Controversy surrounds a proposal to place a copper mine near the Boundary Waters wilderness in Minnesota.

Jim Brandenburg/Minden Pictures/National Geographic

Q&A: Escalating battle over Minnesota mine puts spotlight on studies of potential impacts

Two Democratic lawmakers poised to rise to powerful positions in the U.S. House of Representatives are demanding that President Donald Trump’s administration explain its decision to abruptly abandon a study of the potential environmental impacts of mining on wildlands and waterways in northern Minnesota. The move marks the latest twist in what is becoming a major political battle over mining on U.S. public lands.

In a letter sent last week to Secretary of the Interior Ryan Zinke and Secretary of Agriculture Sonny Perdue, representatives Betty McCollum (D–MN) and Raúl Grijalva (D–AZ) asked the officials to detail why they prematurely ended a 20-month-old environmental assessment aimed at examining the risks that a proposed copper and nickel mine might pose to 95,000 hectares of federal land within the Rainy River watershed. The study began in 2016, after former President Barack Obama’s administration moved to bar mining in the watershed, which sits within the Superior National Forest next to the Boundary Waters Canoe Area Wilderness. But the Trump administration canceled the study in September, saying it had revealed “no new scientific information” on mining risks. It also announced it would renew mineral leases in the watershed. One company, Twin Metals Minnesota based in St. Paul, has long eyed the area for a large open pit mine.

The Trump administration should immediately halt those leasing efforts, say the two lawmakers, who will become senior members of the House when Democrats take control of the chamber in January. McCollum is expected to lead a spending panel that oversees public lands, and Grijalva is expected to lead the House natural resources panel. Agency officials violated federal environmental laws by canceling the study, they alleged in a previous letter sent on 5 November. And they “appear to have disregarded scientific information” in many “new scientific reports detailing the risk of sulfide-ore mining.”

In particular, the lawmakers pointed to an array of studies by state and academic researchers that detail potential problems associated with so-called metallic sulfide mines, such as the one proposed by Twin Metals. When sulfide ores or waste tailings that contain copper and other metals are exposed to air and water, chemical reactions create sulfuric acid, contributing to highly acidic runoff that can harm aquatic life. Many mines continue to produce acidic runoff for decades after they have closed, as water continues to percolate through pits and tailings. Treatment systems can reduce the acidity, but the process can be expensive.

Sulfate released by the mines also can set off a biochemical chain reaction that enables methylmercury, a potent neurotoxin, to build up in fish and other organisms. The sulfate can also reshuffle food webs, killing aquatic plants and helping feed problematic algae blooms. In Minnesota, excess sulfate has been shown to kill wild rice, a culturally and economically important aquatic plant.

One researcher involved in examining mine impacts is biogeochemist Lawrence Baker of the University of Minnesota in St. Paul. In 2013, the nonprofit group Northeastern Minnesotans for Wilderness asked Baker to evaluate the potential impacts of a sulfide mine near the Boundary Waters Canoe Area Wilderness. He recently spoke with ScienceInsider about that work. This interview has been edited for clarity and brevity.

Q: Why has the proposed Twin Metals mine been controversial?

A: We’re not talking about something out in the plains, or on a desert. We're talking about an area that is immediately adjacent to a wilderness area. It’s a major recreational area; 250,000 people a year go into the Boundary Waters.

Q: What are these mines like?

A: Since less than 1% of the rock, the ore, is copper, it means you have an enormous amount of tailings leftover. The rock gets crushed with these enormous grinders basically to a powder. They run it through a series of flotation steps with different densities of fluids … to get different parts of the ore to settle. It’s not just copper. They try to get gold and other things, too. In the end, you end up extracting 0.7% or less of the rock. The rest of it becomes the tailings.

Q: What are some of the potential environmental impacts?

A: Many lakes up here are fairly sensitive to acidification: It wouldn’t take a whole lot [of acid mine drainage] to cause some lakes to suffer.  If [the water] gets below pH 5, your sport fish will go away. The most delicate fish are actually not the game fish. It’s the minnows that they eat that are very sensitive. They’re the first thing to go. If they go, the larger fish will go.

We also have a mercury problem in our state. Mercury is produced mainly by combustion of coal. Additional sulfate tends to make the [mercury] problem worse in a fairly complex way. The sulfate gets reduced to sulfide [in water and sediments]. That tends to mobilize the mercury, converting it into methylmercury, which is more soluble and accumulates in fish. [Sulfate also has] the indirect effect on wild rice. From a sulfate standpoint, it’s the worst place you could put a mine.

We are also concerned with flows in a river. Simply by pulling water out of the river and using it for mining, you would lower the [lowest] flow. If you have a constant input of pollutants, the impact of those pollutants would be worse at low flow. Therefore, for any other pollutant—for example, a leaking septic system—the effects would be worse [at low flows].

Q: What about the mine tailings?

A: Normally, you build some sort of a dam—typically a rock dam or a pond, or a great big pit, or many great big pits. To a varying degree, [the wastes] solidify. When they put it in, it’s a slurry. The problem is that these tailings dams tend to break. There are about two large tailings dam failures in the world per year. The Mount Polley mine in British Columbia, Canada, which is about 100 miles north of the Twin Metals site, failed [in 2014].

Q: Can a company do anything to mitigate the potentially negative effects?

A: You can continue active treatment [of the acidic mine runoff] forever. That’s what you have to do. I don’t think we can predict what’s going to happen 20, 30 years after a mine closes. There has to be vigilance well beyond the closure of the mine.