DULUTH, Minn. — Minnesota regulators will review the proposed expansion of a taconite mine after environmentalists warned of potential water pollution risks.

Northshore Mining wants to expand its mine near Babbitt by about 100 acres. But the Duluth-based environmental group Save Our Sky Blue Waters says the expansion could mean digging taconite out of a new area with rock that has a higher-than-usual sulfide content, which the group says could lead to acid mine drainage off the site.

The group petitioned the Department of Natural Resources for an environmental assessment worksheet, but the DNR declared the request moot this month because Cliffs Natural Resources, which owns Northshore Mining, filed its own request for a review.

The potential for acid runoff from sulfide-bearing rock has fueled the debate over whether the state should allow copper-nickel mining in northeastern Minnesota, because the area's untapped copper and nickel deposits are locked up in sulfur compounds. The issue hasn't come up often in relation to iron mining. However, the taconite mine near Babbitt is only about a mile from the proposed PolyMet copper-nickel mine.

"It's the same problem that we're facing with the copper-nickel mines, and the same potential for water quality impacts," said Jane Reyer of Save our Sky Blue Waters. "I just think we need to look at it regardless of what type of mine it's coming from."

Cliffs spokeswoman Sandy Karnowski described the expansion plan as "conservative" and said the company decided to work with the DNR to address any questions about it.

The Babbitt mine is near the Dunka Pit, an abandoned iron ore mine at the south end of Birch Lake, which contains sulfide-bearing rock. Environmentalists say it has been leaking acid, sulfates, toxic metals and other pollutants for decades.

"It's not necessarily the first time a taconite mine has dealt with the potential for acid mine drainage, but it's the first environmental review that I'm aware of to address the problem," said Kathryn Hoffman, an attorney for the Minnesota Center for Environmental Advocacy. "We talk about sulfide (copper) mining versus taconite mining, but sometimes the line between the two is gray."