Arsenic is a toxic element found in rocks and soil. It enters water through natural erosion and human activities like mining. Long-term exposure causes cancer and organ damage.

Arsenic occurs naturally in the earth's crust and can contaminate groundwater. Industrial processes, pesticides, and wood preservatives are human-made sources. It's measured in parts per billion (ppb), with inorganic forms being more toxic than organic. Arsenic gained notoriety as a poison in historical times. Water treatment methods like reverse osmosis and ion exchange can effectively remove arsenic. The EPA set a maximum contaminant level of 10 ppb in drinking water to protect public health.