U.S. Rep Betty McCollum and two other Minnesota Democrats are asking federal regulators to give the public more time to comment on environmental concerns surrounding a controversial copper-nickel mine proposed by Twin Metals Minnesota LLC.

In a letter released Tuesday, they noted that the original 30-day public comment period, which was set to expire this week, has overlapped with the 32-day partial government shutdown, making it difficult for citizens to register their concerns with the U.S. Interior Department.

McCollum, who now chairs the House Appropriations Committee on Interior and the Environment, also said the project's environmental review deserves "special care" because the mine would stand on the doorstep of the Boundary Waters Canoe Area Wilderness and its pristine chain of lakes and rivers.

"Much is potentially at stake with the renewal of these leases. State and local governments, tribal nations, advocacy organizations, and interested Americans deserve sufficient opportunity to weigh in," McCollum said in a statement accompanying the letter.

A spokeswoman for the U.S. Bureau of Land Management (BLM), which oversees the Twin Metals leases and environmental assessment, said she could provide no updates late Tuesday.

McCollum, along with Reps. Ilhan Omar and Dean Phillips, and four other members of Congress, voiced their concerns in a Jan. 18 letter to Interior Acting Secretary David Bernhardt and Brian Steed, the BLM's deputy director of policy and programs.

The letter asks for as much extra time for comments as the number of days the government was shut down.

The group also urged the BLM to prepare a "detailed and rigorous" environmental review — known as an environmental-impact statement — of the 10-year Twin Metals mining leases, saying such a review is required under the National Environmental Policy Act because of the potential environmental effects.

The Boundary Waters wilderness, the group said, is "uniquely vulnerable to sulfide-ore copper mining pollution." Environmentalists say copper-nickel mining is riskier than Minnesota's incumbent taconite industry because its waste rock can produce acid that leaches heavy metals from the ground, threatening local lakes and rivers with toxins. The letter said the agency's review should include multiple public hearings and meaningful discussions with tribal governments.

Jeremy Drucker, a spokesman for the Campaign to Save the Boundary Waters, applauded the letter. Drucker said the BLM should consider whether hard-rock mining is appropriate for that sensitive region of the state and that his group couldn't get its comments on the draft environmental assessment through to the agency.

"We even tried to FedEx the comments, and FedEx said no one was at the door to pick them up," Drucker said. "It's amazingly frustrating."

Twin Metals Minnesota is a subsidiary of the Chilean conglomerate Antofagasta PLC. The two leases are for land located on Superior National Forest lands just outside the Boundary Waters near Ely.

Twin Metals is the second of two large mining companies to propose copper-nickel mines on Minnesota's Iron Range. The other, PolyMet Mining Corp., has received most of its required Minnesota permits after more than a decade of review. But environmentalists consider the Twin Metals plan riskier because, unlike the PolyMet mine, it would lie within the watershed of the BWCA.

Jennifer Bjorhus • 612-673-4683