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An administrative court has recommended denying a key permit for PolyMet Mining's open-pit copper-nickel project, asserting that its design would risk contaminating too much water.

Administrative Law Judge James E. LaFave wrote in a ruling released on Tuesday that the company's plan to apply bentonite clay to an old taconite tailings basin, and then put waste rock from its hardrock mine on top, would not satisfy the state's rules because it was not a "practical and workable" solution.

As a result, PolyMet's permit to mine should be denied by the Minnesota Department of Natural Resources, LaFave wrote.

"That's an amazing result, and a huge victory," said Paula Maccabee, an attorney with the nonprofit group WaterLegacy, one of several organizations that challenged the tailings basin design.

The ruling is not a final decision — it will be sent to back to DNR, which can accept or reject it. LaFave also wrote that if DNR decides differently and proceeds with the permit, the agency should attach special conditions to manage water seeping through mine tailings.

"We're reviewing the [administrative law judge's] recommendation and evaluating our options at this time," Bruce Richardson, a spokesman for PolyMet and NewRange Copper Nickel, wrote in an email.

PolyMet wants to build a massive, open-pit hardrock mine near Babbitt, Minn., and use the former LTV Steel site in Hoyt Lakes to process the material. Hardrock mining has been controversial in Minnesota because of an increased risk of acid mine drainage, compared to the iron mining that's traditionally dominated the state.

Since proposing its mine, PolyMet, which is owned by the international conglomerate Glencore, formed the NewRange partnership with a subsidiary of the Canadian firm Teck Resources. Teck was developing its own mining project next door.

PolyMet's proposal has been stalled by other permitting issues — most recently, the Minnesota Supreme Court revoked a key water pollution permit in August.

In a statement, DNR Deputy Commissioner Barb Naramore wrote that an unnamed senior leader at the agency will review the recommendations and solicit comments from the parties in the case before making the final decision on the permit.

"They and their legal counsel also have not had, and will not have, any contact with the DNR permitting team regarding the specifics of the case," Naramore wrote.

The ruling comes after a five-day hearing this spring, in which a coalition of environmental groups, the Fond du Lac Band of Lake Superior Chippewa, the DNR and representatives for PolyMet all presented their sides in the case.

The issues covered were narrow: Could the use of bentonite clay on the former LTV Steel tailings basin stop water from filtering through waste rock? If not, acid mine drainage could escape into the Lake Superior watershed.

Bentonite clay is used in many applications, including at other mine sites in Minnesota. It's also an ingredient in some cosmetics. In this case, the clay was supposed to reduce water and oxygen infiltration on the sides and bottom of an already water-filled tailings pond.

Despite recommending denial, LaFave's ruling asserted that bentonite could be used effectively to coat the sides and bottom of the pond, and that it would reduce water and oxygen infiltration.

"I don't quite know what to make of that [part of the ruling]" said Chris Knopf, executive director of the group Friends of the Boundary Waters.

But Knopf pointed to another section which detailed an undisputed fact — that 298 million gallons of water would still seep from the tailings each year.

DNR and PolyMet argued this was just a tiny proportion of all the water in the watershed, and a fraction of a percent of the water contained by the tailings pond. LaFave decided, however, that the design did not stop "substantially all water" from contacting the tailings and flowing away, as required by state rules.

"If a hauler wanted to transport 298 million gallons of water by truck ... it would take 27,091 trucks to carry the water. If those trucks were lined up bumper-to-bumper, the convoy would stretch 271 miles — approximately the distance between St. Paul and Grand Marais," he wrote.

In a statement, Fond du Lac Band of Lake Superior Chippewa Chair Kevin R. Dupuis Sr. wrote that the decision served to safeguard the natural resources that Native people rely on, and that treaty rights protect. Fond du Lac's reservation is downstream of PolyMet's planned mine.

"Today's decision by the ALJ would protect these resources for the Band and all Minnesotans," Dupuis wrote. "DNR must accept the ALJ's decision."