Detection Limits in Water Testing: Why Non-Detect Isn’t Zero
You open your water test report and find ND or
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A water report or Consumer Confidence Report can list two numbers for the same contaminant, and they do not always match. That is the MCL vs MCLG distinction: the MCL is the enforceable limit for public water systems, while the MCLG is a non-enforceable health goal.
For arsenic, for example, EPA lists an MCLG of zero and an MCL of 0.010 mg/L, or 10 parts per billion (ppb). That difference is deliberate and comes from how federal drinking water standards are set.
Our drinking water quality standards overview explains the broader regulatory framework. Here, we focus on MCLs, MCLGs, SMCLs, action levels and treatment techniques, with EPA examples for arsenic, lead, copper and PFAS.
ETR Laboratories reports water-testing results that you can compare with applicable drinking water benchmarks.
An MCL is the highest contaminant concentration permitted under federal drinking water regulations for public water systems. An MCLG is the health-based goal EPA would like exposure to stay below. The MCL is enforceable; the MCLG is not.
| Term | What It Means | Enforceable? | Applies To |
| MCL | Highest level of a contaminant allowed under the drinking water standard | Yes | Public water systems |
| MCLG | Level below which there is no known or expected health risk, with a margin of safety | No | Health goal used when setting standards |
| SMCL | Guideline for aesthetic effects such as taste, odor, color and staining | No federally | Drinking water quality guidance |
| Action level | Concentration used to trigger required system actions under a treatment technique | Yes through the applicable rule | Lead and copper regulation |
| Treatment technique | Required process or actions used instead of an MCL for certain contaminants | Yes | Public water systems |
The important point is that these terms are not interchangeable. Each one answers a different regulatory or health question.
An MCL, or maximum contaminant level, is the highest level of a contaminant allowed in public drinking water under EPA regulations.
EPA sets MCLs as close to the corresponding MCLG as feasible, taking into account available treatment technology and cost. That is why the legal limit may be higher than the health goal.
Meeting an MCL means a public water system meets that particular federal standard. It does not mean the contaminant is completely absent, and it should not be read as a promise that every lower concentration carries zero risk.
States can also adopt drinking water requirements that are more stringent than federal standards.
An MCLG, or maximum contaminant level goal, is the level of a contaminant below which EPA expects no known or anticipated adverse health effects, with a margin of safety.
Unlike an MCL, an MCLG is not enforceable.
EPA bases the goal on health information rather than treatment cost or whether every water system can consistently reach that concentration. That is why an MCLG may be lower than the enforceable MCL and, for some contaminants, may be set at zero.
You may also see MCLGs in your utility’s annual report. Our guide on how to read your consumer confidence report explains how those numbers appear alongside actual monitoring results.
The MCL has to function as an enforceable standard across public water systems. The MCLG is a health goal.
EPA therefore considers treatment feasibility and cost when setting the MCL, while the MCLG is based on health considerations.
Arsenic shows the difference clearly:
A result below 10 ppb meets the federal MCL for public water systems, but the health goal remains zero.
If you want to see where your own water stands, ETR’s arsenic water test costs $45. Our guide to arsenic in drinking water covers the contaminant itself in more detail.
Lead works differently because EPA does not regulate it with a conventional MCL.
Its MCLG is zero, but compliance is based on a treatment technique designed to control corrosion in public water systems. Utilities collect tap samples from selected locations that are more likely to have lead-containing plumbing.
The current federal lead action level is 15 ppb, or 0.015 mg/L. It is a trigger for utility action, not a health-based safe level for one household.
Under the revised Lead and Copper Rule Improvements framework, systems are scheduled to comply with a 10 ppb action level beginning November 1, 2027.
Copper follows a similar regulatory approach. EPA lists:
Because utility monitoring uses selected homes, your property may not be among the sampled locations. Lead concentrations can also vary with service lines, fixtures and premise plumbing from one home to another.
Our guide to how to test for lead with the first-draw method explains household sampling, and ETR’s lead water test costs $45.
A secondary maximum contaminant level, or SMCL, is a non-mandatory federal guideline for drinking water characteristics that primarily affect appearance, taste, odor or staining rather than the health-based primary standards.
