Recently, the world watched another mine tailings dam fail, this time in Florida. The dam at the Piney Point phosphate plant stores phosphogypsum, what remains after the processing of mined phosphate rock, along with its wastewater. Although the dam has not collapsed as feared, it was stabilized only by the intentional release of 215 million gallons of untreated and contaminated wastewater into Tampa Bay (nearly 50% of the wastewater the dam held).
By any standard, this is a major dam failure. The Piney Point tailings dam near-collapse and massive release of polluted wastewater should set off a red alert for anyone concerned about the future of clean water in northeastern Minnesota.
Although this critical fact has been lost in much of the news coverage, all phosphogypsum mine tailings facilities in Florida were built using the upstream method. In 2018, the Minnesota Department of Natural Resources (DNR) permitted PolyMet to use the same cheap and unstable upstream construction method used at Piney Point to store its toxic tailings in its plans for a copper-nickel mine in northern Minnesota.
The upstream method of construction uses compacted tailings (mine waste the consistency of sand or talcum powder) to build a dam on top of the uncompacted tailings and wastewater. In other words, the dam is essentially built on loose, wet sand. There is no concrete or rock brought in to build the dam, only the leftover tailings from the mining process. A failure to manage the water in this structure can cause the dam to liquefy and collapse.
In January 2019, an upstream tailings dam failed near Brumadinho, Brazil, killing 270 people, the majority of whom were mineworkers. In November 2015, an upstream tailings dam failed near Marianas, Brazil, releasing 8.5 billion gallons of tailings and wastewater, killing 19 people, and contaminating waterways for 400 miles until the tailings flood entered the Atlantic Ocean.
In August 2014, the tailings dam at the Mount Polley mine in British Columbia, Canada, failed, releasing 6.2 billion gallons of tailings and wastewater, destroying pristine salmon habitat.
The upstream method of tailings dam construction is now illegal in Brazil, Chile, Ecuador and Peru. A 2021 study in Nature, the world's most prestigious scientific journal, reported that "active upstream facilities report a higher incidence of stability issues (18.3%) than other facility types," and that this elevated risk "persists even when these facilities are built in high governance settings," namely jurisdictions that seem to have strong regulations and regulatory agencies.
The study continues: "While upstream facilities make up 37% of the total, they have declined from a peak of 85% of new facilities in 1920-29 to 19% of new facilities in 2010-19," which puts Minnesota into a kind of time warp.
Why would PolyMet want to use upstream construction? Simple, the upstream construction method is cheap. While most copper ores are at least 1% copper, PolyMet will be mining an ore-body that has about one-fourth as much copper per ton excavated. With such a low-grade ore, PolyMet could not make a profit without cutting corners.
Minnesota's DNR asserts that upstream construction is good enough for Minnesota. What could go wrong? It turns out the answer is: a lot. A Minnesota tailings dam could fail as in Brazil. Or like at Piney Point, a failing dam could require officials to dump contaminated wastewater into the surrounding watershed.
Piney Point was stabilized by releasing nearly half of its 438 million gallons of stored wastewater. The PolyMet tailings dam will store about 41 billion gallons of tailings and wastewater, almost 100 times more than the Piney Point dam.
Is the Minnesota DNR prepared to allow PolyMet to release half — 20 billion gallons — of its toxic tailings and wastewater into the St. Louis River, which flows downstream to Lake Superior?
The PolyMet upstream tailings dam hasn't been constructed yet. It is not too late for the state's leaders to change course. Minnesota has better and safer choices than dangerous upstream tailings dams.
Steven H. Emerman is the owner of Malach Consulting in Spanish Fork, Utah, which specializes in groundwater and mining.