How to read well water test results

Well water test results can be a page of chemical names, units and numbers. Most of it comes down to one question per line: is this result above the benchmark? This page explains the common results, the benchmarks behind them, and what to do when a result is high.

How do you read a well water report?

Labs usually print a result, its unit and a reference value, often EPA's maximum contaminant level (MCL). MCLs are legally binding only for public water systems; private well owners are urged to use them as guidelines, and state or local standards may be stricter. Water Systems Council: wellcare Guide to Evaluating Water Wells (for home inspectors) [WSC-INSPECTOR source note]

Watch the units: mg/L (milligrams per liter, about the same as parts per million) versus µg/L (micrograms per liter, parts per billion). 0.010 mg/L equals 10 µg/L.

What are safe levels in well water test results?

Source for the benchmarks: EPA: National Primary Drinking Water Regulations [EPA-NPDWR source note]

Results such as pH, total dissolved solids, hardness and iron mostly affect taste, staining and plumbing rather than health, and are often compared with non-health "secondary" guidelines. They still matter for treatment choices.

What does a coliform positive well test mean?

The Minnesota Department of Health explains that total coliform bacteria are an easy-to-test indicator, and that their presence suggests there may be disease-causing microorganisms in the water. Its steps: Minnesota Department of Health: Bacterial Safety of Well Water [MDH-BACTERIA source note]

1. Use bottled water, boiled water or another safe source for drinking and cooking until the problem is fixed.
2. Disinfect the well with a chlorine solution, through a licensed well contractor or by following official instructions.
3. Retest after disinfection to confirm no coliform bacteria remain.
4. If disinfection doesn't solve it, work with a contractor; the fix may involve repairs, removing debris, or a new well.

Do not rely on boiling if nitrate or arsenic is also present: boiling can increase their concentration.

What if nitrate in well water is high?

• Confirm it with a second sample if the lab or health department recommends it.
• Use another source for drinking, cooking and infant formula until you have advice.
• Ask your local health department what the result means for your household and what treatment options fit.
• Look for a source: a nearby septic system, fuel tank, fertilizer or animal waste can raise nitrate and other contaminants. Check setbacks in how far a well must be from a septic system.

E. coli vs. total coliform

Labs often report both. Total coliform is a broad indicator group; most types do not cause illness, but their presence signals a pathway for surface contamination. E. coli is a specific member of the group associated with fecal contamination, so a positive E. coli result is treated as more urgent. In either case, follow the steps above: use safe water, disinfect, and retest, and contact your local health department about an E. coli result. Penn State Extension recommends testing annually for coliform and E. coli bacteria. Penn State Extension: Water Well Maintenance and Rehabilitation [PSU-REHAB source note]

Results that point to a well problem

Some patterns suggest the well itself needs attention, not just treatment:

• Bacteria that return after disinfection can mean a damaged cap, casing or seal; see well casing and completion.
• New cloudiness or sediment can signal a declining or damaged well; see signs your well is running dry.
• A rising nitrate trend over several years points to a nearby source.

Keep testing

One clean report is a snapshot. Keep testing every year and after any change; see how often to test well water. If you haven't ordered a test yet, start with private well water testing.