Tap water contains a mix of naturally occurring minerals, added disinfectants, and trace contaminants picked up during treatment and delivery. The exact makeup varies by city, water source, and the pipes in your home, but the broad categories are consistent across the United States. Here’s what’s actually in there and what the levels mean for you.
Minerals From the Source
Before any treatment happens, water picks up minerals as it flows through rock, soil, and underground aquifers. The most common are calcium, magnesium, sodium, and copper. On average, U.S. tap water contains about 20 to 30 mg/L of calcium and around 10 mg/L of magnesium, levels that epidemiological research links to modest health benefits.
These minerals contribute to what’s often called “hard” or “soft” water. If you drink about two liters of tap water a day, you’re getting roughly 10% of the daily value for copper, 6% for calcium, 5% for magnesium, and 3% for sodium. That’s not a major nutritional source, but it’s not nothing either, especially for calcium and magnesium if your diet runs low on dairy or leafy greens.
Chlorine and Chloramine
Water utilities add a disinfectant, either chlorine or chloramine, to kill bacteria and viruses during treatment and to keep the water safe as it travels through miles of pipes to your faucet. Chlorine is the more traditional option. Chloramine, a combination of chlorine and ammonia, lasts longer in the distribution system and is increasingly common in larger cities.
Both are kept at low concentrations. The EPA considers levels up to 4 mg/L (4 parts per million) safe for drinking. Most utilities operate well below that ceiling. The taste and smell some people notice in tap water is usually residual chlorine, and it dissipates quickly if you let a glass sit on the counter or refrigerate it for a few minutes.
Fluoride
About 73% of Americans on community water systems receive fluoridated water. Utilities add fluoride to prevent tooth decay, targeting a concentration of 0.7 mg/L. That number was updated in 2015, down from a previous range of 0.7 to 1.2 mg/L, to reflect that people now get fluoride from other sources like toothpaste.
The EPA sets a secondary safety standard at 2.0 mg/L to prevent cosmetic dental fluorosis (faint white streaks on teeth) and a hard maximum of 4.0 mg/L to protect against bone disease. Naturally occurring fluoride in groundwater can occasionally push levels above the recommended range, which is one reason your local water report is worth checking.
Disinfection Byproducts
When chlorine reacts with organic matter naturally present in the source water, it creates byproducts. The two main groups are trihalomethanes and haloacetic acids. These form in tiny amounts, but because they’re linked to increased cancer risk at higher exposures, the EPA sets strict annual average limits on both. Utilities must test for them regularly and report the results.
The levels in your tap water depend heavily on the source. Systems drawing from rivers or lakes with more organic material tend to produce more byproducts than those using deep groundwater. If your water has a strong chlorine taste, that doesn’t necessarily mean byproduct levels are high, but it does indicate more disinfectant is being used.
Lead and Copper From Your Pipes
Lead and copper aren’t added to water during treatment. They leach in from plumbing materials: lead solder, brass fixtures, copper pipes, and in older homes, lead service lines connecting the house to the water main. The EPA’s goal for lead in drinking water is zero. Its action level, the threshold that triggers mandatory corrective measures, is 15 parts per billion.
Copper has an action level of 1.3 parts per million. At that concentration, some people notice a metallic taste. Higher levels can cause nausea. The risk of lead and copper leaching increases when water sits stagnant in pipes for hours, which is why running your cold tap for 30 seconds to two minutes before drinking in the morning can meaningfully reduce exposure, especially in homes built before 1986 when lead solder was still legal.
PFAS: The “Forever Chemicals”
Per- and polyfluoroalkyl substances, commonly called PFAS or “forever chemicals,” are synthetic compounds used in nonstick coatings, food packaging, and firefighting foam. They’ve entered water supplies nationwide and break down extremely slowly in the environment.
In 2024, the EPA finalized the first-ever enforceable limits for PFAS in drinking water. The two most studied compounds, PFOA and PFOS, are now capped at 4 parts per trillion each. Three other PFAS compounds are limited to 10 parts per trillion. Those are extraordinarily small concentrations, reflecting the concern that even low-level, long-term exposure may affect the immune system, hormone function, and cancer risk. Water systems have several years to comply, so testing and treatment infrastructure is still being built out in many communities.
Bacteria and Other Microorganisms
The disinfection process is designed to eliminate harmful microorganisms, and in the vast majority of cases it works. The EPA sets a maximum contaminant level goal of zero for total coliform bacteria (an indicator of contamination) and for Legionella, the bacterium that causes Legionnaires’ disease. Legionella has been found in water distribution systems of hospitals, hotels, homes, and public buildings, though illness typically occurs when contaminated water is inhaled as a mist rather than swallowed.
Cryptosporidium, a parasite resistant to chlorine, is another concern for certain water sources. Systems at higher risk use additional treatment methods like UV light or ozone to address it.
Microplastics
Tiny plastic particles have been detected in tap water across the globe, though concentrations vary widely depending on the study and the detection method used. Research sampling tap water from the U.S., Europe, and Japan found average concentrations ranging from roughly 29 to 66 particles per liter. Smaller particles tend to be more numerous: studies that measure down to 1 micrometer consistently find higher counts than those that only capture particles visible to the naked eye.
There are currently no regulatory limits for microplastics in drinking water. The health effects of ingesting these particles at typical tap water levels are still being studied, but the World Health Organization has noted that based on limited current evidence, microplastics in drinking water do not appear to pose an immediate health risk.
Other Regulated Contaminants
The EPA regulates more than 90 contaminants in public water systems. Beyond what’s already covered above, the list includes arsenic (limited to 10 parts per billion), nitrate (limited to 10 mg/L, especially important for infant health), mercury, and dozens of pesticides and industrial solvents. Each has a legally enforceable maximum contaminant level, and utilities that exceed them must notify the public and take corrective action.
How to Check What’s in Your Water
Every community water system in the U.S. is required to publish an annual Consumer Confidence Report, sometimes called a water quality report. It lists the source of your water (river, lake, aquifer), every regulated contaminant detected, the levels found, and how those compare to legal limits. It also flags any violations and describes what the utility is doing about them.
Your report should arrive by July 1 each year, often as a link on your water bill or posted on the utility’s website. Key things to look for: any contaminant listed above its maximum contaminant level, lead results if you live in an older home, and the source water assessment that summarizes contamination risks in your area. If you can’t find your report, the EPA’s Safe Drinking Water Hotline (1-800-426-4791) can point you to it. For a more detailed picture of what’s coming out of your specific faucet, home water test kits are available for lead, copper, chlorine, and other common concerns.