State regulators may have to redo a major portion of the environmental analysis of a controversial open pit copper mine proposed for northeast Minnesota, which could add months to the project’s timeline.

New data on the flow of the nearby Partridge River indicate that some of the major assumptions used in the analysis may be three times too low, which would throw off many of the conclusions about the mine’s potential impact on water. Fixing it could require redoing the complex computer model the entire analysis is based on, officials said.

Steve Colvin, who has headed the environmental review for the Minnesota Department of Natural Resources, said state scientists are now reviewing the analysis to determine whether it needs to be redone.

“That’s one of the things our experts will have to evaluate,” he said. “Do they think there is likely to be a sufficient difference such that we would remodel it?” He said that building a model with the new data could take “some months. I don’t know if it’s a few or several. ”

The glitch came to light just as thousands of people are weighing in on it during the legally required public comment period now underway for the $650 million project.

The state also may decide to add other elements as new data and new information rise up through the public comment period, which ends in March. Officials also may decide to add another year’s worth of flow data to the model to increase its accuracy, he said, but he declined to say how that could change the timeline of the project.

Colvin said continually updating an environmental-impact statement with new information is an expected part of the process. But environmental groups and the Indian tribes who have said since 2008 that the water flow assumptions were wrong pounced on the news. It was first reported Thursday by the Timberjay, an Iron Range weekly newspaper.

“We have tried to raise a red flag on it for a long time,” said Nancy Schultz, environmental specialist for the Fond du Lac Tribe of Chippewa, which has been a participant on the environmental analysis along with other tribes and state and federal agencies. “This is one of the key parameters that you build a model on.”

Bruce Richardson, a spokesman for PolyMet Mining Corp., which has proposed the mine, said, “It’s premature for us to speculate on what the DNR might or might not do with the information they are gathering.”

Meeting in St. Paul up next

PolyMet’s Northmet project has drawn intense scrutiny and controversy. It would be the first copper-nickel mine built in the state. And while it promises 300 to 350 jobs and a resurgence of mining along the state’s Iron Range, it also brings what many view as unacceptable environmental risks.

Last week, about 1,300 people braved frigid weather for a five-hour nighttime meeting in Duluth to hear about the project from state and federal regulators. This week about 650 turned up at a meeting in Aurora, and next week many more are expected to attend the last of the public meetings in St. Paul.

The 2,200-page document analyzes all aspects of what could happen to the air, wildlife, surface and groundwater if the open pit mine is built. The analysis relied on data from a gauge 17 miles downstream from the site in the Partridge River, which starts near the mine site and flows into the St. Louis River and, eventually, Lake Superior. The original data showed an average flow of 0.5 cubic feet per second. But a new gauge was recently installed closer to the site in connection with another project, Colvin said. And that one showed that in 2011 and 2012 flows averaged 1.3 to 1.8 cubic feet per second.

“It’s an extremely important input,” said Kathryn Hoffman, an attorney with the Minnesota Center of Environmental Advocacy, a nonprofit law firm. She commended the DNR for being willing to consider rebuilding the model, because if it’s wrong “their conclusion that this mine won’t violate water quality standards could be incorrect.”

Paula Maccabee, an attorney who works with the tribes, said the scientific accuracy of the analysis is critical for a major and controversial project that has split opinions in the state.

“The only way that Minnesotans can think of proceeding is if the science is accurate and complete,” she said. “Something this fundamental casts aspersions on all the science.”

Once the public comment period is closed, the regulatory agencies will review the comments and incorporate them into a final environmental impact statement. After that is complete, the company would apply for permits, which state and company officials have said could happen late next year.