The water that flows from your sink tap is a complex mixture of substances that reflects its journey from the source through treatment and distribution. For most people, this water originates from a municipal supply, though some rely on private wells, and in both cases, the composition is never pure H2O. The quality of the water reaching your glass is determined by a balance of intentional additions and the unintentional presence of various contaminants picked up along the way. Understanding what is in your water involves looking at components purposefully added for safety, as well as substances that enter the supply from environmental and infrastructure sources.
Intentional Components: Treatment Chemicals and Essential Minerals
Public water systems intentionally include certain chemicals to ensure the water remains safe for consumption throughout the distribution network. Chlorine and chloramine are the most common disinfectants added to kill disease-causing microorganisms before the water reaches your home. A small residual amount of these chemicals is maintained in the pipes to prevent microbial regrowth, though this can sometimes result in a noticeable taste or odor. Many municipal systems also add fluoride to promote dental health and reduce tooth decay.
Beyond these treatment chemicals, tap water naturally contains various minerals picked up from the source water, often groundwater that has flowed over rock formations. The most common are calcium and magnesium, which contribute to “water hardness.” While hard water is generally considered harmless and contributes positively to dietary mineral intake, these minerals can precipitate out when heated, leading to scale buildup in appliances and plumbing.
Unwanted Chemical Contaminants and Pollutants
Chemical contaminants are substances not intentionally added but enter the source water from human activity. Agricultural runoff introduces pollutants like nitrates from fertilizers and various pesticides and herbicides. These substances can seep into groundwater or wash into surface water sources, requiring complex treatment processes to reduce concentrations before distribution.
Modern industrial and consumer practices have also introduced emerging contaminants, which are increasingly a focus of water quality monitoring. Per- and polyfluoroalkyl substances (PFAS), often called “forever chemicals,” are widespread due to their persistence in the environment. These chemicals, used in products ranging from non-stick coatings to fire-fighting foams, are estimated to be present in the tap water of nearly half of the U.S. population. Additionally, trace amounts of pharmaceuticals, hormones, and microplastics can enter the water supply from personal care products and wastewater.
Biological and Microbial Presence
The primary goal of water treatment is the removal or inactivation of biological pathogens, which are the main cause of waterborne disease. Treatment processes are highly effective at eliminating harmful bacteria like E. coli, viruses such as Rotavirus, and parasitic protozoa like Giardia and Cryptosporidium. Filtration and disinfection steps reduce these organisms to undetectable or non-infectious levels.
However, the microbial landscape is dynamic, and certain organisms can still pose risks. Failures in the treatment system, such as a malfunction in disinfection or a break in a distribution pipe, can allow pathogens to enter the system, often resulting in a public “boil water” advisory. Furthermore, some opportunistic pathogens, including Legionella and certain species of Mycobacteria, survive low-level disinfection and grow within distribution pipes and household plumbing. These organisms frequently reside in biofilms, protective layers that form on pipe surfaces, making them difficult to eliminate with standard protocols.
The Role of Home Plumbing and Infrastructure
The water’s final journey through the service line and plumbing within the building can significantly alter its quality. Water leaving the treatment plant is typically free of heavy metals like lead and copper, but these elements can leach into the water through contact with household materials. Lead enters the water primarily from older lead service lines, lead solder used in plumbing installed before 1986, and some brass fixtures.
The leaching process is accelerated by corrosive water, which may have a lower pH or mineral content that strips metal from pipe surfaces. Copper can also enter the water from residential copper pipes, often showing higher concentrations in newer systems before a protective mineral scale has formed. The time the water spends sitting motionless, known as “water age,” is a major factor, as increased contact time allows more metal to dissolve. To reduce exposure, especially after water has been stagnant for several hours, running the cold water tap for 30 seconds to two minutes flushes standing water from the plumbing. Point-of-use filters can also remove specific contaminants like lead and improve taste by reducing disinfectant residuals.