What should you test for if you drink well water?

Why wells are different. A public water system is tested, treated and reported on by law. A private well is not: the owner is responsible for making sure the water meets drinking-water standards, and more than 23 million U.S. households rely on private wells¹. In a Wisconsin survey, about half of well owners had tested in the past ten years, and only 10% of those had tested in the past 12 months⁴.

The yearly four. The CDC advises testing at least once a year, at a state-certified lab, for total coliform bacteria, nitrates, total dissolved solids and pH. Test again after flooding or land disturbance nearby, after repairs to the well system, when taste, color or smell changes, when someone becomes pregnant, or when a child starts living with you³.

What your area adds. Most contaminants above health benchmarks in wells come from local geology; nitrate and bacteria are the main exceptions, tied to land use. In the USGS survey, radon topped the lower of two proposed limits in about 65% of wells, nitrate, arsenic and manganese were each above benchmarks in roughly 5–7%, and total coliform bacteria turned up in about a third². An estimated 2.1 million people in the contiguous U.S. use well water with predicted arsenic above 10 micrograms per liter (µg/L)⁵.

Nitrate and radon. Nitrate comes from fertilizer, septic systems and livestock and can lower the blood's ability to carry oxygen, especially in infants⁶. Radon forms in groundwater from natural uranium and radium; breathing in high levels over time can cause lung cancer, and it needs treatment where water enters the house, since point-of-use devices will not reduce it⁶. See radon.

Manganese, uranium, PFAS. In 362 children, a tenfold higher manganese level in home tap water went with 2.4 fewer IQ points⁷. In 325 people using drilled wells, uranium in water was weakly linked to changes in how kidney tubules handle calcium and phosphate⁸. PFAS (long-lasting industrial chemicals) appeared in 15% of wells in an area with no known sources and in 53–88% of wells at sites with known sources⁹.

Treating what you find. Match treatment to the contaminant and use certified equipment. If bacteria show up, ask your health department how to disinfect the well and retest³. The CDC lists reverse osmosis, ultrafiltration, distillation or ion exchange for arsenic, ion exchange, distillation or reverse osmosis for nitrate, and filters certified under NSF/ANSI 53 or 58 for PFAS; boiling will not remove chemicals⁶. See choosing a filter and, for city water, testing your water.

National survey. The USGS sampled as many as 2,167 domestic wells in 30 aquifers between 1991 and 2004; most contaminants were measured in 1,389 wells. About 23% of wells had at least one contaminant above an enforceable limit or a health-based screening level, though no single contaminant exceeded its benchmark in more than 8% of wells. Radon was above the lower proposed limit (300 pCi/L) in about 65% of wells but above the higher one (4,000 pCi/L) in about 4%. Nitrate, arsenic, manganese, strontium and gross alpha radioactivity were each above benchmarks in roughly 5–7%, uranium in about 1–2%, and manufactured organic compounds in 0.8%. Total coliform bacteria were detected in about 34% of wells and E. coli in about 8%². The data are two decades old and describe a sample of aquifers, not your well.

A local study. Murray and colleagues sampled 118 wells in four Maryland counties: 43.2% failed at least one federal health-based standard. Total coliform, fecal coliform, enterococci and E. coli were found in 25.4%, 15.3%, 5.1% and 3.4% of wells, and about 3.4% exceeded the nitrate standard. Nearby animal feeding operations showed no statistically significant link to bacteria, a reminder that the cause of contamination is often not obvious¹⁰.

Arsenic. Ayotte and colleagues built a model from arsenic measurements in 20,450 domestic wells. About 44.1 million Americans use domestic wells, and 2.1 million (95% CI 1.5–2.9 million) in the contiguous states are predicted to be using water above 10 µg/L. This is a prediction for areas, not a measurement at each home, and some regions had sparse data⁵.

Nitrate. A review of epidemiologic studies since 2005 (more than 30 studies) found that, besides infant methemoglobinemia, the strongest evidence links drinking-water nitrate to colorectal cancer, thyroid disease and neural tube defects. Many studies saw higher risk at levels below the legal limit, but exposure assessment was often weak¹¹.

Manganese. Bouchard and colleagues studied 362 children aged 6–13 in groundwater-supplied communities. Median tap manganese was 34 µg/L (range 1–2,700). Each tenfold increase was associated with a 2.4-point lower IQ (95% CI −3.9 to −0.9) after adjusting for maternal intelligence, family income and other factors, and children in the highest quintile scored 6.2 points lower than those in the lowest. Hair manganese tracked water intake, not diet. The study was cross-sectional, so it cannot prove cause⁷.

Uranium. Kurttio and colleagues measured uranium in water and urine in 325 people on drilled wells (median 28 µg/L, maximum 1,920). Higher urinary uranium was associated with greater fractional excretion of calcium and phosphate, signs of altered kidney tubule function, but not with creatinine clearance or urinary albumin. The authors said the clinical relevance was not easy to establish⁸.

PFAS. Wood and colleagues had 271 well users in four states collect tap water for analysis of 25 PFAS. PFAS were found in 15% of wells in an area with no known sources, some above a health guideline, and in 53–88% of wells at sites with known sources. Concentrations were higher closer to production facilities, Superfund sites, spill sites and federal facilities. When filters were recommended, 49% of households installed one⁹.

Null and conflicting results. Findings for the lesser-known contaminants are modest or uncertain: uranium showed no effect on glomerular function⁸, the manganese link is cross-sectional⁷, and nitrate associations below the legal limit may reflect confounding¹¹. None of these changes the baseline advice to test for bacteria and nitrate every year.

Bottom line (confidence 4 of 5). Private wells have no regulator, and a meaningful share fail at least one health benchmark. Yearly testing for bacteria, nitrate, pH and dissolved solids is the sound baseline, with arsenic, radon, uranium, manganese and PFAS added where your geology or neighbors suggest them. Evidence on lower-dose effects of manganese, uranium and PFAS is still observational.