How to Read Your Well Water Test Results
A lab report full of abbreviations and mg/L numbers isn't useful if you don't know what counts as safe. Here's how to read yours in five minutes, and which numbers to act on first.
What's Actually on a Standard Lab Report
Most certified lab reports list each contaminant tested, your result, the unit of measurement (usually mg/L or ppb), and a reference value — often labeled MCL, action level, or secondary standard. Some labs flag exceedances in red or bold; others just print the numbers and expect you to compare them yourself.
If your report doesn't flag anything visually, don't assume everything is fine — always compare each result to its reference value manually, especially for bacteria, nitrate, arsenic, and lead.
Understanding MCLs and Action Levels
An MCL (Maximum Contaminant Level) is the highest concentration the EPA allows in drinking water for substances with established health risks. An action level works similarly for a smaller group of contaminants, like lead, where the "safe" level depends partly on plumbing rather than the source water alone.
Results below the MCL are generally considered safe for that specific contaminant. Results at or above it mean the water exceeds a legal, health-based threshold — not a suggestion, an enforceable limit used for public water systems and a strong benchmark for private wells too, even though private wells aren't regulated the same way.
Reading Results Contaminant by Contaminant
| Contaminant | Safe Range | Concern Threshold |
|---|---|---|
| Total coliform bacteria | Absent | Any positive result |
| E. coli | Absent | Any positive result |
| Nitrate | Below 10 mg/L | 10 mg/L or higher |
| Arsenic | Below 10 ppb | 10 ppb or higher |
| Lead | Below 15 ppb | 15 ppb or higher |
| Iron | Below 0.3 mg/L | Staining above 0.3 mg/L |
| Manganese | Below 0.05 mg/L | Staining above 0.05 mg/L |
| pH | 6.5–8.5 | Below 6.5 or above 8.5 |
| Hardness | Below 60 mg/L soft | Above 180 mg/L very hard |
Results That Mean "Stop Using the Water Now"
A few results warrant switching to bottled or treated water immediately, before you even schedule a retest: any positive bacteria result, nitrate above 10 mg/L in a household with infants or pregnant women, arsenic significantly above 10 ppb, or lead well above 15 ppb. These aren't cosmetic issues — they carry documented, immediate health risks.
Results That Are Borderline vs. Truly Concerning
Secondary contaminants like iron, manganese, hardness, and TDS affect taste, smell, staining, and appliance life, but they're not acute health hazards at typical well-water levels. A borderline result here is worth planning treatment around, not panicking over. Primary contaminants (bacteria, nitrate, arsenic, lead) deserve the opposite response: even a borderline positive should be retested quickly and treated as a real signal, not noise.
What to Do After You Get Results
Start by separating your results into three buckets: results well within safe range (no action needed), results that are borderline (retest to confirm before spending on treatment), and results that clearly exceed a limit (treat or switch water sources now, and retest after treatment to confirm it worked). Keep a copy of every report — you'll want the history if you sell the home, file an insurance claim, or need to show a lender the water is safe.
Decode Your Test Results — Free
Use our free decoder to understand what your specific well water test results mean.
Use the Free Decoder →Frequently Asked Questions
What does MCL mean on a well water test report?
MCL stands for Maximum Contaminant Level — the highest concentration of a substance the EPA allows in drinking water. Results below the MCL are considered safe for that contaminant; results at or above it mean the water exceeds the legal safety threshold.
Which well water test results should I worry about first?
Prioritize any positive result for total coliform or E. coli bacteria, nitrate above 10 mg/L, arsenic above 10 ppb, or lead above 15 ppb. These have direct, immediate health implications, especially for infants and pregnant women, and should be addressed before cosmetic issues like hardness or staining.
What's the difference between a primary and secondary contaminant on my report?
Primary contaminants (bacteria, nitrates, arsenic, lead) have enforceable health-based limits because they can cause illness. Secondary contaminants (iron, manganese, hardness, TDS, pH) mostly affect taste, odor, staining, or plumbing — they're regulated as guidelines, not strict health limits.
Do I need to retest if a result is just barely over the limit?
Yes. Levels for contaminants like nitrate and bacteria can fluctuate with rainfall, season, and well use. A single borderline result is worth confirming with a second sample before you invest in treatment, unless the contaminant is bacteria — any positive bacteria result should be treated as actionable immediately.
Should I share my well water test results with anyone?
Keep a copy for real estate transactions, insurance, and your own records. If any result exceeds an MCL, your state or county health department can help interpret it and may want a copy, especially for bacteria or nitrate exceedances near private wells.