In Simple Terms
Researchers found pollution trapped in Arctic ice and traced much of it to a copper mine in Montana, thousands of kilometers away. The pollution peaked and then fell, even though the mine kept operating, likely because pollution-control equipment was finally installed. The researchers also estimate that millions of people in the United States and Canada were exposed to pollution from the mine.
An Ice Record Raises a Puzzling Question
For decades, Joe McConnell, a climate and environmental scientist at the Desert Research Institute, has studied ice samples from Greenland, northern Canada, and the Russian Arctic. Snow builds up year after year, then gradually compresses into ice. Tiny particles that settle from the air become trapped between the layers, turning ice into a natural archive of environmental change—including traces of human activity.
In their records, the researchers noticed that levels of bismuth and antimony rose in the early 20th century. Both metals can be released into the air during mining and metal smelting. At first, the increase seemed easy to explain: industrial activity was expanding at the time. But industry did not stop or shrink afterward. It continued to grow, while concentrations of the two metals in the ice peaked early and then declined. That raised the central question: Why did the signal in the ice fade while industry carried on?
The signals alone could not identify the source of the pollution. Finding a metal in an ice sample does not automatically reveal which factory or mine released it, and different sources can produce similar pollutants. The team needed another substance that could provide an additional clue. They also needed to compare the timing and characteristics of the deposits with the history of metal production and the locations of possible sources.
Arsenic Adds a New Clue
That additional clue was arsenic. A few years ago, McConnell’s lab developed a method for measuring it in ice, even though arsenic is harder to detect than metals such as lead. It occurs in extremely small amounts in the samples and has no isotope fingerprint that can identify its source in the way possible for some other elements. Copper smelting, however, can release arsenic, as well as lead, bismuth, and antimony.
For the new study, published in Nature Sustainability, the team analyzed 10 polar ice cores. These are cylindrical columns drilled from ice that reveal its internal layers. The researchers did more than measure the concentration of a metal in each sample: they compared deposition patterns at different sites with the history of copper production and information about the characteristics of emitted metals. They also used models of air movement to estimate the routes that could have carried the particles.
Putting these lines of evidence together pointed to one leading source: the Anaconda copper mine in Montana, thousands of kilometers from the ice-sampling sites. Measurements showed that arsenic levels in the ice in 1914 were about 15 times higher than preindustrial levels. The timing closely matched the mine’s production history, lending strong support to the idea that a particular source was responsible, rather than merely suggesting that the pollution came from North American industry in general.
How the Researchers Narrowed Down the Suspects
The investigation began with the pattern of deposition. The heavy metals in the study travel through the air as particles, not as stable gases. According to the researchers, the particles remain airborne for only days or weeks before settling on a surface. Their arrival in ice is therefore closely connected to when they were emitted. That makes it possible to compare the ice record with years of industrial production, while also accounting for air movement.
The distribution of deposits across the ice sites suggested that the source was in North America. The team then examined copper-production records from the United States and Canada and tested a range of possible industrial sources. A smelter in Tacoma, Washington, was ruled out because it was not large enough to explain the quantities found in the ice. The lead-isotope ratios associated with mining in the Sudbury region of Canada also did not match the measured signal.
The researchers also examined mines on Michigan’s Keweenaw Peninsula. But most of the copper extracted there was native copper, so emissions of the heavy metals under study were relatively low. Production data from the Anaconda mine, by contrast, aligned more clearly with the arsenic changes recorded in the ice, including the sharp rise in 1914. Air-transport models strengthened this interpretation by showing plausible routes for pollution to spread across North America.
Karl Krutz, an earth scientist at the University of Maine, described the work as a striking scientific investigation. He said that using arsenic to link the pollution to a specific major mine was one of the study’s compelling features. Francieli Schwank Carlos, a geoscientist at the Polar Climate Center at the Federal University of Rio Grande do Sul in Brazil, noted that tracing emissions to a single source is difficult. Still, she found a strong fit between historical production data from Anaconda, the ice measurements, and the air-transport models. Neither researcher was involved in the study.
Falling Pollution Did Not Mean the Mine Had Closed
The story matters not only because it identifies a likely pollution source, but also because it offers an explanation for why pollution levels fell after peaking. The Anaconda mine operated until 1947, yet pollutant levels in the ice began to decline after 1919. The study team suggests that installing electrostatic precipitators at the mine’s smelter helped bring emissions down. These devices capture particles from an airflow, limiting how much escapes into the environment.
