Iron is a metal that naturally occurs in soil and rocks. It enters water through erosion or corrosion of pipes, causing rusty color and metallic taste in tap water.

Iron is one of the most abundant elements in the Earth's crust and an essential nutrient for human health. In water, it exists in two forms: ferrous (Fe2+) and ferric (Fe3+). Iron is measured in milligrams per liter (mg/L) or parts per million (ppm). While not a health concern, it's classified as a secondary contaminant due to aesthetic issues. High levels can stain laundry and plumbing fixtures. Common treatment methods include filtration, ion exchange, and oxidation processes.