EPA’s secondary drinking water standards include:
That means water can meet federal primary health standards and still have noticeable aesthetic or plumbing-related issues.
SMCLs also should not be confused with MCLs. They serve a different purpose and are not federally enforceable in the same way as primary standards.
PFAS provide a recent example of the same framework at very low concentrations.
EPA’s 2024 federal PFAS rule established:
The 2024 rule remains the current federal framework, but parts of it are under active EPA rulemaking. In 2026, EPA proposed retaining the PFOA and PFOS limits while changing their compliance timeline and separately proposed rescinding requirements for PFHxS, PFNA, HFPO-DA and the Hazard Index.
Because that process is still changing, check EPA’s current PFAS drinking water page before relying on the compliance dates or long-term status of those requirements.
Our guide to new PFAS limits for wells explains what these numbers mean for private well owners. ETR’s PFAS water test covers 18 compounds and costs $415.
Here is the current federal framework discussed in this guide:
| Contaminant | MCLG | MCL or Treatment Technique |
| Arsenic | 0 | MCL 0.010 mg/L, or 10 ppb |
| Lead | 0 | Treatment technique; action level 15 ppb, with 10 ppb scheduled from Nov. 1, 2027 |
| Copper | 1.3 mg/L | Treatment technique; action level 1.3 mg/L |
| PFOA | 0 | MCL 4.0 ppt |
| PFOS | 0 | MCL 4.0 ppt |
| PFHxS | 10 ppt | MCL 10 ppt |
| PFNA | 10 ppt | MCL 10 ppt |
| HFPO-DA | 10 ppt | MCL 10 ppt |
| Certain PFAS mixtures | Hazard Index 1 | MCL Hazard Index 1 |
PFAS requirements are under active rulemaking, so verify the current EPA status before using this table for a regulatory decision.
MCLs and MCLGs commonly appear in Consumer Confidence Reports from public water systems and on laboratory reports that provide regulatory benchmarks alongside results.
EPA requires community water systems to provide an annual CCR. Our guide on how to read your consumer confidence report explains where to find those numbers.
On a laboratory report, you can compare your result with the drinking water limit listed for that parameter.
The reporting limit matters too. A laboratory cannot give useful reassurance about a very low regulatory benchmark if its own reporting limit is too high to evaluate that level. Our guide to non-detect results and detection limits explains that distinction.
Private wells are different.
Federal MCLs regulate public water systems under the Safe Drinking Water Act. Private wells are not routinely monitored under that federal public-water framework, so owners generally need to arrange their own testing unless state, local, lender or other requirements apply.
Our guide on who sets private well standards explains how public drinking water standards are commonly used as reference points for private wells.
For more terminology you may see on a report, our guide to water quality parameters provides additional context.
An MCLG is usually equal to or lower than the MCL because it is a health goal that does not account for treatment feasibility or cost. Arsenic is a clear example: EPA sets the MCLG at zero, while the enforceable MCL for public water systems is 0.010 mg/L, or 10 ppb.
Not exactly. An MCL is an enforceable drinking water standard that takes treatment feasibility and cost into account. A result below the MCL meets that particular regulatory limit, but the corresponding MCLG may be lower. For lead, for example, EPA sets the health goal at zero.
Federal MCLs apply to public water systems, not private wells. Private well owners commonly use them as benchmarks when interpreting laboratory results, but state, local, lender or property-related requirements may also apply. Ask your state or local health department which standards are relevant to your well.
A public water system that violates an MCL must follow EPA or state requirements for corrective action and public notification. How quickly customers must be notified depends on the type and severity of the violation. Lead and copper use action levels rather than MCLs, with their own required responses.
ETR Laboratories can test your water so you have a result to compare with the appropriate drinking water benchmark and discuss in context.
If you are deciding between a single-contaminant test and a broader screen, our panel guide can help you compare the options. ETR also offers individual testing for contaminants such as arsenic, lead and PFAS.
Private well owners generally need to arrange their own testing unless another state, local, lender or property requirement applies.
Call 800-344-9977 if you want help choosing the test that fits your water concern.
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A water report or Consumer Confidence Report can list two
Blue-green stains around a drain, a pinhole leak in a
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