MINNEAPOLIS (AP) — Opponents of the proposed PolyMet copper-nickel mine in northeastern Minnesota charged Thursday new data show that the state’s new draft environmental review for the project underestimates how much contaminated water could flow from the mine, raising serious questions about the long-term costs of treating it.

Hydrologists working for American Indian tribes in the area, which are critical of the proposed mine, have been telling regulators for some time that the hydrological model on which the analysis rests underrepresents the true “baseflows” of water at the site near Babbitt and need to be fixed. The Timberjay newspaper, which first reported the concerns about the model, said redoing the work could delay completion of the review and push back the start of the permitting process.

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A memo to regulators from John Coleman, environmental section leader with the Great Lakes Indian Fish and Wildlife Commission, said the model was based on “unrealistically low baseflows,” undermining the model’s conclusions about how contaminants might flow from the site, which is near the headwaters of the Partridge River.

“It is unlikely that any accurate predictions of water movement, transport of contaminant mass, or contaminant levels can be made when the characterization of the hydrologic system is so out-of-kilter,” he wrote in 2012.

But the Department of Natural Resources defended its review, which runs nearly 2,200 pages and is formally known as a “supplemental draft environmental impact statement.” The modeling was based on good data, DNR spokesman Chris Niskanen said. But one year’s worth of data from a new monitoring station closer to the mine site has become available and hydrologists are discussing what to make of it, he said.

The new data show a flow rate of 1.3 to 1.8 cubic feet per second, compared with the 0.5 cubic feet per second the model uses, which was extrapolated from data from a gauge farther downstream, according to a DNR memo from December.

“Does this mean we have to re-run the model? We’re not sure yet. Is it a deal-breaker for the SDEIS? It is not,” Niskanen said.

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But Polymet critics said the new data confirm that the tribal hydrologists were right and suggest that the project’s planned treatment plants will have to process larger volumes of water than expected.

“This environmental plan seems to be standing on pretty shaky ground,” said Paula Maccabee, an attorney for the group WaterLegacy.

“This is not a small problem,” said Betsy Daub, policy director for Friends of the Boundary Waters Wilderness. “This is a significant problem with the water model. We are hopeful the agencies will take the time they need to rectify this problem.”

Niskanen said concerns expressed about the water modeling will help the agency create a better final version of the report.

“This is precisely the way the process should work and these are precisely the kinds of conversations we should be having,” he said.

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