Selenium is a mineral found in soil and rocks. It enters water through erosion and industrial processes. While essential in small amounts, high doses can be toxic.

Selenium naturally occurs in soil and rocks, but industrial processes and agricultural runoff can increase its presence in water. While it's an essential nutrient in trace amounts, excessive intake can be harmful. The EPA sets a maximum contaminant level of 50 parts per billion in drinking water. Selenium is typically measured using atomic absorption spectroscopy. Water treatment methods like reverse osmosis can effectively remove selenium from drinking water.