The technology had been developed before it was installed at Anaconda. It was patented in 1908, and its inventor had drawn up a plan to install it at the facility. But putting that plan into practice took years. The study estimates that the devices reduced heavy-metal emissions by about 65%. This offers a possible answer to the original puzzle: a facility can keep operating without releasing pollution at the same rate if its emission-control methods change.
McConnell links the delay to a regulatory environment that, in his description, allowed polluters to install pollution controls when doing so made economic sense for them. In the researchers’ calculations, the delay was more than an administrative matter: they estimate that it resulted in an additional 60,000 tons of arsenic and 24,000 tons of lead being released by the mine. These are estimates of what might have been avoided if emissions had been controlled sooner.
Estimates Reveal How Far Human Exposure May Have Reached
To estimate how many people may have been exposed, the researchers combined United States and Canadian census records from 1910 and 1911 with particle-transport modeling. They then estimated the populations living in areas that may have received pollution levels linked to the mine’s emissions between 1898 and 1918. This is a reconstruction of past exposure, not a direct count of people who became ill or a clinical measurement of exposure in specific individuals.
According to their calculations, about 55% of the United States population at the time—50.5 million people—was exposed to arsenic levels that could increase the risk of lung cancer. In Canada, the estimate covered 11% of the population, or 600,000 people. The researchers also suggest that about 1.4 million people were affected by elevated levels of lead pollution. These figures do not mean that everyone counted developed cancer or lead poisoning. They describe the scale of potential exposure based on the production records and population and air-transport data used by the team.
Lead is especially important in assessing health risks because the United States Environmental Protection Agency says there is no safe level of lead exposure for children. In the study’s estimates, arsenic was linked to an increased risk of lung cancer. The findings show how emissions from an industrial facility can travel well beyond its immediate surroundings, spreading across distant regions and affecting large populations. But determining the health risk to any one person requires information that ice samples alone cannot provide.
Concerns about the Anaconda mine do not come from the ice analysis alone. In the past, the facility faced lawsuits from nearby farmers and ranchers, and later became involved in a dispute with the U.S. Forest Service. The allegations included damage to agriculture, poisoned livestock, and forest destruction caused by the mine’s activities. The area is now home to the largest Superfund site in the United States, a designation for polluted locations requiring extensive cleanup. This history is consistent with the picture of a pollution source, while the new study adds evidence drawn from the ice archive.
What the Ice Shows—and What It Cannot
The strength of the study lies in the fact that its source attribution does not rest on a single clue. Copper production at Anaconda coincides in time with rising arsenic levels; several metals associated with smelting appear in the record; deposition patterns at polar sites point toward North America; and particle-transport models support the proposed route. Comparing Anaconda with other possible sources, including Tacoma, Sudbury, and Keweenaw, also helps explain why the researchers favor this mine.
Even so, the conclusion should be understood as identifying the leading source supported by the evidence—not as a claim that every polluted particle in the ice came from one mine. Air carries particles over long distances, historical measurements and models reconstruct possible routes, and available population records do not track exposure for each person. Research of this kind shows how difficult it is to trace emissions to a single facility, while also showing how independent clues, considered together, can make an explanation much stronger than a single measurement would.
The study also demonstrates the value of natural archives when industrial or regulatory records are incomplete, or when the distance between a pollution source and the place where it is detected is vast. Ice layers do more than preserve a signal that metals were present. When compared with production histories and air movement, they can help establish when events happened and identify a likely geographic source. In the case of Anaconda, this approach shows that emissions from a facility in Montana left a detectable trace in Arctic ice—and that a change in industrial controls may explain why the trace began to fade decades before the mine closed.
The Bottom Line
Joe McConnell’s team at the Desert Research Institute turned measurements of arsenic and other metals in 10 polar ice cores into an investigation that points to the Anaconda copper mine in Montana as the dominant source of the pollution found in the ice. Arsenic levels recorded in 1914 were about 15 times higher than before the industrial era. Emissions then declined after 1919, probably because pollution-control devices were installed, even though the mine remained open until 1947. The study estimates that the delay in installing those devices led to an additional 60,000 tons of arsenic and 24,000 tons of lead being released, and that millions of people in the United States and Canada may have been exposed to concerning levels between 1898 and 1918. More broadly, the findings show that industrial pollution can travel thousands of kilometers—and that installing controls in time can change how much reaches the environment and the people who live in it.