Tap water is mostly pure H₂O, but it contains a mix of dissolved minerals, added disinfectants, fluoride, and trace amounts of other substances picked up during treatment and delivery. The exact composition varies by region, source, and the pipes it travels through, but every glass of tap water in the U.S. is regulated to meet federal safety standards covering more than 90 potential contaminants.
Dissolved Minerals
Water is an excellent solvent. As it moves through soil, rock, and underground aquifers on its way to a treatment plant, it dissolves naturally occurring minerals. The most common ones found in U.S. tap water are calcium, magnesium, sodium, potassium, iron, copper, manganese, phosphorus, and zinc. A nationally representative USDA sampling found that if you drink about two liters of tap water a day, four minerals contribute meaningfully to your diet: copper provides roughly 10% of the daily value, calcium about 6%, magnesium about 5%, and sodium about 3%.
Calcium and magnesium are the two minerals that determine “hardness.” Soft water contains less than 100 mg/L of calcium carbonate, while very hard water tops 300 mg/L. Hard water leaves white scale on faucets and inside kettles, but it’s not a health concern. In fact, epidemiological research suggests that water with at least 20 to 30 mg/L of calcium and 10 mg/L of magnesium may carry modest health benefits, and the average U.S. tap water meets those thresholds.
Disinfectants: Chlorine and Chloramine
Before water reaches your faucet, utilities add a disinfectant to kill bacteria, viruses, and parasites. The two most common choices are chlorine and chloramine (a compound made by combining chlorine with ammonia). Chloramine breaks down more slowly, so utilities often choose it when water has to travel long distances through pipes. A few systems use chlorine dioxide instead, though this is less common.
The EPA considers chlorine or chloramine levels up to 4 mg/L safe in drinking water. Most systems keep levels well below that ceiling. These disinfectants are what give tap water its faint bleach-like smell. If your water has a noticeable chlorine taste, letting it sit in an open pitcher for 30 minutes or running it through a carbon filter will reduce it.
Fluoride
Most community water systems in the U.S. 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 benefits while minimizing the risk of dental fluorosis (faint white spots on teeth that can develop in children who get too much fluoride). Some water sources contain naturally occurring fluoride at or above this level, in which case the utility doesn’t need to add more. Your annual water quality report will tell you the exact fluoride concentration in your supply.
What Your Pipes Add
Water can pick up additional substances between the treatment plant and your glass. Copper dissolves from copper plumbing, especially in newer homes where the interior of the pipes hasn’t yet developed a protective mineral coating. In older homes, the bigger concern is lead. Lead enters drinking water when lead service lines, lead solder in joints, or brass fixtures corrode. Several factors affect how much lead leaches in: the acidity of the water, how long the water sits in the pipes, water temperature, and whether the utility applies corrosion control treatment.
The EPA’s goal for lead in drinking water is zero, with an action level of 0.015 mg/L that triggers additional steps if exceeded. Utilities are required to treat water so it’s less corrosive to the plumbing it passes through. If your home was built before 1986, running the cold tap for 30 seconds to two minutes before drinking or cooking flushes out water that has been sitting in contact with older plumbing.
pH and Other Baseline Properties
Tap water isn’t perfectly neutral. The EPA’s recommended pH range is 6.5 to 8.5, which spans slightly acidic to slightly alkaline. This is a secondary standard, meaning it’s a guideline rather than an enforceable rule, but utilities pay close attention to pH because water that’s too acidic corrodes pipes and water that’s too alkaline can taste flat or leave deposits. Other secondary standards cover things like color, odor, and total dissolved solids, qualities that affect how your water looks and tastes without posing a direct health risk.
Regulated Contaminants
The EPA sets legally enforceable maximum contaminant levels for dozens of substances that could end up in drinking water. A few notable ones: arsenic is capped at 0.010 mg/L, nitrate (a common agricultural runoff chemical) at 10 mg/L, and lead at an action level of 0.015 mg/L. These limits apply to every public water system in the country.
A newer category of concern is PFAS, often called “forever chemicals” because they don’t break down easily in the environment. In April 2024, the EPA finalized the first national drinking water standards for two of the most studied PFAS compounds, PFOA and PFOS, giving public water systems until 2029 to comply. These synthetic chemicals aren’t added intentionally but can be present in source water due to industrial contamination.
How to Find Out What’s in Your Water
Every public water system serving more than 25 people is required to publish an annual Consumer Confidence Report. This document lists the source of your water (whether it comes from a river, lake, or aquifer), every regulated contaminant detected, the levels found compared to legal limits, and any violations along with what the utility did to fix them. It also includes health information about lead, arsenic, and nitrate in areas where those are relevant concerns.
Your utility mails or emails this report once a year, and most post it online. If you can’t find yours, the EPA’s Safe Drinking Water Hotline (800-426-4791) can point you to it. Reading the report is the single most practical step for understanding exactly what’s dissolved in the water coming out of your faucet, since the composition varies significantly from one zip code to the next.