Well Water Contaminants:
Understand Your Lab Results

A result describes one bottle collected at one place and time. This guide shows you how to verify that context, understand common report terms, and separate health questions from taste, staining, and equipment concerns.

• Observe. Check the property, faucet, collection date, raw or treated status, and any water change that led to the test.
• Test. Confirm the analyte name, units, reporting limit, and flags with the state-certified laboratory that ran the analysis.
• Compare. Use the applicable state or local guidance and compare only with earlier samples collected for the same purpose. Do not choose treatment from a symptom or an isolated number.

Have your report in hand? Enter it in our free well water test results checker to see which results are above EPA health or aesthetic reference levels, with unit conversion and next steps.

Understand what a laboratory report actually describes

A result describes one sample collected at a specific place and time. Every result is defined by four things that appear on the form: the sample itself, the analyses requested, the location where the bottle was filled, and the date and time of collection. Read all four before you react to any number.

The sample includes how it was collected, preserved, and kept until analysis. The location matters because it reframes what the number means. A sample from the kitchen tap describes the water as it reaches the home, after it has sat in your own plumbing. A sample taken at the pressure tank or a raw line describes the source itself, before much of the household has touched it. Change the location and you can change the result, which is exactly why collection details appear on the report. A change over time, comparing this year to last, only makes sense if the two samples were drawn the same way at the same kind of point.

Know what a "not detected" result means

When a report shows "not detected," often shortened to ND, it means the laboratory did not find the substance above the smallest amount that its method could reliably measure for that sample. That smallest reliable amount is called the reporting limit. A not-detected result is bounded by that limit; it is not a statement of absolute zero.

Reporting limits vary by compound, by method, and by laboratory. Two labs can report the same sample with different limits, so you need both the result and the limit to compare them fairly. When you review a report, ask the lab what its reporting limit was for each analysis, in the same units as the result, and treat ND as "below this lab's reporting limit for this sample" rather than as "nothing at all."

Review your results with the right people

Interpret results with the people who can speak to the specifics of your report and your region. That means the certified laboratory that ran the analysis, or your state or local health authority and county health department that oversee private wells. Use pages like this one as orientation, not as a replacement for an informed reading of your own report.

When you call or write, bring the full report and ask which current federal and state guidance applies to the substance, your location, and your household. Ask what the reporting limits were and what the sampled location really represents. Keep the original report, note the date of your conversation, and record the guidance you received so you can trace it later. Guidance changes over time, so a conversation tied to the date is worth more than a number pulled from memory.

Tell health issues from taste, staining, and operational issues

Private-well problems fall into two broad baskets, and it helps to know which one you are looking at before you choose a next step.

Health-related concerns involve contaminants that can matter at low levels and that often leave no clue in how the water looks, tastes, or smells. Aesthetic and operational issues involve minerals and other material that change how water behaves in the home: staining, cloudy water, hardness, an odor, or deposits that shorten the life of appliances. You can have both at once. A well can deliver clear water that smells fine while carrying something you cannot sense, and that same well can be hard or stained enough to complicate daily chores. Senses alone cannot separate the two, so testing is the tool that does.

Learn about coliform bacteria and E. coli in a water well

The most common private-well microbial tests look for total coliform bacteria and for E. coli. Total coliform is a broad family of bacteria that lives widely in soil, plants, and the intestines of animals and people. Its presence in a well points to a possible pathway for contamination, such as surface water seeping in, a damaged wellhead, or a nearby septic system. It is an indicator, not a diagnosis by itself.

E. coli is a more specific subgroup whose detection points more directly to human or animal waste. Both groups are tested as indicators because they signal a route that deserves attention and inspection, not because the bacteria themselves are a single fixed threat. The health concern is the pathway these indicators expose. The practical response is to inspect the well and the area around it, correct whatever lets surface water or waste in, and then retest under proper sampling conditions. Before you sample at all, review our water testing guide and, for the rhythm of routine checks, our annual well testing article. Ongoing household practices appear in the well care guide.

Understand nitrate and nitrite results

Nitrate and nitrite usually reach groundwater from fertilizer, animal waste, or septic systems moving through soil. Both dissolve readily and travel with water, which is why they can appear far from the source of the nitrogen. Nitrate is the form most often measured for a private well.

Nitrate has no taste, odor, or color, so observation cannot detect it. The reason families with infants sample for it and ask a specific question is that health guidance for nitrate is different for infants than for older children and adults, and that guidance also varies by state. Testing is the only way to know whether nitrate is present in your water. If a result comes back, read it with your laboratory or health department before acting, and treat any local guidance as the figure that applies to you.

Labs report nitrate either "as N" or "as NO3", and the two differ by a factor of about 4.4. The test results checker converts between them.

