Drinking water is rarely pure H₂O, naturally containing dissolved solids and gases acquired during its journey through the environment. These dissolved substances are a mixture of minerals that can be beneficial, neutral, or pose a risk if present at high concentrations. The specific composition of minerals in tap water depends heavily on the source water’s geology.
Essential Minerals: The Foundation of Hard Water
The most abundant minerals found naturally in drinking water are Calcium (Ca) and Magnesium (Mg), which define “hard water.” Hardness is a measure of the concentration of these metallic cations, typically leached from geological formations like limestone and chalk.
Magnesium and Calcium serve important biological functions and contribute to daily dietary intake. Magnesium intake has been linked to a potential reduction in the risk of stroke, while Calcium supports bone health and density.
While hard water can cause scale buildup in pipes and appliances, it is generally considered safe for consumption and may offer health advantages. Public water systems do not regulate these minerals with health-based limits, as they are not known to cause adverse health effects in typical concentrations.
Intentional Addition: The Role of Fluoride
Fluoride (F) occurs naturally in many water sources but is often intentionally added during treatment, a practice known as fluoridation. This public health measure aims to prevent tooth decay by slowing the demineralization of tooth enamel and enhancing its remineralization.
The optimal concentration recommended by public health agencies is typically 0.7 milligrams per liter (mg/L). Water systems often use compounds such as sodium fluoride or fluorosilicic acid to adjust the level to this target. The decision to add fluoride is made at the local municipal level, not mandated by federal regulation.
The US Environmental Protection Agency (EPA) has set a Maximum Contaminant Level (MCL) of 4.0 mg/L, an enforceable standard designed to protect against the long-term risk of skeletal fluorosis. A secondary standard of 2.0 mg/L is also in place to limit the aesthetic concern of dental fluorosis, which can cause cosmetic discoloration or pitting of tooth enamel in children.
Monitored Minerals and Potential Health Risks
Certain minerals and metals are closely regulated because they pose specific health risks or cause aesthetic problems. Arsenic (As), a naturally occurring semi-metal and known human carcinogen, often dissolves into groundwater from natural rock deposits. Long-term exposure above the EPA’s Maximum Contaminant Level of 0.010 mg/L has been linked to an increased risk of cancer, skin damage, and circulatory issues.
Lead (Pb) rarely originates from the source water itself; instead, it enters the supply through the corrosion of aging infrastructure, such as service lines and plumbing fixtures. Because lead exposure can cause neurological problems, especially in young children, the EPA has set the Maximum Contaminant Level Goal (MCLG) at zero, reflecting that there is no safe level of exposure.
Other naturally occurring metals, Iron (Fe) and Manganese (Mn), are regulated primarily for aesthetic reasons, as they are not considered a health risk at standard concentrations. Iron (Secondary MCL of 0.3 mg/L) can cause an unpleasant metallic taste and reddish-brown staining. Manganese (Secondary MCL of 0.05 mg/L) is known for causing black staining and a bitter taste.