Tap water is a complex mixture containing a variety of dissolved substances. Water collected from surface sources like rivers and lakes, or from underground aquifers, picks up minerals and other compounds on its journey. Before reaching the faucet, it undergoes a treatment process that intentionally adds certain chemicals for public health while removing unwanted elements. The water flowing from your tap is a dynamic solution, constantly monitored to ensure it meets safety standards.
Essential Minerals and Intentional Additives
The natural journey of water through soil and rock layers dissolves minerals, leading to water hardness. Calcium and magnesium are the primary minerals responsible, and their concentration varies significantly by location. These minerals are not a health concern and contribute to the daily intake of essential nutrients. High levels of these minerals cause aesthetic issues such as scale buildup in pipes and appliances, and they reduce the effectiveness of soap.
Water utilities deliberately add certain chemicals to protect public health against waterborne diseases. Chlorine is a powerful disinfectant used to kill disease-causing organisms like bacteria and viruses. Many systems use chloramine, a compound of chlorine and ammonia, as a secondary disinfectant because it is more stable and persists longer throughout the distribution system. This disinfectant residual remains in the water as it travels to your home, safeguarding against potential contamination within the pipes.
Another intentional additive is fluoride, a naturally occurring mineral adjusted to an optimal level for public health protection. The U.S. Public Health Service recommends a concentration of 0.7 milligrams per liter to maintain its cavity-preventing benefits. Community water fluoridation protects all ages against tooth decay, regardless of access to dental care.
Unwanted Contaminants and Emerging Concerns
Despite rigorous treatment, water can pick up unintended contaminants from the distribution system itself, most notably lead and copper. These metals typically leach from aging service lines, household plumbing, and solder found in older homes. Lead is a potent neurotoxin that accumulates in the body and is especially harmful to the mental and physical development of children. Utilities manage this risk by implementing corrosion control treatments, which adjust the water chemistry to create a protective mineral coating inside the pipes, reducing the rate of metal leaching.
Other environmental contaminants infiltrate the water supply through agricultural and industrial runoff, including substances like nitrates and pesticides. Nitrates, largely from fertilizer use and animal waste, are of particular concern in groundwater sources, as high concentrations can cause methemoglobinemia, or “blue baby syndrome,” in infants. Beyond these regulated contaminants, emerging contaminants are increasingly being detected at very low concentrations. These include per- and polyfluoroalkyl substances (PFAS), often called “forever chemicals” due to their persistence, and trace amounts of pharmaceuticals.
Pharmaceuticals enter the water supply primarily through human excretion and the improper disposal of unused medications. While often found in nanograms per liter, conventional wastewater treatment is not designed to remove them completely. Microplastics are another emerging concern; these tiny plastic particles are ubiquitous in the environment and have been found to potentially transport other chemical contaminants like PFAS.
How Water Treatment Shapes the Final Composition
The specific treatment process a water utility uses depends heavily on its source water. Surface water from reservoirs and rivers typically requires intensive treatment due to higher levels of suspended solids, microorganisms, and fluctuating quality. Conversely, groundwater is naturally filtered as it moves through soil and rock, but it often requires treatment to remove dissolved minerals like iron and manganese.
The comprehensive treatment process for surface water begins with a physical removal stage. Chemicals called coagulants, often aluminum or iron salts, are added to neutralize the negative charge of fine particles. This process, called coagulation and flocculation, causes the tiny particles to clump together into larger, heavier masses known as floc. The heavy floc then settles out during sedimentation, which significantly reduces the material processed in the next stage.
The water then moves to filtration, where it passes through layers of materials like sand, gravel, and activated carbon to physically trap remaining suspended solids and fine particles. This filtration step removes particles that could shield microorganisms from the final step: disinfection. Disinfection, using chlorine or chloramine, is applied to kill any remaining pathogens and is designed to leave a persistent residual that maintains the water’s safety as it travels through the distribution network.
Understanding Local Water Quality and Regulation
The safety and quality of public drinking water are overseen by the Environmental Protection Agency (EPA), which establishes national standards for contaminants. This oversight is founded on the Safe Drinking Water Act (SDWA), a federal law that regulates more than 90 contaminants that could pose a risk to public health. State and local authorities implement rules that are at least as stringent as the EPA’s to maintain primary enforcement authority.
Water quality varies significantly based on the local source, the age of the infrastructure, and the specific treatment methods employed. To keep the public informed, the EPA requires all community water systems to produce an annual Consumer Confidence Report (CCR). This report details the water’s source, lists the level of detected contaminants, and compares those levels to the EPA’s health-based standards.
The CCR is a valuable tool for consumers, providing a transparent look at their local water system’s compliance with federal standards. Homeowners typically receive this report automatically, or it can be accessed online through the local water utility’s website. While CCRs provide a general overview of the quality leaving the treatment plant, they may not reflect specific issues, such as lead leaching from the plumbing within an individual home.