Calcium is a mineral naturally present in water from rock dissolution. It's essential for health but can contribute to hard water and scale buildup in pipes at high levels.

Calcium enters drinking water as it flows through or over limestone and other calcium-rich rock formations. While essential for bone health, excessive calcium can lead to hard water, causing scale in pipes and appliances. It's typically measured in milligrams per liter (mg/L) or grains per gallon. Water treatment plants may use ion exchange or reverse osmosis to reduce calcium levels in areas with very hard water. However, moderate amounts of calcium in drinking water can contribute positively to daily dietary intake.