This is part one of a two-part series focusing on the safety of PolyMet’s tailings basin design.  

A major consideration in any new mine development is what to do with the crushed and ground rock that’s left over after the economic metals have been removed from the ore. Called tailings, this leftover silt and sand-like material is usually permanently stored in large basins or impoundments.

PolyMet has a big advantage over other new mine development projects because we plan to use a former taconite tailings basin that already exists at the site to store our tailings. This gives us the benefit of more than 40 years of stable tailings basin operations to build on and the added benefit of not having to disturb new land.

Our tailings basin structures such as berms and dams will be built from taconite tailings left from the previous operation. Once dams are constructed to the appropriate new elevations, PolyMet tailings will be pumped to the basin where they will settle and be permanently stored. Water that collects and pools in the basin during operations will be constantly recycled back to the plant for reuse.


Tailings basins are highly engineered structures that are continuously maintained, monitored and inspected for stability. Minnesota is home to dozens of large tailings basins, many of which have been in existence for decades.

 Here are six important things you need to know about the PolyMet tailings basin:

  1. PolyMet will re-use an existing tailings structure. We will use a tailings basin that began use in 1957 and was idled in 2001 when the LTV Steel taconite mine closed. The eastern portion of the basin to be reused is about 2.1 square miles in size and up to 125 feet high. It will rise an estimated 120 additional feet over the 20-year permitted PolyMet mine life, ultimately reaching the height already reached by the western portion of the basin.
  2.  Process water that accumulates in the tailings basin along with rainwater is predicted to have about one-tenth of one percent sulfur content, and will react with tailings naturally containing neutralizing carbonate, so that it does not produce acidic water or a condition commonly referred to as acid rock drainage. Put another way, the water will be pH neutral.
  3. There will be limited ability for tailings to release mercury and other heavy metals. Because most of the new tailings will be deposited underwater in the basin, the amount of tailings that will be exposed to oxygen and thus have the ability to release heavy metals will be limited. In addition, the non-acidic tailings water will help keep some of the trace amounts of copper, nickel and heavy metals that occur in the rock naturally and are not extracted during processing, from dissolving in the water. Some of the metals that do dissolve will attach to other tailings material and sink out of the water. And finally, any water that may be eventually discharged from the tailings basin will first pass through an on-site treatment plant where any remaining metals will be removed.

    All water on the PolyMet project site will have low concentrations of mercury, and any water discharges will comply with the Great Lakes Initiative standard of 1.3 ng/L for mercury. In fact, mercury concentrations from PolyMet discharges will be about three times lower than what is found in natural runoff from the watershed and about eight times lower than found in rain falling on the watershed.

  1.  A groundwater containment system will be constructed at the beginning of operations along the northern and western perimeters of the tailings basin to capture and pump back any water that seeps from the tailings basin. This barrier wall, which will extend from the ground surface to bedrock, is made of a clay-like material called bentonite and will extend nearly five miles around the portion of the tailings basin where groundwater is known to migrate. This also serves to address legacy groundwater issues created by former taconite mining in the area. Barrier walls such as these are a proven technology for containment.
  2. A permanent pond will minimize future leakage and potential for water quality impacts. At closure, a permanent pond that will support plant and wildlife will be formed at the tailings basin. The pond will continue to be monitored for water quality, and the basin will continue to be monitored for stability.
  3. Amended pond bottom. The pond bottom and exposed beach areas will be amended with bentonite to limit oxygen infiltration and water percolation into the tailings after mining ends.

Find out more on the PolyMet tailings. Next: in Part 2 of our tailings blog we’ll examine the steps PolyMet is taking to ensure our tailings basin remains stable and safe over the long term.

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