You got your lab report back. Now what? Water test results can be confusing — different units, technical terms, and rows of numbers that don't obviously mean anything. Here's how to make sense of them.
Reading the report format
Most lab reports list each analyte tested along with:
- Your result — the measured concentration in your water
- Units — how the concentration is expressed (see below)
- Detection limit (MDL) — the lowest concentration the lab can reliably measure
- MCL or action level — the EPA health-based limit for that analyte
- Result flag — sometimes shown as ND (not detected), <MDL, or a specific value
Understanding units
Water quality results are typically reported in:
- mg/L (milligrams per liter) — also equivalent to ppm (parts per million). Common for nitrate, fluoride, hardness.
- µg/L (micrograms per liter) — also equivalent to ppb (parts per billion). Common for arsenic, lead, uranium, metals. 1 mg/L = 1,000 µg/L.
- pCi/L (picocuries per liter) — used for radioactive contaminants like radium and radon.
- CFU/100mL (colony forming units) — used for bacteria.
- Standard units (SU) — used for pH (a dimensionless scale from 0–14).
What "not detected" means
"Not detected" (ND) does not mean the contaminant is absent. It means the concentration is below the lab's method detection limit. The actual level could be anywhere from zero to just below the MDL. For contaminants with very low health limits (like arsenic at 10 µg/L), even an ND result is generally reassuring.
Comparing to EPA limits
The EPA sets Maximum Contaminant Levels (MCLs) for regulated contaminants in public water systems. These limits also serve as reference points for private well owners, though they are not legally enforceable for private wells.
- Result below MCL — Generally considered acceptable, though some contaminants have health effects below the MCL
- Result above MCL — Action is warranted; consult our treatment guide or a water quality professional
Some contaminants (like lead) have an "action level" rather than an MCL. Others (like PFAS) have health advisory levels that are not legally binding MCLs.
When to take action
Consider treatment or retesting if:
- Any result exceeds the EPA MCL or health advisory
- Bacteria are detected (any coliform detection warrants immediate action)
- Lead is detected at any level if children or pregnant women are in the household
- Nitrate exceeds 10 mg/L and infants under 6 months drink the water
Use our Test Result Interpreter to paste in your results and get a plain-language explanation and recommended next steps.