Tap water is never found in a pure state outside of a laboratory. As water travels from its source—whether a river, lake, or underground aquifer—it acts as a near-universal solvent, dissolving and carrying various elements and compounds. This results in a complex chemical profile that includes naturally occurring minerals, intentionally added treatment chemicals, and trace amounts of environmental contaminants. Understanding what is in your tap water requires looking beyond the basic chemical formula to the various categories of substances it holds.
The Foundation: Water’s Natural Components
Tap water’s composition begins with the geology of its source, where it naturally picks up dissolved solids as it flows over and through rock and soil. These elements are primarily essential minerals that contribute to the water’s taste and mineral content. The most common are Calcium (Ca) and Magnesium (Mg), often dissolved from limestone and other rock formations.
The concentrations of these two minerals are responsible for “water hardness”; high concentrations result in hard water, while low concentrations result in soft water. Hard water is not a health concern and can contribute positively to dietary mineral intake. Other naturally occurring minerals include Sodium (Na) and Potassium (K), which are electrolytes found in trace amounts important for maintaining fluid balance in the body.
These natural components vary significantly by region depending on the local water source, whether it is surface water or groundwater. Groundwater, for example, often has a higher total dissolved solid content because it spends more time in contact with mineral-rich underground rock formations. Trace elements like Iron (Fe), Manganese (Mn), and Zinc (Zn) are also present, often dissolving from natural deposits, though they are usually found at levels too low to have a health impact.
Elements Intentionally Added for Safety
Before water reaches your home, municipal treatment facilities introduce elements to protect public health and maintain the water distribution system. The primary purpose of this addition is disinfection to eliminate harmful microorganisms. Disinfectants like Chlorine (Cl) and Chloramine (a compound of chlorine and ammonia) are added to kill pathogens and maintain a residual disinfecting effect as the water travels through the pipes.
The amount of these disinfectants is controlled, typically allowed up to 4 milligrams per liter (parts per million). This residual presence prevents microbial growth within the water lines, ensuring the water remains safe until it flows from your tap. Additionally, Fluoride (F) is often added as a public health measure to help prevent tooth decay. Treatment plants may also add corrosion inhibitors, such as various forms of phosphates, to form a protective layer on the inside of pipes and prevent metals from dissolving into the water.
Understanding Trace Contaminants
A separate category of substances consists of unwanted elements and compounds that enter the system accidentally from various human and natural sources. One major source of contamination is the existing infrastructure, where heavy metals can leach from older plumbing materials. The most recognized of these is Lead (Pb), which primarily enters the water from lead service lines or lead-containing solder and brass fixtures, especially in homes built before 1986. Similarly, Copper (Cu) can dissolve into the water from copper pipes and fittings, often due to water chemistry that causes corrosion.
Environmental and agricultural practices introduce other contaminants, such as Nitrates, often found in water supplies due to fertilizer runoff and septic system discharge. These chemicals can be particularly concerning for vulnerable populations, like infants. Elements like Arsenic (As) can dissolve into groundwater from natural deposits in the earth’s crust, though industrial activity can also contribute to its presence. Trace amounts of industrial chemicals, pesticides, and residues from pharmaceuticals can also find their way into water sources, requiring filtration and monitoring by water utilities.
How Your Water Quality Is Monitored
Public water systems are required to regularly test and monitor for hundreds of contaminants to ensure the water meets safety standards. The regulatory framework establishes Maximum Contaminant Levels (MCLs), which represent the highest amount of a contaminant allowed in drinking water. These standards are set to be as close as feasible to the Maximum Contaminant Level Goals (MCLGs), which are the levels at which no known or expected health risk exists.
The results of this testing are made accessible to the public through annual documents called Consumer Confidence Reports (CCRs), or Water Quality Reports. These reports detail the source of the local water, list the regulated contaminants that were detected, and show their measured concentrations compared to the established MCLs. Reviewing your local CCR is the most direct way to find specific data on the elements and compounds present in your tap water, providing transparency about the safety and quality of the supply.