Well water lab report decoder
Lab reports mix units and codes: mg/L, ppb, pCi/L, grains per gallon, ND, TNTC, "as N". Convert any number below, then look up the codes. Everything runs on your phone; nothing is sent anywhere.
Type a value to see it in other units.
Codes and abbreviations on lab reports
- ND / Not detected
- The lab did not find the substance above the lowest level its method can report. It does not always mean zero.
- < (less than), e.g. <0.005
- The result is below the reporting limit shown. Treat it as not detected at that level.
- MDL / RL / MRL / PQL
- Detection or reporting limits: the lowest amount the lab's method can detect or reliably report.
- MCL
- Maximum Contaminant Level: EPA's legal limit for public water systems. Private wells aren't regulated, but it's the usual yardstick.
- MCLG
- EPA's health goal, a non-enforceable level. For some contaminants, such as lead and arsenic, it is zero.
- SMCL / secondary standard
- EPA guideline for taste, odor, color or staining, not health (for example iron and manganese).
- Action level (AL)
- For lead (15 µg/L) and copper (1.3 mg/L), the level that triggers action at public water systems.
- MAC / AO (Canada)
- Health Canada's maximum acceptable concentration (health) and aesthetic objective (taste, odour, colour).
- P/A, Present / Absent
- Bacteria result: present or absent in the sample, usually 100 mL.
- CFU/100 mL, MPN/100 mL
- Bacteria counts: colony-forming units, or most probable number, per 100 mL. Any E. coli is a concern.
- TNTC
- Too numerous to count: so many bacteria colonies that the lab couldn't count them. Treat like a positive result and retest.
- as N / as NO3
- Nitrate can be reported as nitrogen (as N) or as nitrate (NO3). EPA's limit is 10 mg/L as N, which is about 45 mg/L as NO3.
- as CaCO3
- Hardness or alkalinity expressed as calcium carbonate. Use the converter for grains per gallon.
- pCi/L, Bq/L
- Units for radioactivity (radon, radium, gross alpha). 1 pCi/L = 0.037 Bq/L.
- J flag / estimated
- The value is an estimate, often because it's between the detection and reporting limits.
- H flag / hold time exceeded
- The sample reached the lab too late for an exact result. Ask the lab whether to resample.
Unit cheat sheet
- 1 mg/L = 1 ppm = 1,000 µg/L (ppb) = 1,000,000 ng/L (ppt)
- Nitrate: mg/L as NO3 ÷ 4.43 = mg/L as N
- Hardness: 1 grain per gallon = 17.1 mg/L as CaCO3
- Radioactivity: 1 pCi/L = 0.037 Bq/L; 1 Bq/L = 27 pCi/L
Questions
Is mg/L the same as ppm?
Yes, for water: 1 mg/L equals 1 part per million. 1 µg/L equals 1 part per billion.
What does ND mean on a water test?
Not detected: the substance wasn't found above the lowest level the lab's method can report.
How do I convert grains per gallon to mg/L?
Multiply by 17.1. For example, 10 grains per gallon is about 171 mg/L as calcium carbonate.
My nitrate says 'as NO3'. How do I compare it with the EPA limit?
Divide by 4.43 to get mg/L as N, then compare with EPA's 10 mg/L limit.
What does TNTC mean?
Too numerous to count: too many bacteria colonies to count. Treat it as a positive bacteria result and talk to the lab about retesting.