Spot the metals to know: arsenic, iron, manganese, and lead

Metals do not behave alike, so it helps to group them by where they come from and what they do.

Arsenic occurs naturally in some aquifers and dissolved bedrock. It has no taste, odor, or color, so it will not announce itself, and it does not change the appearance of the water. That is why the health-related metals are included in routine analysis rather than left to be noticed at the tap.

Iron and manganese are common natural minerals in groundwater. At levels usually seen in wells they are not a routine health concern, but they are a steady source of annoyance: staining of sinks, tubs, and laundry, buildup in pipes and water heaters, and a metallic taste or, with manganese, dark or black particles. Their main effect is aesthetic and operational. If rust-colored stains are the issue you actually live with, the iron staining article covers how those build up and what to look at.

Lead is different in origin. It rarely comes from the aquifer itself; it usually enters water by dissolving out of household plumbing, solder, or fixtures after water has sat in the pipe. That means the location and timing of the sample change the meaning of the result. A sample drawn right after a long standing period is not the same as one after a flush. If lead is on your mind, coordinate with the laboratory on exactly where and when a meaningful sample should be taken, and connect the result to the plumbing it reflects.

Minerals behind taste, staining, and hardness

Hardness comes from dissolved calcium and magnesium. Hard water is common in private wells. It is not a health concern, but it makes soap lather poorly, leaves whitish scale on fixtures and appliances, and can shorten the useful life of a water heater. It sits squarely in the operational column.

If scale is the problem, the water softener size calculator sizes a unit from your hardness, iron and manganese results.

Other minerals shape how water looks and smells. Iron and manganese drive the rust and black staining described above. Sulfur-related compounds can give water a rotten egg odor, often one that fades after the lines run and returns after the water stands, and they can contribute to corrosion. For the smell itself, the sulfur odor article goes into more detail. Cloudy or milky water has its own set of likely causes covered in the cloudy well water article, and testing remains the way to confirm rather than assume the source.

Learn about organic chemicals and PFAS

Human-made organic chemicals can reach groundwater from leaking tanks, solvents, pesticides, industrial sites, or firefighting foam. As a group they span many chemical families with very different behavior, so it is rarely useful to treat them as one thing.

PFAS is a broad term for thousands of synthetic compounds used in nonstick coatings, waterproofing, firefighting foam, and other products. They resist breakdown, move through soil and water, and can enter wells near where they were used or released. The scientific and regulatory picture for PFAS is still moving: targets and methods are updated as understanding grows. If you test for PFAS, use a laboratory that follows current methods, ask what the newest guidance is for your state, and review the result with a current source, because this is the category where an outdated number or an outdated limit does the most harm.

Learn about radionuclides

Naturally occurring radionuclides such as uranium and radon can be present in some bedrock and dissolved groundwater, most often in regions underlain by certain rock types. Whether they appear in your sample depends on the geology where your well draws its water and on which analyses you chose to run.

Radon forms in rock and soil and can move from water into indoor air, so it is sampled and considered with care. For any radionuclide result, review the report with the laboratory and with your state health or radiological authority. Interpretation is specialized and tied to location and geology, so no single general description fits every region. The measured value alone does not answer the question for your well; the surrounding context and an informed reading do.

When to use another water source

Use a known-safe water source while you get local guidance if sewage or manure may have entered the well, if flooding reached it, if the water has a strong chemical odor or a sheen, or if illness may be related to the water. This is the one situation where an interim step is obvious, and even then the plan is to contact your health department and follow their instruction, not to decide on safety from a page of general guidance.

Where this guidance comes from

The U.S. Environmental Protection Agency and the U.S. Centers for Disease Control and Prevention publish reliable guidance for private-well owners.

• EPA Private Drinking Water Wells: an overview of testing, wellhead protection and common contaminants.
• EPA Protect Your Home's Water: practical actions to reduce contamination risk.
• CDC Well Water Safety: testing and health considerations for private wells.

Choose your next step

Test results, not symptoms or marketing, should drive treatment choices. A system selected before you know the contaminant and its level can be the wrong tool or undersized for your household. So the realistic order is: test first, read the results with the laboratory or health authority, and compare treatment only afterward.

• Check your numbers with the Test Results Checker.
• Use the Symptom Finder to organize a visible or noticeable change.
• Use the water testing guide to plan a valid sample and a certified laboratory.
• Keep a routine with the annual well testing guide.
• Review treatment categories only after you have the relevant results in hand.
• Maintain the well itself with the well care guide.

And keep the distinction from this page in mind the whole way: aesthetic problems show themselves and health-related ones often do not. Sample the right point, ask the right people, and let the report, read in context, point you to the correct next step.