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A new study involving Prof Graham Bird and colleagues reveals the global scale of this problem for the first time. Researchers compiled a massive database of over 180,000 active and abandoned metal mines across six continents. Using models and on-the-ground data, they estimated the extent to which rivers and floodplains downstream of these mines exceed safe levels of contaminants like lead, zinc, copper, and arsenic.

The results are stark - over 480,000 kilometres of river channels worldwide are polluted by metal mining, affecting a land area the size of Iceland. Most concerning is that an estimated 23 million people live on these contaminated floodplains.

In the developing world, especially Africa and Asia, modern mining pollution threatens vulnerable communities already burdened by poverty and lack of infrastructure. But the rich nations of North America and Europe aren't off the hook either - legacy contamination from centuries of mining affects over 12 million people in these regions.

The study also puts tailings dam failures in perspective. Failures like those seen recently in Brazil grab headlines for good reason - they can be catastrophic for nearby towns and ecosystems. But the new study shows that slow, ongoing leakage from mines and tailings waste actually exposes 50 times more people to unsafe levels of metals over the long term.

With mining expanding globally and more frequent flooding expected worldwide, contaminated sediment may be remobilized from floodplains and expose even more communities in the future. The authors call for expanded monitoring of mining impacts, especially in poorly regulated regions, to better understand health and environmental risks.

While metals form the foundation of the modern world, this new study is a sobering reminder that they often come with heavy costs. Better oversight and reduced waste from mining operations will be crucial to prevent countless millions more from paying the toxic toll.