Short answer: do not average results until you know what each sample measured
If two laboratory reports for a private well disagree, first compare the sample locations, collection conditions, analyte definitions, analytical methods, units, and reporting limits. A difference does not automatically mean one lab is wrong: one bottle may represent raw well water and another treated tap water, or the reports may use different methods or detection capabilities. Keep both original reports, contact the laboratories, and ask the local health department or environmental office how to confirm a result that could affect health. Do not average unlike measurements or choose the reassuring number without checking comparability.
If either report indicates a potentially harmful result, use an alternate drinking and cooking source while you promptly seek advice from the health department, lab, or clinician as appropriate. CDC advises private-well owners to use state-certified laboratories and notes that local officials can help interpret results. Boiling is not a universal answer to a water-quality concern; it does not remove dissolved chemicals and can concentrate some of them. Follow public-health advice for the specific contaminant rather than improvising treatment.
Illustrative scenario: the same well has two different nitrate results
Imagine an owner finds a recent nitrate result that is higher than a report from the prior year. The older sample came from a kitchen tap after treatment equipment; the newer sample was collected at an untreated sampling tap. The owner remembers that a filter cartridge was changed and heavy rain occurred before the second test. This is an illustrative scenario, not a report of a measured installation. The two values may answer different questions, so the owner should not calculate an average or conclude that the well abruptly changed based on the numbers alone.
The owner lists each report's collection date, tap, treatment status, bottle, instructions, laboratory, method, result units, and any qualifier such as “less than” or “not detected.” The owner then calls both laboratories to ask whether the samples and methods can be compared and whether the reporting limits differ. If a repeat is warranted, the lab or health office specifies the sample point, bottle, preservation, delivery time, and whether another sample should be collected. The same well can have changing quality, but a controlled follow-up is more informative than comparing undocumented samples.
Check that the samples represent the same water
Start with the sample point. A sample near the well, a pressure-tank tap, an untreated tap before equipment, a kitchen faucet after treatment, and an outdoor hose connection may represent different parts of the plumbing. Treatment can change selected constituents, while stagnant water inside a fixture or pipe can affect some measurements. Mark each sampling point on a simple plumbing sketch and record whether a softener, sediment filter, reverse-osmosis unit, ultraviolet device, or other treatment was operating. A before-and-after result is useful only when its purpose and sampling points are clear.
Compare how the samples were collected. Laboratories may supply different bottles, preservatives, cooling requirements, sterile containers, flushing times, and maximum holding periods. These details vary by analyte and analytical method. A bacterial sample collected with a bottle that was opened or handled contrary to instructions may not be comparable with a carefully collected sample. A volatile-compound bottle can have different handling needs from a metals bottle. Do not rinse, combine, transfer, or preserve a sample unless the laboratory's current instructions specifically say to do so.
Read the analyte, method, units, and reporting limit line by line
Confirm that both reports name the same substance or measurement. A laboratory might report total versus dissolved metal, a specific chemical form versus a broad group, or a screening indicator rather than the contaminant itself. The laboratory method defines how a sample is prepared and measured. EPA explains that analytical methods specify collection, preservation, storage, measurement, quality-control criteria, and result reporting. A “not detected” result is not proof of absolute zero; it means the substance was not reported above the method's applicable reporting or detection capability.
Compare units before comparing values. Reports may express a concentration in milligrams per liter, micrograms per liter, parts per million, or parts per billion. The laboratory can confirm how to convert them and whether the stated benchmark uses the same unit and form. Record any “less than,” estimated, qualified, or censored result exactly as printed. Do not treat a reporting limit as the measured concentration, or compare a numerical detection from one method with a less-sensitive “not detected” from another as if both methods had equal ability to find low levels.
Ask the laboratories and health department precise questions
Contact the laboratories that issued the reports. Provide the report numbers and ask which method and analyte were used, whether that method is appropriate for drinking-water testing in your state, what “not detected” means on that report, whether the result is qualified, and what sample handling could explain the difference. Confirm that the laboratory is currently certified by the state for that particular analyte, not simply that it offers some drinking-water testing. EPA's certification resources explain the laboratory program; state and local programs identify laboratories and scope.
