Well water contains whatever the groundwater picks up as it filters through layers of rock, soil, and sediment on its way to your aquifer. That means a mix of naturally occurring minerals like calcium, magnesium, and sodium, along with potential contaminants ranging from bacteria to heavy metals to industrial chemicals. Unlike municipal tap water, private well water isn’t monitored or treated by any government agency, so what’s in it depends entirely on your local geology, nearby land use, and how well your system is maintained.
Minerals That Occur Naturally
As rainwater seeps underground, it dissolves minerals from the surrounding rock. The most common ones in well water are calcium, magnesium, sodium, and potassium. Calcium and magnesium are the two minerals responsible for “hard” water, the kind that leaves white scale on faucets and makes soap harder to lather. Wells drawing from limestone or chalk formations tend to have especially high levels, sometimes exceeding 150 mg/L of calcium or 50 mg/L of magnesium.
Sodium levels vary widely. Wells in coastal areas or regions with certain clay formations can have sodium above 200 mg/L, which may matter if you’re watching your salt intake. Iron and manganese are also extremely common in well water. You’ll notice iron before you ever test for it: concentrations above 0.3 mg/L produce a rusty color, metallic taste, and reddish-orange stains on sinks and laundry. Manganese causes similar problems at even lower levels. Above 0.05 mg/L, it turns water dark and leaves black stains with a bitter metallic taste.
None of these minerals are necessarily dangerous. In fact, calcium and magnesium in drinking water contribute meaningfully to your daily intake. But high concentrations of any mineral can affect how your water tastes, looks, and interacts with your plumbing and appliances.
Bacteria and Other Microorganisms
The contaminant that catches most well owners off guard is bacteria. Total coliform bacteria are the standard screening test for biological contamination. Coliforms live in soil, on plants, and in the digestive systems of people and animals. Their presence in your well doesn’t always mean you’ll get sick, but a high count signals that harmful germs, including viruses and parasites, may have found a path into your water supply.
The more specific concern is fecal coliforms, particularly E. coli. A positive test for either one likely means that animal or human waste has entered the well. This can happen through cracks in the well casing, a poorly sealed cap, flooding, or a nearby septic system that’s leaking. There is zero acceptable level of E. coli in drinking water. If it shows up, the water shouldn’t be used for drinking or cooking until the problem is identified and resolved.
Nitrates From Fertilizer and Manure
Wells in agricultural areas face a particular risk from nitrates, which leach into groundwater from fertilizers, animal manure, and septic systems. The EPA’s legal limit for nitrate in drinking water is 10 parts per million, a standard set back in 1962 to prevent blue baby syndrome, a condition that starves infants of oxygen by interfering with how their blood carries it.
More recent research has raised concerns well below that threshold. Studies now link nitrate levels as low as 5 ppm to elevated risks of certain cancers and birth defects. If you live near farmland, regular nitrate testing is particularly important, especially if anyone in the household is pregnant or there are infants drinking the water or using it in formula.
Arsenic and Heavy Metals
Arsenic occurs naturally as a trace component in many rocks and sediments. Whether it ends up in your well depends on the chemical makeup of your aquifer, how long the water has been sitting underground, and the local soil chemistry. Wells in the Southwest are especially vulnerable. In basin-fill aquifers there, arsenic exceeds the EPA’s maximum contaminant level more than twice as often as in wells nationwide, because the groundwater moves through volcanic and granitic rock that releases arsenic over time. High pH, arid climates, and long groundwater residence times all increase the risk.
Arsenic isn’t just a geological issue. Mining, industrial activity, and the historical use of arsenic-based pesticides and wood preservatives can also push levels higher. The metal is odorless and tasteless, so you won’t detect it without a test. Lead is another invisible threat. It rarely comes from the aquifer itself. Instead, it leaches from old plumbing, solder joints, or well components, particularly when the water is acidic and corrosive.
Industrial Chemicals and Solvents
If your well is near current or former industrial sites, gas stations, dry cleaners, or military bases, volatile organic compounds (VOCs) are a real concern. The most common VOCs in groundwater are benzene, toluene, xylene, and ethylbenzene (collectively known as BTEX), plus chlorinated solvents like trichloroethylene and vinyl chloride. Trichloroethylene, widely used as an industrial degreaser, is one of the most frequent groundwater contaminants and is classified as carcinogenic. Vinyl chloride is similarly toxic.
These chemicals enter groundwater through industrial discharge, leaking underground storage tanks, improper waste disposal, and even the use of certain pesticides and fertilizers. Once in an aquifer, they can persist for decades and travel significant distances from the original source.
PFAS: The “Forever Chemicals”
Per- and polyfluoroalkyl substances, commonly called PFAS, are synthetic chemicals used in nonstick coatings, waterproof fabrics, firefighting foam, and food packaging. They earned the nickname “forever chemicals” because they don’t break down naturally in the environment. PFAS have been detected in groundwater across the country, and private wells near airports, military installations, landfills, and manufacturing facilities are at the highest risk.
The EPA does not regulate private wells, so there are no enforceable PFAS standards for well owners. However, as of 2025, the EPA advises well owners who suspect contamination to contact their state environmental or health agency for guidance and a list of certified labs that can test for PFAS using approved methods. Treatment options include certified in-home filtration systems and filtered pitchers designed to reduce PFAS levels. If testing confirms contamination, using an alternate water source for drinking, cooking, and brushing teeth is a reasonable precaution while you address the issue.
Radioactive Elements
Certain geological formations release naturally occurring radioactive materials into groundwater. Radium, uranium, and radon are the most common. The EPA has set enforceable standards for public water systems: combined radium-226 and radium-228 must stay below 5 picocuries per liter, and uranium must remain below 30 micrograms per liter. These standards don’t legally apply to private wells, but they provide a useful benchmark if you test your water.
Wells drilled into granite, sandstone, or certain sedimentary formations are more likely to contain elevated levels of these elements. Radon in well water is a particular concern because it can be released into indoor air during showering or running hot water, adding to overall radon exposure in the home.
pH and Water Hardness
Two physical properties shape your day-to-day experience with well water more than almost anything else: pH and hardness. The EPA recommends keeping drinking water between a pH of 6.5 and 8.5. Water on the low end of that range (acidic) tends to be naturally soft but corrosive, meaning it can dissolve metals from your pipes and fixtures, potentially adding copper, lead, or zinc to your water over time. Water above 8.5 typically signals high levels of alkalinity minerals and can leave deposits throughout your plumbing.
Hardness is driven by calcium and magnesium content. Very hard water isn’t a health risk, but it creates practical headaches: scale buildup in water heaters, reduced soap efficiency, and spotted dishes and glassware. Many well owners install water softeners for these reasons, though softeners work by swapping calcium and magnesium for sodium, which slightly increases the sodium content of your drinking water.
How Often to Test Your Well
Because no government agency monitors private wells, testing is entirely your responsibility. The CDC recommends testing for total coliforms at least once a year. If coliforms are found, follow up with a test specifically for fecal coliforms or E. coli. Beyond the annual bacteria screen, testing for nitrates is important if you’re in an agricultural area, and testing for pH, hardness, and metals like iron, manganese, copper, lead, and arsenic gives you a fuller picture of your water quality.
You should also test outside your regular schedule after any event that could compromise the well: flooding, nearby construction, changes in taste or smell, a new septic system installed nearby, or any repair to the well itself. State health departments can provide lists of certified labs in your area, and many offer basic well water test kits at low or no cost. A comprehensive panel covering bacteria, nitrates, pH, hardness, and common metals typically costs between $100 and $300, which is a small investment given that you’re the only line of defense between your aquifer and your family’s drinking glass.