Tap water is mostly water molecules, of course, but it also contains a mix of dissolved minerals, disinfectant chemicals, dissolved gases, and trace amounts of other substances picked up during treatment and delivery. The exact composition varies by city, water source, and even the pipes in your home, but the core ingredients are surprisingly consistent across the United States.
Minerals Dissolved in Tap Water
Water is an excellent solvent, so by the time it reaches your glass it has picked up minerals from soil, rock, and the infrastructure it travels through. A USDA analysis of municipal water across the U.S. found nine minerals present in measurable amounts: sodium, potassium, calcium, magnesium, iron, copper, manganese, phosphorus, and zinc.
The concentrations are low. If you drink about two liters of tap water a day, only four minerals contribute more than 1% of your daily nutritional needs: copper at roughly 10%, calcium at 6%, magnesium at 5%, and sodium at 3%. Even the highest concentrations found in the study would supply only about 20% of your daily calcium and 23% of your magnesium. Tap water contributes to your mineral intake, but it’s not a significant source for most people.
The mineral content also determines whether your water is “hard” or “soft.” Hardness is measured by the concentration of calcium carbonate. Water under 60 mg/L is classified as soft, 61 to 120 mg/L is moderately hard, 121 to 300 mg/L is hard, and anything above 300 mg/L is very hard. Hard water leaves scale buildup on faucets and makes soap harder to lather. It’s not a health risk, but it’s one of the main reasons tap water tastes and behaves differently from one city to the next. Chemical water softeners significantly reduce calcium and magnesium levels, which is why softened water feels slippery and tastes different.
Disinfectants That Keep Water Safe
The most important intentional additive in tap water is a disinfectant, almost always chlorine or chloramine. These chemicals kill bacteria, viruses, and parasites, and they’re meant to remain active in the water so germs can’t regrow as the water travels through miles of pipes to your home.
Chlorine is the traditional choice and the one you’re most likely to smell or taste. It works fast but gets used up quickly, sometimes leaving too little protection by the time water reaches distant taps. That’s one reason many utilities have switched to chloramine, a compound made from chlorine and ammonia. Chloramine lasts longer in the distribution system and produces fewer chemical byproducts, though it can affect lead and copper pipes by changing the water’s chemistry.
Both are considered safe at concentrations up to 4 parts per million, which is the federal limit. Most tap water contains well below that. A less common alternative, chlorine dioxide, has a stricter limit of 0.8 parts per million.
Fluoride
Many U.S. water systems add fluoride to help prevent tooth decay. The U.S. Public Health Service recommends a concentration of 0.7 mg/L, a level chosen to maximize dental health benefits while minimizing the risk of dental fluorosis (faint white spots on teeth that can develop in children exposed to too much fluoride). Not every community fluoridates its water, so whether yours does depends on local policy. Your annual water quality report will tell you.
Disinfection Byproducts
When chlorine reacts with naturally occurring organic matter in the water source (things like decomposing leaves and soil), it creates secondary chemicals called disinfection byproducts. The two most common groups are trihalomethanes and haloacetic acids. The presence of bromide or iodide in the source water can increase the variety and amount of these byproducts.
These compounds are regulated because long-term exposure at high levels is linked to health concerns. Utilities manage them by controlling how much organic matter remains in the water before disinfection and by adjusting chlorine doses. Switching to chloramine is another strategy, since it generates fewer byproducts than chlorine alone.
Dissolved Gases
Tap water contains dissolved nitrogen, oxygen, and carbon dioxide, all absorbed from contact with air. These gases are the reason you sometimes see tiny bubbles form on the inside of a glass of water or notice a cloudy, milky appearance when you first fill a glass from the tap. That cloudiness clears from the bottom up as the gas bubbles rise and escape. Carbon dioxide slightly lowers the pH of water, which is one factor that influences taste.
What Your Pipes Add
The composition of tap water can change in the last stretch of its journey: your home’s plumbing. Lead and copper enter drinking water primarily through pipes, solder, and fixtures rather than from the water source itself. Older homes built before the mid-1980s are more likely to have lead solder or lead service lines connecting them to the water main.
The EPA sets an action level of 15 parts per billion for lead and 1.3 parts per million for copper, measured at customer taps. When more than 10% of sampled homes in a system exceed those levels, the utility is required to take additional steps to control corrosion, such as adjusting water chemistry to make pipes less likely to leach metals. The federal goal for lead in drinking water is zero, because no amount is considered safe.
Substances That Affect Taste, Color, and Odor
Some things in tap water aren’t dangerous but are noticeable. The EPA sets non-mandatory guidelines for 15 of these nuisance substances. Iron above 0.3 mg/L can give water a metallic taste and a rusty color, and it stains sinks and laundry reddish-orange. Manganese above 0.05 mg/L turns water dark and leaves black stains. Sulfate above 250 mg/L makes water taste salty. These guidelines aren’t enforced the way health-based standards are, but most utilities try to stay within them because customers notice when they don’t.
Trace Contaminants and Newer Concerns
Federal drinking water regulations cover more than 90 contaminants, organized into categories: microorganisms like Cryptosporidium and Giardia (both regulated to a goal of zero), inorganic chemicals like arsenic (limited to 10 parts per billion), nitrate (limited to 10 mg/L), and mercury (limited to 0.002 mg/L), along with dozens of synthetic organic chemicals and pesticides.
One of the newest additions to the regulated list is PFAS, a group of synthetic chemicals sometimes called “forever chemicals” because they don’t break down easily in the environment. In 2024, the EPA finalized the first-ever national limits for six PFAS compounds. The two most well-known, PFOA and PFOS, now have a maximum contaminant level of 4 parts per trillion, an extremely tight standard. The health goal for both is zero. Water systems have several years to come into compliance.
How to Find Out What’s in Your Water
Every public water system in the U.S. is required to publish an annual Consumer Confidence Report, also called a water quality report. It lists every detected contaminant, its concentration, and how that compares to the legal limit. You can find yours on your utility’s website or by searching the EPA’s database with your zip code. If you’re on a private well, no one tests your water for you. State health departments typically recommend testing at least once a year for bacteria, nitrates, and any contaminants common in your area.