📌 Key Point
Most private well contaminants have no taste, color, or odor at harmful concentrations. The only way to know what’s in your water is to test it. Don’t rely on sensory indicators alone.
Biological Contaminants
Coliform Bacteria
Total coliform bacteria are the most commonly detected contaminant in private wells. They serve as indicators of broader contamination. Their presence means the well may be vulnerable to more dangerous pathogens. Sources include cracked well casings, flooding, nearby septic systems, and animal activity.
About 1 in 5 private wells tested nationally shows some form of bacterial contamination in a given year, though rates vary enormously by region and well construction. See what to do if you test positive →
E. coli (Fecal Coliform)
E. coli specifically indicates fecal contamination—human or animal waste has entered the water supply. This is a more serious finding than total coliform alone. Sources include nearby septic failure, livestock operations, and surface-water infiltration through an inadequate wellhead seal.
Giardia and Cryptosporidium
Parasitic protozoans that can survive standard chlorination. More common in surface water (dug wells, springs) than deep drilled wells. Causes gastrointestinal illness. Standard water tests don’t screen for these automatically—you need to specifically request protozoan testing if you have a shallow well or suspect surface water intrusion.
Chemical Contaminants
Nitrates
Nitrate is one of the most widespread chemical contaminants in private wells, particularly in agricultural areas. Sources: fertilizer application, livestock manure, failing septic systems. The EPA MCL of 10 mg/L is set specifically to protect infants under 6 months, in whom high nitrate levels can cause methemoglobinemia (“blue baby syndrome”).
Nitrates are tasteless and odorless. Boiling water does not remove nitrates—it concentrates them. Reverse osmosis or ion exchange at the point of use is the standard treatment.
Arsenic
Arsenic occurs naturally in bedrock in many U.S. regions, including New England, the Upper Midwest, and parts of the Southwest. Long-term exposure above the EPA MCL of 0.010 mg/L (10 ppb) is associated with increased risk of bladder and lung cancer, cardiovascular disease, and diabetes. It has no taste or odor.
Arsenic levels can vary significantly even between neighboring wells in the same area, depending on which rock formations the well draws from. If you’re in an arsenic-prone region, test specifically for it at least every 2 years.
Lead
Lead is rarely present in the groundwater itself. Most lead contamination in well water comes from lead solder used in plumbing installed before 1986, or from brass fixtures containing lead alloys. Acidic water (low pH) dramatically accelerates lead leaching from these materials.
There is no safe level of lead exposure for children. Even “low” lead levels below the EPA Action Level of 15 ppb can contribute to developmental impacts. If your home has plumbing installed before 1986 and your water is acidic, test for lead and consider point-of-use NSF-53 certified filtration.
Radon
Radon dissolved in well water off-gases into the air when you use water (showering, washing dishes). In homes where radon-in-water is a concern, the airborne radon risk from the water may exceed the direct ingestion risk. New England, parts of Appalachia, and areas with granite geology have the highest radon potential.
Radon in water is measured in picocuries per liter (pCi/L). The EPA recommends treatment if radon in water exceeds 4,000 pCi/L (the level that typically contributes 0.4 pCi/L to indoor air). Point-of-entry aeration or granular activated carbon (GAC) systems are the primary treatment methods.
Volatile Organic Compounds (VOCs)
VOCs include gasoline components (BTEX: benzene, toluene, ethylbenzene, xylene), dry cleaning solvents (PCE, TCE), and industrial chemicals. Sources include leaking underground storage tanks, dry cleaners, and industrial facilities. Many VOCs are known or suspected carcinogens at elevated concentrations.
If you live within a mile of a gas station, dry cleaner, industrial facility, or former agricultural chemical storage site, request a VOC panel as part of your testing.
Naturally Occurring Inorganic Minerals
Iron
One of the most common aesthetic issues with well water. Iron causes orange-red staining on fixtures, laundry, and appliances, and gives water a metallic taste above about 0.5 mg/L. The EPA SMCL is 0.3 mg/L. Not a health risk at concentrations typically found in wells, but damaging to appliances and plumbing over time.
Manganese
Similar to iron in aesthetic effects (black or dark brown staining). The EPA issued a health advisory of 0.3 mg/L for manganese based on concerns about neurological effects with long-term exposure at higher levels, particularly in infants fed formula mixed with well water. The SMCL for taste/odor/staining is 0.05 mg/L.
Hardness (Calcium and Magnesium)
Hard water is not a health concern but causes scale buildup in water heaters and pipes, reduces soap effectiveness, and shortens the life of appliances. Water above 120 mg/L as CaCO3 is considered “hard.” Water softeners (ion exchange) are the standard treatment.
Hydrogen Sulfide
The source of the “rotten egg” smell in some well water. Produced by sulfur bacteria or occurs naturally in some geological formations. Not a direct health hazard at concentrations typically found in private wells, but unpleasant and can corrode plumbing.
Emerging Contaminants
PFAS (Per- and Polyfluoroalkyl Substances)
Often called “forever chemicals,” PFAS are increasingly detected in private wells near industrial facilities, military bases (AFFF fire-suppression foam), landfills, and some agricultural areas. The EPA in 2024 set MCLs for six PFAS compounds—the first federal drinking water limits for these substances.
If your well is within 1–2 miles of a known PFAS-contaminated site, test specifically for PFAS using EPA Method 533. Granular activated carbon (GAC) or reverse osmosis filters can reduce PFAS at point of use.