Non-Detect (ND)
ND means the laboratory ran the test and the contaminant's concentration fell below the lowest level the method can reliably measure — so the result is reported as not detected rather than as a number.
Where you'll see it on a CCR
Your Consumer Confidence Report includes a water quality table listing every regulated contaminant tested, alongside the result, the applicable MCL, and the units of measurement. When a contaminant shows ND in the result column, the lab found no measurable concentration. Many contaminants appear as ND in most CCRs — that's normal for well-managed systems. The CCR glossary covers the other terms in that table.
What "non-detect" actually means
ND does not mean zero. It means the concentration fell below the laboratory's Method Detection Limit (MDL) or, more commonly in CCR tables, its Method Reporting Level (MRL).
The MDL is the lowest concentration measurable with 99% confidence that the value is greater than zero, as defined at 40 CFR Part 136, Appendix B. Labs often report against the slightly higher MRL, which ensures results are both detectable and quantifiable with acceptable precision.
When a result is ND, the actual concentration could be anywhere from zero up to (but not including) the MRL. For most regulated contaminants the MRL sits far below the MCL, so ND still means the water passed the health standard by a wide margin. But ND is not a guarantee of literal absence — it reflects the resolution limit of the analytical method.
Related terms
- MDL (Method Detection Limit) — the lowest concentration measurable at 99% confidence; defined at 40 CFR Part 136, Appendix B
- MRL (Method Reporting Level) — a higher, lab-set threshold used for routine reporting; what most CCR tables apply
- MCL (Maximum Contaminant Level) — the legal upper limit; an ND result is always compliant with the MCL
- Units of measurement — mg/L, µg/L, ppt, and other scales used alongside ND in CCR tables
Citations
- 40 CFR Part 136, Appendix B — Method Detection Limit procedure: https://www.law.cornell.edu/cfr/text/40/part-136/appendix-B
- 40 CFR § 141.151–141.156 (CCR Rule, Subpart O): https://www.law.cornell.edu/cfr/text/40/part-141/subpart-O
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