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Listen to part of a lecture in an environmental science class. Professor: Today we're going to talk about a surprising source of pollution: abandoned mines. You might think that once a mine is closed, the environmental damage stops. But that's not true. Many old mines leak acid and heavy metals into nearby rivers and groundwater. This happens when rainwater seeps into the mine shafts and reacts with sulfur-bearing rocks, forming sulfuric acid. The acid then dissolves metals like lead and arsenic from the rock, creating a toxic solution called acid mine drainage. This pollution can persist for decades, even centuries, if not treated. However, there is a relatively new technique called 'passive treatment' that uses natural materials, such as limestone and wetlands, to neutralize the acid and trap the metals. Unlike traditional chemical treatment, which requires ongoing energy and maintenance, passive systems are designed to work with minimal human intervention. Some communities have successfully implemented these systems, turning polluted sites into clean water sources and even wildlife habitats. This approach shows that we can remediate old mines in a cost-effective and sustainable way.