For more than 25 years, Minntac Corp.'s enormous taconite waste pit in Mountain Iron has operated with an expired state pollution permit, allowing contaminated water to seep out into nearby lakes and trout streams.

Now, after years of appeals and lawsuits, the Minnesota Pollution Control Agency (MPCA) has issued a final, updated water permit to the mine's parent company, Pittsburgh-based U.S. Steel Corp.

The agency says the five-year permit, effective Dec. 1, sets vital new standards for the company and its tailings basin, which was built in the 1960s before many of the federal pollution regulations that are in place today.

Shannon Lotthammer, the MPCA's assistant commissioner for water policy, said the permit "lays out a path for further environmental improvement."

Among other things, the permit calls for U.S. Steel to reduce sulfate, a mining byproduct that can pollute water and damage aquatic plants, in the tailings basin within 10 years. It also requires the company to install a system to collect seeping pollutants on the western side of the basin and return the discharge back to the basin.

"The biggest element of this permit is the requirement to meet drinking water standards for sulfate in groundwater at the property boundary, and also meet surface water standards for salty pollutants," Lotthammer said in an interview Friday.

But at least one environmental group called the final permit a major disappointment — a sign that the matter is far from closed.

"This permit sets no standards that will adequately protect surface water from sulfate and other pollutants that impair wild rice, harm aquatic life and increase toxic mercury contamination of fish," said Paula Maccabee, advocacy director and lawyer for Duluth-based WaterLegacy. "It simply doesn't have limits on pollutants of surface water."

She said she was particularly troubled that the U.S. Environmental Protection Agency issued no comments. In the past, she noted, the federal agency has played a backstop role, encouraging the MPCA to place tighter restrictions on the tailings pit.

"This was really exceptional to me," Maccabee said. "That's what's really new."

A second conservation group, the Minnesota Center for Environmental Advocacy, sued the MPCA over the expired permit in 2016, alleging that the agency was failing to enforce clean water law. A spokesman said Friday that the group's lawyers hadn't had time to fully review the new permit.

Lotthammer called the sulfate reduction specified by the new permit "significant." The permit gives U.S. Steel 30 months to tell the MPCA how long it will take to meet surface and groundwater standards. The company must start construction or implement treatment systems within 54 months.

U.S. Steel applied for a variance from some of the standards, and that was denied, Lotthammer said.

In a statement late Friday, the company said: "U.S. Steel is pleased that MPCA acted to reissue the administratively extended Minntac Tailings Basin NPDES permit. We are currently reviewing the permit and evaluating the conditions."

There is a 30-day window for an appeal. The 96-page permit is posted on the MPCA website.