What Does “ND” Mean on a Water Test Report?

When you receive a water test report, you might some parameters listed as “ND.” ND means “Not Detected.”

What does “Not Detected” actually mean? Does ND mean there is zero of that contaminant in your water? And how does a non-detect result compare with EPA drinking water standards?

ND doesn’t necessarily mean zero. It means the contaminant was not detected at or above the laboratory’s detection or reporting limit.

What Is a Detection Limit?

A detection limit is the lowest concentration of a substance that the laboratory’s testing method can reliably detect.

Here is an example, if a water test report shows:

Parameter: Lead
Result: ND
Detection Limit: 0.5 ppb

It means the laboratory didn’t detect lead at a concentration of 0.5 parts per billion (ppb) or higher.

The actual concentration could be zero, but it could also be a small amount below 0.5 ppb. The test cannot reliably determine the exact concentration below its stated detection or reporting limit.

Detection limits can vary between laboratories and testing methods. The detection limit depends on factors such as the contaminant being tested, the analytical method, and the laboratory’s equipment and procedures.

Does “Not Detected” Mean Zero?

No. A result of ND does not mean that the contaminant is completely absent from your water.

Instead, ND means the contaminant was not detected at or above the laboratory’s stated detection or reporting limit.

For example, if the detection limit for arsenic is 1 ppb and your result is ND, the arsenic concentration could be:

0 ppb
0.2 ppb
0.5 ppb
0.9 ppb

The test cannot reliably report a specific concentration below 1 ppb using that testing method.

This is why the detection limit is important when interpreting a non-detect result.

Why Does the Detection Limit Matter?

A non-detect result is most useful when the laboratory’s detection limit is low enough to provide meaningful information about the contaminant.

For example, imagine that a contaminant has an applicable drinking water benchmark of 10 ppb.

Example 1: Low detection limit

EPA benchmark: 10 ppb
Laboratory detection limit: 1 ppb
Test result: ND

This is a useful result because the laboratory was able to determine that the contaminant was well below the 10 ppb benchmark.

Example 2: High detection limit

EPA benchmark: 10 ppb
Laboratory detection limit: 25 ppb
Test result: ND

This result provides much less information. The laboratory did not detect the contaminant at or above 25 ppb, but the test cannot tell you whether the concentration is 5 ppb, 15 ppb, or 24 ppb.

What If a Contaminant Is Detected but Below the EPA Standard?

A contaminant can be detected in your water but still be below the EPA Limit

A detected result does not automatically mean that your water is unsafe.

Some parameters like iron and copper are actually healthy at low levels. Other contaminants, like lead, don’t have a safe level of consumption and should ideally be zero.

It’s important to remember that EPA standards are set based off of a combination of health standards and what is realistic for public water utilities to treat to.

Does a Detected Result Mean My Water Is Unsafe?

Not necessarily.

A detected result should be evaluated based on:

What contaminant was detected
The concentration that was detected
The laboratory’s reporting or detection limit
The EPA standard
How the water is being used
Whether the water is from a regulated public water system or a private source
Who is consuming the water (for example is there an infant in the home?)

For some contaminants, even very low concentrations may be dangerous. For others, a detected concentration may primarily affect taste, odor, staining, or other aesthetic characteristics rather than indicate a health risk.

This is why a water test report should be interpreted parameter by parameter rather than treating every detected or non-detected result the same way.

What Is the Difference Between ND and a Low Detected Result?

An ND result means the laboratory did not detect the contaminant at or above its stated detection or reporting limit.

A detected result means the laboratory was able to measure the contaminant at a concentration that can be reported.

The exact terminology and reporting practices can vary between laboratories, so always refer to the definitions provided with your specific water test report.

Key Takeaway: What Does ND Mean?

ND means “Not Detected.” It does not necessarily mean that the contaminant is completely absent from your water.

Instead, it means the contaminant was not detected at or above the laboratory’s stated detection or reporting limit.

When evaluating an ND result, look at the detection limit and compare it with the appropriate EPA standard or health-based benchmark for that specific contaminant.

A good water test report should provide enough information for you to understand not only whether a contaminant was detected, but also how sensitive the testing method was and whether the result is meaningful for your water quality concerns.