tough to have a really good handle on what is going to happen."

In Kline's tests, run on the Auke Bay floor during his doctoral studies, little aquatic bugs successfully colonized the Kensington tailings.

Other toxicity testing, run in an independent laboratory, came back with spotty results. Some testing with amphipods (small crustaceans that are not common in Lower Slate Lake) had poor results. A more common Lower Slate Lake species, the midge, also had a poor result in a test involving emergence, a critical point in its life cycle in which it swims to the surface.

Such aquatic bugs are an important part of the food chain in the lake.

The Environmental Protection Agency repeatedly questioned the poor test results, saying that "the potential toxicity of the tailings cannot be ruled out. Further testing is necessary to determine the actual cause of toxicity," a regional EPA director wrote in a letter to the U.S. Forest Service.

At that point, the EPA favored putting the tailings in dry storage, where there would be less exposure to the environment.

Coeur contends that it will save money, reduce the size of the mining operation and use less fossil fuel with the lake disposal option.

After some controversy over the matter, EPA dropped its previous stance and granted a permit for the water discharge from Lower Slate Lake to the stream.

Coeur has not run any more tests on its tailings. The company told regulators that it had used all of the available Kensington tailings during prior tests, said Kenwyn George, an environmental engineer for the Alaska Department of Environmental Conservation.

The lake must, at closure, support life similar to what it has now, George said.

Coeur believes strongly that the permits were properly approved by the agencies, said Luke Russell, vice president of environmental services for Coeur d'Alene Mines Corp., which owns the Kensington Mine.

Russell declined to address most questions for this story, citing concern from the company's attorneys.

If Coeur can't prove that the tailings are habitable, it will likely be required to cover the bottom of the new, larger lake with clean sediment, according to federal documents.

State officials said last week that the toxicity studies of the Kensington tailings will resume after the mine commences operations. The tailings will be placed in trays in nearby Upper Slate Lake, to see how lake organisms respond to them.

"It's a continuing study to ensure that the tailings will be habitable and the lake will be restorable - which we think it will be," George said.

As Kline suggested last January, however, the state will not require routine analysis of the toxicity of the tailings during mine operations.

• Elizabeth Bluemink can be reached at elizabeth.bluemink@juneauempire.com.