Your drinking water contains a mix of minerals, disinfectants, and trace contaminants, some intentionally added and others that find their way in through pipes, soil, agriculture, and industrial discharge. The EPA regulates over 90 contaminants in public water systems under the Safe Drinking Water Act, but many other substances show up at levels too low to be regulated or too new to have formal limits. Here’s what’s actually flowing through your tap.
Minerals Your Body Can Use
Not everything in your water is a concern. Municipal water naturally contains calcium, magnesium, sodium, potassium, zinc, copper, and other trace minerals picked up as water moves through rock and soil. On average, U.S. drinking water contains about 20 to 30 mg/L of calcium and around 10 mg/L of magnesium, concentrations that epidemiological research links to modest health benefits. These amounts won’t replace what you get from food, but they do contribute to your daily mineral intake.
If you use a chemical water softener, it strips most of that calcium out. One national analysis found that softened water had calcium levels roughly half those of untreated water, with median values dropping close to zero. Softeners also tend to increase sodium content, since they swap calcium and magnesium ions for sodium.
Chlorine and Its Byproducts
Water treatment plants add chlorine (or a related disinfectant called chloramine) to kill bacteria and viruses before water reaches your home. That disinfection is essential, but it comes with a tradeoff. When chlorine reacts with naturally occurring organic matter in the water, it creates disinfection byproducts, the two main groups being trihalomethanes (THMs) and haloacetic acids (HAAs).
The International Agency for Research on Cancer has classified some THMs as probable human carcinogens. Several epidemiological studies have linked long-term THM exposure to increased bladder cancer risk. More recently, researchers at the National Cancer Institute found a novel association between HAAs in drinking water and endometrial cancer risk. The concentrations involved are low, and the risk increase is small for any individual, but it’s a population-level concern because virtually everyone drinks treated water every day.
Fluoride
About 73% of Americans on public water systems receive fluoridated water. The U.S. Public Health Service recommends a fluoride concentration of 0.7 mg/L, a level chosen to maximize protection against tooth decay while minimizing dental fluorosis, a cosmetic condition that causes white spots on teeth. Your water utility’s annual Consumer Confidence Report will tell you the exact fluoride level in your supply.
Lead and Other Heavy Metals
Lead doesn’t typically exist in source water. It enters your drinking water when plumbing materials corrode, particularly in older homes and cities. The most significant source is lead service lines, the pipes connecting homes to the water main, which are more common in houses built before 1986. Brass faucets and lead solder on copper pipes are the next most common culprits. Water with high acidity or low mineral content corrodes these materials faster, releasing more lead.
The EPA’s action level for lead is 15 parts per billion. If more than 10% of sampled taps in a water system exceed that threshold, the utility must take corrective steps. But there is no truly safe level of lead exposure, especially for young children. Arsenic is another metal of concern. It enters water both from natural rock erosion and from industrial runoff, including old orchard land where arsenic-based pesticides were once used.
Nitrates From Agriculture
In farming regions, nitrates are one of the most common groundwater contaminants. They come primarily from fertilizer runoff and livestock waste. The EPA’s maximum contaminant level is 10 mg/L for nitrate-nitrogen (equivalent to about 50 mg/L as nitrate). At higher concentrations, nitrates interfere with your blood’s ability to carry oxygen. The body converts nitrate to nitrite, which binds to hemoglobin and forms methemoglobin, a form that can’t deliver oxygen to tissues. Infants are especially vulnerable because their digestive systems convert nitrate to nitrite more readily than adults do.
Bacteria, Viruses, and Parasites
Chlorination and filtration eliminate most pathogens, but failures happen, particularly during infrastructure breakdowns, flooding, or in systems serving small communities. The organisms of greatest concern in water include Cryptosporidium and Giardia, two parasites that cause prolonged diarrhea and are resistant to standard chlorine treatment. Cryptosporidium can be life-threatening for people with weakened immune systems.
Bacterial contaminants include E. coli strains that can cause severe illness progressing to kidney failure in children, Salmonella, Campylobacter (a leading cause of bacterial gastroenteritis), and Legionella, which grows in warm water systems and causes Legionnaires’ disease, a serious form of pneumonia. Viruses like norovirus, rotavirus, and hepatitis A can also contaminate water, though modern treatment catches most of them. Boil-water advisories exist for the times it doesn’t.
PFAS: The “Forever Chemicals”
Per- and polyfluoroalkyl substances, known as PFAS, are synthetic chemicals used in nonstick coatings, waterproof fabrics, food packaging, and firefighting foam. They earned the nickname “forever chemicals” because they don’t break down in the environment. PFAS have been detected in drinking water systems across the country. In 2024, the EPA finalized the first-ever national limits on several PFAS compounds in drinking water, setting maximum contaminant levels in the single-digit parts per trillion for the most studied varieties, PFOA and PFOS. Water systems have several years to comply. Exposure has been linked to thyroid disease, immune suppression, certain cancers, and developmental effects in children.
Pharmaceutical Residues
Trace amounts of medications show up in treated water supplies. The most commonly detected classes are painkillers and anti-inflammatory drugs (ibuprofen, naproxen, diclofenac), antibiotics, antidepressants, and hormones including estrogen compounds. These enter the water cycle mainly through human excretion and improper disposal of unused medications. Conventional water treatment removes some but not all of these compounds. Analgesics, anti-inflammatories, and beta blockers tend to be the most persistent, with the lowest removal rates during treatment.
The concentrations are extremely low, typically measured in nanograms per liter, which is thousands of times below a therapeutic dose. The concern isn’t acute toxicity but rather the unknown effects of constant low-level exposure to a cocktail of pharmaceuticals over decades, along with the potential for hormone-disrupting compounds like estradiol and ethinylestradiol to affect aquatic ecosystems and, possibly, human endocrine function.
Microplastics
Tiny plastic fragments, mostly smaller than a grain of sand, have been found in both tap and bottled water worldwide. A review of 21 studies found that microplastic concentrations were actually higher in bottled water than in tap water, likely because plastic packaging itself sheds particles. Tap water studies have found concentrations ranging from less than 1 to roughly 440 particles per liter depending on the particle size measured and the method used. The smaller the particle size researchers look for, the more they find.
The health effects of ingesting microplastics are still poorly understood, but concerns center on the chemicals embedded in or absorbed onto the plastic particles, including plasticizers and other additives. No regulatory limits exist for microplastics in drinking water yet.
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 (sometimes called a Water Quality Report) that lists detected contaminants and their levels. You can find yours on your utility’s website or by searching the EPA’s database. If you’re on a private well, no one tests your water unless you do. State health departments generally recommend testing well water at least once a year for bacteria, nitrates, and any contaminants common in your area.
For anyone concerned about specific contaminants, home filtration can help. Activated carbon filters (including pitcher filters) reduce chlorine, THMs, and some organic chemicals. Reverse osmosis systems remove a broader range of contaminants including lead, PFAS, nitrates, and many dissolved solids. No single filter removes everything, so matching the filter to your specific water quality results gives you the best outcome.