Mercury is a toxic metal from industrial pollution and fossil fuel burning. It accumulates in fish and can damage the nervous system, especially in children and fetuses.

Mercury enters water from both natural sources (e.g., volcanic activity) and human activities like mining and industrial processes. It exists in various forms, with methylmercury being the most concerning for human health. Water treatment plants use activated carbon filtration to remove mercury, which is typically measured in parts per billion. The EPA has set a maximum contaminant level of 2 ppb for mercury in drinking water. Regular testing and proper disposal of mercury-containing products help reduce contamination risks.