How to read and interpret your well water test results
Most reports have the same few columns: the test, your result, the unit, and a limit to compare against. Read those four, set aside the quality control lines, and check that your result and the limit use the same unit.
The columns
| Column | What it means |
|---|---|
| Parameter, Analyte, Test | What was measured, like nitrate or arsenic. |
| Result | Your number. "Present" or "Absent" for bacteria. |
| Units | mg/L, ug/L (also written µg/L), ppm, ppb, or per 100 mL for bacteria. |
| MCL, Limit, Standard, MAC | The limit to compare with. MCL is the US health limit; MAC is Canada's and Ontario's. |
| RL, RDL, MDL | The lowest amount the lab can reliably measure. Not a limit. |
| Method | The lab method, like EPA 200.8. You can ignore it. |
Units, made simple
1 mg/L is 1 part per million (ppm). 1 ug/L is 1 part per billion (ppb). 1 mg/L equals 1,000 ug/L, so an arsenic result of 14 ug/L is 0.014 mg/L, above the 0.010 mg/L limit. Always convert before comparing.
Nitrate is sometimes reported "as N" and sometimes "as NO3". Limits use nitrate as N. Divide an NO3 number by 4.43 to compare.
ND and the less than sign
ND means not detected. A result like "<0.001" means less than 0.001, the lowest amount the lab can measure. Both usually mean good news. The exception: if that detection limit is above the health limit, the result can't confirm you're under it, so ask the lab to retest with a lower one.
Lines you can set aside
Many reports include the lab's own checks: trip blank, field blank, method blank, duplicate, matrix spike, surrogate recovery, LCS. They test the lab's process, not your water. You can ignore them when reading your results.
Health limits and cosmetic guidelines
Some limits protect health, like nitrate, arsenic, lead and bacteria. Others are about taste, stains and scale, like iron, manganese at low levels, hardness and pH. An asterisk or a "secondary" label usually marks the cosmetic ones. Go over a health limit and you should act; go over a cosmetic guideline and treatment is up to you.
Common questions
What does ND mean on a water test?
Not detected: the lab didn't find any above the lowest amount it can measure.
What does the < sign mean?
Less than. "<0.001 mg/L" means below 0.001 mg/L, the lowest amount the lab could measure. If that number is above the limit, ask for a lower detection limit.
Is mg/L the same as ppm?
Yes. 1 mg/L equals 1 part per million. 1 ug/L equals 1 part per billion, and 1000 ug/L equals 1 mg/L.
Guides to each result
Sources: EPA drinking water standards; Ontario Drinking Water Quality Standards, O. Reg. 169/03.