What does 'ND' or 'not detected' mean on a water test?

ND (not detected) means the contaminant was not found at or above the laboratory's reporting limit (also called the method detection limit or MDL). It does not mean the contaminant is entirely absent — it means the level, if any, is below the threshold the method can reliably measure. For many contaminants, ND is the expected and acceptable result.

What is an MCL and how does it differ from a health advisory?

An MCL (Maximum Contaminant Level) is an enforceable federal standard. If a public utility exceeds an MCL, it must notify customers and take corrective action. A Health Advisory (HA) is a non-enforceable guidance level set by EPA for contaminants without a finalized MCL. Health Advisories represent EPA's best estimate of a safe level but do not trigger the same regulatory obligations.

My result is below the MCL — does that mean the water is safe?

MCLs represent a regulatory compliance threshold — they are set based on a combination of health risk and technical feasibility of treatment. Some contaminants, like lead, have an MCL goal (MCLG) of zero, meaning any level is considered to pose some risk. Being below an MCL is a positive data point but does not guarantee zero health risk for every contaminant.

What does 'ppb' and 'ppm' mean in water test results?

ppb means parts per billion — micrograms per liter (µg/L). ppm means parts per million — milligrams per liter (mg/L). One ppm = 1,000 ppb. PFAS results are typically reported in ppt (parts per trillion) — nanograms per liter (ng/L). Lead results are typically in ppb. Nitrate results are typically in mg/L (ppm).

What should I do if a contaminant is detected above the MCL?

Contact your state health department or local health department for guidance. For private well users, this typically means investigating the source, considering treatment options, and possibly using an alternative water source for drinking and cooking while you address the issue. For public utility users, your utility is required to notify you and take corrective action if an MCL exceedance is confirmed in compliance monitoring.

Do I need a certified lab to get reliable results?

For health decisions, yes. State-certified or NELAP-accredited laboratories follow standardized collection, chain-of-custody, and analysis procedures that ensure accuracy and traceability. Home test kits can provide useful screening but are not equivalent to certified lab analysis for regulatory or health-decision purposes.

Water Testing Labs

Lab Report Guide

How To Read Water Test Results

Water quality lab reports use technical units and regulatory terms that can be hard to interpret. This guide explains what each element of a lab report means, how to compare results to EPA limits, and what to do next.

Last updated: 2026-05-13 · Source: EPA, NELAP, state lab programs

Direct Answer

A water test report shows detected concentrations (or ND = not detected) in specific units (mg/L, µg/L, ng/L). Compare each result to the EPA MCL for that contaminant — the lab report may include reference limits, or you can look them up using the table below. A result below the MCL is within the regulatory standard. A result of ND means the contaminant was not found at or above the lab's detection threshold — not that it is definitively absent.

Understanding Units in Water Test Results

Units matter. A result of 10 mg/L and 10 µg/L differ by a factor of 1,000. Make sure you are comparing results to benchmarks in the same units.

Unit	Full Name	Also Known As	Typical Use
mg/L	Milligrams per liter	Same as ppm (parts per million)	Nitrate, nitrite, hardness, iron, manganese
µg/L	Micrograms per liter	Same as ppb (parts per billion)	Lead, arsenic, many VOCs, some PFAS
ng/L	Nanograms per liter	Same as ppt (parts per trillion)	PFAS (PFOA, PFOS, PFNA — EPA MCLs at ppt scale)
CFU/100mL	Colony-forming units per 100 mL	—	Coliform bacteria, E. coli
pH units	Dimensionless scale 0–14	7 = neutral; <7 = acidic; >7 = alkaline	pH; secondary standard 6.5–8.5

What Each Element of a Lab Report Means

Sample ID / Chain of Custody

Unique identifier linking your sample to lab analysis records. Verify this matches your submission.

Analyte name

The contaminant or water quality parameter tested. Chemical names may differ from common names (e.g., 'perfluorooctanoic acid' = PFOA).

Result / Detected value

The measured concentration, or 'ND' (not detected) / '<MDL' (below method detection limit). ND means the contaminant was not found above the lab's detection threshold.

Units

The unit of measurement for the result (mg/L, µg/L, ng/L, etc.). Critical for correct interpretation — 10 mg/L and 10 µg/L differ by a factor of 1,000.

MDL / Reporting limit

Method Detection Limit — the lowest level the method can reliably detect. Results below this appear as ND or '<[number]'. This is not zero.

MCL / Health benchmark

The regulatory limit or reference benchmark provided by the lab. Not all labs include this — you may need to look up the MCL separately.

QC / Method reference

Quality control indicators and the EPA or standard method used. Confirms the lab followed a validated analytical procedure.

Quick Reference: EPA Limits for Common Contaminants

Use these benchmarks to interpret your results. MCL = Maximum Contaminant Level (enforceable). MCLG = Maximum Contaminant Level Goal (health-based target, not always enforceable).

Contaminant	EPA Limit	Result Units	Notes
Lead	15 ppb (action level; MCLG = 0)	µg/L (ppb)	No safe level per CDC
Nitrate (as N)	10 mg/L MCL	mg/L (ppm)	Critical for infants
PFOA	4 ppt MCL (2024)	ng/L (ppt)	Applies to public utilities
PFOS	4 ppt MCL (2024)	ng/L (ppt)	Applies to public utilities
Arsenic	10 µg/L MCL	µg/L (ppb)	Naturally occurring in some regions
Total coliform	0 detections per 100mL (MCL)	CFU/100mL or P/A	Presence/absence may be reported
E. coli	0 (MCL: no detectable E. coli)	CFU/100mL or P/A	Any detection is a violation for utilities
pH	6.5–8.5 (secondary standard)	pH units	Affects corrosivity; non-enforceable

Source: EPA National Primary Drinking Water Regulations (current). For the full list: EPA NPDWR

What To Do After Reviewing Results

Results are ND (not detected) for all tested contaminants

No immediate action needed for those contaminants. Keep a copy of results for future comparison. Repeat testing annually or per state guidance for well users.

A result is below the MCL but above zero

Within regulatory standards. Note whether the contaminant has an MCLG of zero (lead, nitrite, coliform), as any level may carry some health consideration. Review whether treatment is warranted based on your specific situation.

A result exceeds the MCL or health advisory

Contact your state health department or local health department for guidance. Consider using an alternative water source for drinking and cooking while you investigate treatment options. For private wells: contact the certifying lab for interpretation.

You have questions about your specific results

Contact the certifying laboratory — they can explain what the results mean for your situation. Your state drinking water program or local health department can also provide guidance.

Next Steps

PFAS testing guide

Lab methods and interpreting PFAS results in ppt

Lead testing guide

First-draw protocol and what ppb results mean

Nitrate testing guide

MCL, infant risk, and what mg/L means

Water testing cost guide

What certified lab tests cost by contaminant

Water treatment options

Filters and systems matched to specific contaminants

Find a certified testing lab

State-certified and NELAP labs by state

Water Test Results FAQs

Data Sources and Methodology

Benchmarks and regulatory guidance are drawn from EPA National Primary Drinking Water Regulations, EPA analytical methods documentation, NELAP accreditation standards, and CDC health guidance. MCL values reflect current published standards as of 2026. Full methodology →

EPA NPDWR (full MCL list) NELAP Accreditation EPA Certified Lab Directory

Last updated: 2026-05-13