Got your lab report back? Enter a contaminant and your detected value below to find out if it's safe, worth watching, or needs treatment — compared to the EPA's drinking water limit.
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How to read your lab report
What each column means
Your result — the amount found in your water.
Units — how that amount is written. See below; this is where most confusion starts.
Reporting or detection limit (RL, MDL) — the smallest amount the lab can measure.
MCL or action level — the federal limit for that substance.
Flag — a note such as ND (not detected), J (estimated) or a "<" value.
Units
mg/L = ppm — milligrams per liter, same as parts per million. Common for nitrate, fluoride, iron and hardness.
µg/L = ppb — micrograms per liter, same as parts per billion. Common for arsenic, lead and uranium. 1 mg/L = 1,000 µg/L.
ng/L = ppt — nanograms per liter, parts per trillion. Used for PFAS.
pCi/L — picocuries per liter. Used for radon, radium and gross alpha.
CFU or MPN per 100 mL — bacteria counts. Many labs simply report Present or Absent.
gpg — grains per gallon, used for hardness. 1 gpg = 17.1 mg/L.
Mixing up mg/L and µg/L moves your result by a factor of 1,000, which is why the tool above asks for the unit instead of guessing.
Nitrate: "as N" or "as NO₃"
Nitrate is reported two ways. The federal limit is 10 mg/L measured as nitrogen (as N), which is the same as 44.3 mg/L measured as nitrate (as NO₃). A result of 20 mg/L is over the limit on one basis and well under it on the other, so check the wording on your report.
What "not detected" means
ND, "non-detect" or a value such as <0.5 means the lab found nothing above the smallest amount it can measure. It does not prove the substance is absent — the true level is somewhere between zero and that limit. The tool keeps your reporting limit rather than treating a non-detect as a zero.
Limits are not all the same kind
MCL — an enforceable health limit for public water systems.
Action level — used for lead and copper; it triggers action rather than defining safety.
Health advisory — guidance, not enforceable. Manganese is the common one.
Secondary standard (SMCL) — about taste, smell and staining, not health. Iron, chloride, sulfate and pH are secondary.
Private wells are not covered by any of these rules, so no one tests your water but you. The limits are still the best yardstick available.