A mining company was pumping water out of an active pit north of Duluth when its crew spotted in the shallows what looked like a mature colony of Minnesota's most notorious invader — the zebra mussel.

Now the question for the Department of Natural Resources (DNR) is how the damaging mussels were able to get a foothold in the Rouchleau Mine Pit in Virginia, Minn.

It's not unheard of for the tiny mussels to hitch a ride on boats, pipes or other equipment to establish themselves in abandoned mines that have long been filled with groundwater and turned into lakes. But the Rouchleau pit is closed to the public and its waters have seen little, if any, activity, said Richard Rezanka, a DNR invasive species specialist.

"It's weird and kind of an oddball thing that we are looking into," Rezanka said.

He stressed that the DNR isn't trying to place blame, but rather to figure out how the mussels made it to the pit and then devise a plan for slowing their spread and preventing it from happening again.

Zebra mussels have been plaguing waterways throughout the Midwest for decades. Once established, they kill or out-compete native mussels and filter out much of the nutrients and smallest edible material in a lake, robbing minnows and native organisms of essential food. They also produce razor-sharp shells that can make beaches and lake fronts unusable, while excreting a carpet of waste that produces toxic algae.

In large numbers, they form such thick clusters that they can clog pipes and drains, causing cities and companies across Minnesota to spend millions of dollars a year installing screens and scraping them out of water supply lines.

The mining company that owns the property, Cleveland Cliffs, had been pumping water from the pit into a drainage ditch that eventually flows through man-made channels into Long Lake Creek and the St. Louis River, which runs into Lake Superior. While the St. Louis has been infested with zebra mussels for many years downstream, near Superior, the invaders have never been spotted so far north.

After discovering the mussels, the company notified the DNR and immediately stopped pumping the water, Rezanka said.

The DNR will inspect the surrounding waterways to see if any mussels survived the pumping process and were able to establish a home downstream.

"That's pretty unlikely, quite frankly," Rezanka said. "But we're going to look downstream and see what we see."

It's unclear how long the zebra mussels have been in the pit. It can take three or four years for their populations to bloom to a point where they are easily spotted, Rezanka said.

"Based on what I saw, this is not a first- or second-year infestation, but a little more mature," he said.

Greg Stanley • 612-673-4882