DMR Non-Detect Reporting: What to Enter When Your Lab Says "<"

The lab report comes back and there it is: <0.010 mg/L. Or <0.005. Or any number preceded by a less-than sign. You've probably seen hundreds of these results over your career. And if you're honest, you've probably wondered more than once whether you're handling the DMR entry correctly — because this is one of those things that seems simple until you start asking questions about it.

This matters for one specific reason: whatever you enter on your Discharge Monitoring Report gets compared directly against your permit limit in ICIS, EPA's Integrated Compliance Information System. The system does not know your lab reported a non-detect. It only sees the number you submitted. Enter the wrong value, and you can create an apparent violation where none exists, distort your monthly averages, or leave yourself exposed during a compliance review.

Let's go through this carefully, from the basics up through the averaging complications that catch even experienced operators off guard.

What "Non-Detect" Actually Means

A non-detect result does not mean the analyte is absent. It means the concentration is below the threshold at which the laboratory's analytical method can reliably measure it. That threshold can be defined two different ways, and the distinction matters for DMR non-detect reporting.

Method Detection Limit (MDL)

The method detection limit, or MDL, is the lowest concentration at which the analyte can be detected with statistical confidence — specifically, at a 99% confidence level that the signal is distinguishable from background noise. MDL is determined by running a series of low-level standard spikes and calculating the result from the standard deviation of those measurements. It is a statistical concept, not a practical one. The lab can detect a signal at the MDL, but quantifying it with acceptable precision at that level is a different matter.

Practical Quantitation Limit (PQL) / Minimum Quantitation Level (MQL)

The practical quantitation limit (PQL), also called the minimum quantitation level (MQL), is higher than the MDL — typically by a factor of 3 to 10. This is the lowest concentration at which the lab can report a quantitative result with acceptable accuracy and precision under routine conditions. When your lab reports a result as <0.010 mg/L, that 0.010 is often the PQL, not the MDL. The MDL might be something like 0.003 mg/L.

This distinction is critical because the value after the less-than sign determines your substitution math, and different permit conditions or state guidance documents reference one or the other.

What You Cannot Enter: Zero

Entering zero for non-detect values creates several problems. First, it artificially deflates your reported averages. If you have three results in a month — 0.018, a non-detect, and 0.022 — and you enter 0 for the non-detect, your reported average drops to 0.0133 instead of the correctly calculated value. That may look good on paper, but it misrepresents actual conditions and can mask real analytical trends.

Second, zero is not accepted by EPA as a valid substitution approach for non-detect results. EPA's guidance documents on statistical treatment of environmental data have consistently rejected zero substitution as a practice that biases analyses downward without scientific justification. If an inspector or auditor reviews your bench sheets and sees zeros on the DMR where the lab reported non-detects, it raises questions about your overall data quality and reporting practices.

What You Cannot Enter: Nothing

This is one of the more painful automatic violations in NPDES compliance because it has nothing to do with whether your effluent actually exceeded a limit. You sampled, you sent the sample to the lab, the lab ran the analysis — but the DMR field was left empty. ICIS sees it as unreported monitoring data, which is a separate category of violation from an exceedance. Some state agencies treat missing data violations as seriously as limit exceedances during enforcement prioritization.

The Standard Approach: Half-MDL Substitution

The method that EPA recommends in its guidance, and that most state NPDES programs have adopted as a default, is one-half MDL substitution. When a result is reported as a non-detect, you substitute a value equal to half the method detection limit for purposes of DMR reporting and averaging calculations.

The scientific rationale is straightforward. You know the true concentration is somewhere between zero and the MDL. If you assume a uniform distribution of possible values across that range, the expected value is MDL/2. It is not a perfect estimate — no substitution value can be — but it is a defensible, unbiased central estimate that is widely accepted by regulators.

Half-MDL substitution also prevents the systematic downward bias of zero substitution while avoiding the upward bias of substituting the full detection limit value. It sits in the middle, which is why it has become the regulatory standard.

When Your Permit or State Specifies Something Different

The half-MDL approach is a default, not a universal rule. Before you apply it, you need to check two things in this order:

1. Your NPDES permit. Some permits include specific language about non-detect handling. The permit might require half-PQL substitution, full MDL substitution, or even a specific numeric value for a particular parameter. Permit language controls over any general guidance.
2. Your state NPDES program's guidance documents. Many states have published specific non-detect reporting guidance that applies to all permittees. Some states require half-PQL rather than half-MDL. Others have adopted the full MDL as the substitution value. A few states have parameter-specific rules — metals might be handled differently than nutrients, for example.

Common state variations you may encounter:

• Half-PQL substitution: Used in several states, particularly for parameters where the MDL is considered unreliably low relative to permit limits. If your lab reports <0.010 mg/L (PQL) with an MDL of 0.003, you would enter 0.005 rather than 0.0015.
• Full MDL substitution: Less common but used by some states as a conservative approach. You enter the MDL itself, not half of it.
• Report as non-detect with qualifier: Some electronic DMR systems allow you to enter the detection limit value with a "<" qualifier flag. NetDMR supports this in certain configurations — check with your state on whether this is acceptable in lieu of a numeric substitution.

The bottom line: check your permit first, then state guidance. When in doubt, the permit controls.