The health department can advise whether the result triggers an immediate precaution, whether a repeat should use a particular sampling point, and whether the household should test for other contaminants based on local geology or land uses. CDC recommends at least annual testing for total coliform bacteria, nitrate, total dissolved solids, and pH and additional locally relevant testing. That recommendation is a baseline, not a universal panel for every site. If a health-relevant contaminant is detected, contact the relevant public-health office instead of relying solely on equipment sales advice.
Plan a controlled confirmation sample
When officials or a certified lab recommend a repeat, request a written collection plan for the exact analyte. It should say which tap to use, whether to sample untreated or treated water, how long to flush if any, which bottle to use, whether the bottle must remain chilled or preserved, the delivery deadline, and how the sample should be labeled. Do not assume the correct procedure for bacteria also works for chemicals, or that a household sampling kit uses the same method as a state compliance laboratory. Read the kit's instruction sheet immediately and ask before the collection window closes if anything is unclear.
For a disputed result, ask the laboratory or health authority whether it wants a new independent sample, paired samples, or a split sample handled under controlled procedures. Splitting one sample can be inappropriate for some analytes or containers, and taking two bottles at different times is not the same as splitting a single, properly homogenized sample. Avoid sending duplicate bottles unless the receiving laboratory confirms its protocol. Record date, weather, recent well or plumbing work, treatment status, and collection steps so a later reviewer can understand the conditions.
Do not make a treatment decision from a mismatched comparison
Treatment selection comes after the result is understood. Different technologies target different contaminants, and some need a pre-treatment sample, a post-treatment sample, or both to verify performance. A device's label, marketing claim, or demonstration strip does not settle what is in a particular well. Ask a qualified water-treatment professional to cite the contaminant-specific performance standard and operating conditions, and ask the laboratory or health department what follow-up test would verify it. Keep the raw result, treated result, service records, and equipment model together.
Do not change a pressure valve, bypass a treatment device, shock-disinfect a well, or add chemicals just to make a repeat number look better. A visible change in odor, taste, color, or pressure may warrant attention, but appearance cannot identify the laboratory cause. If water quality changed after flooding, a well repair, or new nearby construction, tell the local health office; it may recommend additional tests. For basic collection planning, review the site's private-well sample collection guide and follow the receiving laboratory's current kit instructions.
Safety limits and jurisdiction caveats
A laboratory report does not replace a health consultation when the result could affect a household member. Follow any boil-water, do-not-drink, or alternate-source notice from the proper authority. Different precautions apply to germs, metals, volatile compounds, nitrate, and other substances. Do not assume a countertop filter, water softener, ultraviolet lamp, or boiling will remove a contaminant unless an appropriate design and verification process support that use. Keep original reports and document who advised each action.
Private-well testing, accreditation, disclosure, and comparison criteria vary by state and sometimes by county. CDC's annual baseline and EPA drinking-water analytical-method resources are federal guidance, while the state certifies laboratories and local authorities advise on site-specific concerns. The EPA laboratory-certification manual primarily addresses compliance laboratories, so a homeowner should confirm the state's private-well laboratory list and analyte scope. This guide is educational; it cannot interpret an individual report without the complete documents and the lab's method details.
Frequently asked questions
Should I average the two results? No, not until a qualified lab confirms that the analyte, units, method, sample point, collection, and reporting limits are comparable. Does “not detected” mean the water contains none? No; the result is limited by the method and reporting level shown on the report. Should I repeat the sample at the same kitchen tap? Ask the lab or health department; raw-versus-treated sampling may be the question. Can a store test strip settle a disagreement? It may be a screening tool for a limited property, but it does not replace a state-certified laboratory for decisions about drinking-water safety.
Who should I call first? Call the laboratory that ran the result and the local health or environmental department; contact a clinician for advice about personal health concerns. Should I stop drinking the water while I wait? If a report indicates a potentially harmful result, follow public-health advice and use an alternate source while clarifying it. Can a second lab automatically be trusted more? No. Ask what each method measures and whether the lab is certified for that analyte. Keep the reports and collection records together for the next call.
Related homeowner guides
The site's sampling-mistakes guide describes collection problems that can distort a result, while the well-testing guide helps frame a locally appropriate panel. These pages do not supersede a laboratory or health-department instruction for a specific substance. A controlled sample and a clear question make a follow-up more useful than another unplanned kit.