Long after a mine closes, the toxic by-products of resource extraction can linger in the environment and be spread by rivers and floods. Just how far those contaminants spread may have been underestimated for years, according to a new study.
The research, published in Science, indicates more than 23 million people could be living in floodplains contaminated by heavy metals originating from mines. It’s a warning that mining companies and governments need to expand their efforts beyond mines to protect citizens from harm, said study coauthor Mark Macklin, a geomorphologist at the University of Lincoln in the United Kingdom.
“A huge amount of work has gone into cleaning up mine sites themselves,” he said. “But in many cases, the principal issue is the floodplains and channels downstream.”
Although scientists and policymakers knew mining released heavy metals into soils and waterways, no comprehensive, global database of contamination levels existed until now. With better knowledge of which areas are more likely to be contaminated, Macklin said policymakers can begin targeting cleanup efforts and designing smarter development policies.
A Toxic Legacy
When valuable metals such as iron, copper, and gold are wrested from Earth, they are often accompanied by traces of other metals, many of them toxic. The list includes mercury, lead, and arsenic, and mining operations have long struggled with what to do with these contaminants.
Early mining in England and throughout Great Britain released toxic metals into the atmosphere, which settled in soils. And until the 1800s or so, mining waste was simply discharged into rivers, Macklin said. More recent mining activities in the country during the 19th century left some areas so heavily polluted by lead that their soils are effectively low-grade ores, Macklin said.
Once these toxic metals are released into the environment, they enter soils along riverways and in floodplains, where they can stay for hundreds or even thousands of years. Subsequent floods can erode soils, causing the pollutants in them to leach back out, after which they pollute drinking water and make their way into the plants and animals we eat.
To understand how many people are at risk from these pollutants, Macklin and his coauthors first had to find out where the mines are. They drew from multiple existing mining databases, including those from the U.S. Geological Survey and the British Geological Survey, and conducted an exhaustive search of academic, government, and industry literature, eventually compiling what Macklin said is the most comprehensive database of mines—both historical and active—to date.
In total, the database includes 22,609 abandoned mines, 159,735 active mines, and 11,587 tailings storage facilities. The work, which kept two postdoctoral students busy for 3 years, is a “step change in knowing where these mine sites are,” Macklin said.
“To really understand the global implications, having these databases at our hands is vitally important,” said Karen Hudson-Edwards, a geochemist at the University of Exeter in the United Kingdom and a coauthor on the paper. “We consider ourselves really fortunate to have it.”
The researchers then paired that mine data with global maps of watersheds, including floodplains and the historical extent of floods. The results showed them which mines sat near areas where water is likely to remobilize contaminated sediments.
Macklin and his colleagues say metal mining contaminants are likely to exceed safe levels across 164,000 square kilometers (63,500 square miles) worldwide—an area roughly the size of Wisconsin. These toxins also pose a danger to almost 480,000 kilometers (300,000 miles) of river channels.
In all, 23.5 million people live on floodplains contaminated by heavy metal toxins, the researchers estimated. These areas also host close to 6 million livestock and include some 65,000 square kilometers (25,000 square miles) of irrigated land.
North America has the largest area affected by mining contamination, at 43,000 square kilometers (16,600 square miles), whereas Asia hosts the most people living in contaminated floodplains, at 14.5 million.
“This is an important piece of research,” wrote Jamie Woodward, a geomorphologist at the University of Manchester who wasn’t affiliated with the research, in an email. “While major tailings dam failures have attracted much media attention, this paper shows how the ‘silent pollution’ stored in floodplains may actually pose a much greater threat.”
Waste Around the World
The threat varies in different regions of the world, Macklin said. In North America and Europe, where mining activity boomed during the Industrial Revolution, as well as in South America and Oceania, waste from inactive, legacy mines poses the largest threat. In Africa and Asia, ongoing mining activity is the biggest driver of contamination.
Their database isn’t yet complete, Macklin warned. Data from China and Russia are incomplete, and the true impact of small-scale unregulated artisanal mining from South America, Africa, and elsewhere is also difficult to quantify. That means the true impact of heavy metal pollution is likely to be higher than currently estimated.
Metal mining is also poised to expand in many regions as the green energy transition demands more cobalt, lithium, and other inputs for making batteries.
“One thing we must avoid is the contamination that we’ve seen from previous mining phases,” Macklin said. “We must make sure that we don’t repeat the mistakes of the past.”
That could be made more difficult as climate change makes heavy rainfall ever more likely. Bigger or more frequent floods can move contaminated sediments back into the water supply and spread them downstream to new areas, meaning old contaminants can become a new threat to human health.
The database will help forecast where increased flooding could pose the most risk to human health by increasing heavy metal contamination, Hudson-Edwards said. It will also aid government efforts to clean up polluted floodplains by helping them target areas with the highest levels of pollution or where the most people are likely to be affected, Macklin added. Their maps may even help point out previously unrecognized hot spots of pollution.
“Very often, the regulators, or even just local communities or local governments, don’t really know they have a problem,” he said.
—Nathaniel Scharping (@nathanielscharp), Science Writer
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