Worked Example: Copper Non-Detect

Your permit requires monitoring effluent copper monthly. The permit limit is 0.013 mg/L (daily maximum). The lab reports the result as <0.005 mg/L. You confirm with the lab that 0.005 mg/L is the MDL for this method (EPA Method 200.8 by ICP-MS in this case). Your state follows EPA's half-MDL guidance and your permit has no specific non-detect language.

The calculation:

Non-detect reported at MDL = 0.005 mg/L
Substitution value = MDL / 2 = 0.005 / 2 = 0.0025 mg/L

DMR entry: 0.0025 mg/L

On your DMR calculation worksheet, you would record:

• Lab-reported result: <0.005 mg/L (non-detect)
• MDL: 0.005 mg/L (per lab analytical report, Method 200.8)
• Substitution method: half-MDL per EPA guidance
• Substituted value used for DMR: 0.0025 mg/L
• Reference: [cite your state guidance document or permit condition]

That documentation trail is not optional — we will come back to why in a moment.

The Averaging Complication

Here is where operators most often get tripped up: when you have a mix of detected and non-detected results in the same reporting period and your permit requires a monthly average.

The rule is simple but frequently violated: all results — detected and substituted non-detect values alike — go into the average. You do not average only the detected results and ignore the non-detects. Every sampling event that occurred goes into the denominator and the numerator.

Consider this scenario. Your permit requires four weekly effluent samples per month for total phosphorus, with a monthly average limit of 1.0 mg/L. The lab results come back:

Week 1:  0.87 mg/L  (detected)
Week 2: <0.10 mg/L  (non-detect, MDL = 0.10 mg/L)
Week 3:  1.12 mg/L  (detected)
Week 4:  0.94 mg/L  (detected)

Apply half-MDL substitution to Week 2: 0.10 / 2 = 0.05 mg/L.

Now compute the monthly average including all four values:

Average = (0.87 + 0.05 + 1.12 + 0.94) / 4
        = 2.98 / 4
        = 0.745 mg/L

If you had incorrectly computed the average using only the three detected results:

Incorrect average = (0.87 + 1.12 + 0.94) / 3
                  = 2.93 / 3
                  = 0.977 mg/L

That is a meaningful difference — 0.745 versus 0.977 — and the incorrect calculation is the higher one in this case. But the direction of the error depends on the specific data set; sometimes the incorrect approach produces a lower reported average, which would be understating your actual calculated result. Neither direction is acceptable. The correct approach is always to include all sampling events with appropriate substituted values.

One more nuance: if your permit specifies the number of required samples per month and you collected all of them, every result — detected or not — goes in. If you collected more samples than required, some states allow you to include all of them and some require you to use only the minimum required number. Check your permit language on this point.

Why You Must Document Your Substitution Methodology

Documenting your non-detect substitution approach is not a suggestion. It is an operational necessity for three specific reasons.

Audit defense. If your facility is inspected or if a compliance review is conducted, the inspector will compare your DMR values against your bench sheets and lab reports. If a DMR shows 0.0025 mg/L but the lab reported <0.005 mg/L, the inspector needs to see documentation that explains the transformation. Without it, the difference looks like a data entry error or, worse, a data manipulation issue. A written calculation worksheet with the methodology cited eliminates that ambiguity immediately.

Corrected DMR reconstruction. If you ever need to submit a corrected DMR — which happens more often than anyone likes — your original calculation worksheet tells you exactly what you did and why. Reconstructing the correct methodology months later from memory alone is unreliable and creates risk.

Permit renewal negotiations. Permit writers review your historical DMR data when drafting your next permit. If your non-detect substitution was done inconsistently, the apparent data pattern can influence the permit writer's assumptions about your typical effluent quality — sometimes in ways that lead to more stringent limits. Consistent, documented substitution methodology ensures your historical data accurately represents your effluent and gives you a defensible record if you need to contest a proposed limit.

Your documentation does not need to be elaborate. A simple calculation worksheet — a spreadsheet or even a paper form — that records the lab result, the MDL, the substitution method applied, the substituted value used, and the regulatory basis for the substitution is sufficient. Keep it with your DMR records for the same retention period your permit requires for monitoring data (typically five years).

When to Call Your State NPDES Authority

There are situations where the right move is to pick up the phone and get a documented answer from your state agency rather than proceeding on your own interpretation. Specifically, call your state if:

• Your permit is completely silent on non-detect handling and you cannot find applicable state guidance in writing.
• Your permit limit for a parameter is at or near the MDL or PQL, meaning non-detect substitution values could be a significant fraction of the limit.
• Your lab changes its analytical method mid-permit cycle, resulting in a different MDL for the same parameter.
• You receive non-detect results for a parameter that has never non-detected before, and the permit limit context is ambiguous.

When you call, ask for the answer in writing — an email response is sufficient. Document the date of the conversation, the name of the person you spoke with, and the guidance they provided. That written record becomes part of your compliance file and protects you if the interpretation is ever questioned later.

Non-detect reporting is one of those areas where the regulatory framework is clear in principle but leaves real implementation gaps that get filled differently by different state programs, different permit writers, and sometimes different inspectors within the same agency. Getting the applicable guidance in writing from your specific regulatory authority is the only way to be certain you are meeting your permit obligations.

The good news: once you have your methodology established, documented, and approved, it becomes routine. The complexity is in the setup, not the ongoing execution.