What is an MCL on a water test report?

MCL stands for Maximum Contaminant Level. It's the legally enforceable limit for a contaminant in public water systems, set by the EPA. If a result exceeds the MCL, the water system is in violation and must notify customers. Private well owners are not subject to MCLs but should use them as reference limits.

What does "not detected" mean on a water test?

"Not detected" (ND) means the contaminant was not found above the method detection limit (MDL) for that test. It doesn't mean the contaminant is absent. For PFAS with an MDL of 2 ppt, "not detected" means any PFAS present was below 2 ppt, not that there's zero.

How do I know if my water test results are bad?

Compare each detected contaminant against both the MCL (the legal limit) and the MCLG (the health goal). Any result above the MCL requires action. Results above the MCLG but below the MCL, like arsenic between 0 and 10 ppb, warrant consideration of a point-of-use filter. Any positive E. coli result requires immediate action regardless of level.

What is the difference between MCL and MCLG?

The MCL is the legally enforceable standard. The MCLG is the Maximum Contaminant Level Goal, a health-based target with no legal force. For many contaminants they're the same. But for arsenic, the MCLG is 0 (no safe level established) while the MCL is 10 ppb, because treatment to lower levels isn't technically feasible for most systems. The gap represents a known health compromise.

What should I do if my water test shows lead?

Lead has an MCLG of 0 and an action level (not an MCL) of 15 ppb. Any detected lead in drinking water is worth taking seriously, particularly for households with children or pregnant women. An NSF 53-certified faucet filter or under-sink filter removes lead effectively. Never use hot tap water for cooking or infant formula, as heat concentrates lead from pipes.

How to Read Your Water Test Results: What the Numbers Actually Mean

A water test result has four elements that matter: the contaminant name, the detected concentration, the unit of measurement, and a reference level to compare against. Once you know how to read each one, the results stop looking like random numbers.

Understanding the Units

Units trip up most people. They’re not interchangeable.

mg/L (milligrams per liter) is the same as ppm (parts per million). Used for minerals, nitrates, iron, hardness, chlorine. The EPA MCL for nitrates is 10 mg/L.

µg/L (micrograms per liter) is the same as ppb (parts per billion). Used for metals: lead, arsenic, mercury, manganese. The EPA MCL for arsenic is 10 µg/L.

ng/L (nanograms per liter) is the same as ppt (parts per trillion). Used for PFAS. The EPA MCL for PFOA and PFOS, finalized in 2024, is 4 ppt.

CFU/100mL means colony-forming units per 100 milliliters. Used for bacteria testing. Any positive result for E. coli in drinking water requires action, full stop.

The differences matter. 10 mg/L and 10 µg/L look the same on paper but are 1,000 times apart. The arsenic MCL is 10 µg/L. If a result showed 10 mg/L arsenic, that would be 1,000 times the limit.

MCL vs. MCLG: The Gap That Matters

Every water test result you get from a certified lab will reference MCLs. What many reports leave out is the MCLG.

The MCL (Maximum Contaminant Level) is the legally enforceable standard for public water systems. Water utilities must keep levels below this limit or face violation.

The MCLG (Maximum Contaminant Level Goal) is what the EPA considers the health-based target, the level at which no known or anticipated health effects would occur with adequate margin of safety. It has no legal force. It’s a target.

For some contaminants, they match. Nitrates: MCL = MCLG = 10 mg/L. For others, there’s a gap, and that gap is deliberate.

Arsenic: MCLG = 0. MCL = 10 ppb. The EPA acknowledges that any arsenic exposure carries some risk, but set the MCL at 10 ppb because achieving lower levels consistently across all water systems isn’t currently feasible. A result of 7 ppb is below the MCL and technically compliant. It’s still above the MCLG.

Lead: MCLG = 0. There’s no actual MCL for lead at the tap. Instead, the EPA uses an “action level” of 15 ppb. If testing across 90th percentile samples exceeds 15 ppb, the utility must act. But any detected lead in drinking water for households with young children deserves attention, regardless of how it compares to 15 ppb.

EWG Health Guidelines: A Second Set of Numbers

The Environmental Working Group maintains health guidelines for drinking water contaminants that are typically stricter than EPA MCLs. Their guidelines are based on more conservative health risk assessments and aren’t law.

EWG’s PFAS guideline: 1 ppt vs. EPA’s 4 ppt MCL. Their arsenic guideline: 0.004 ppb vs. EPA’s 10 ppb MCL.

These aren’t regulations. But comparing your results against both sets of numbers tells you something. A result that’s below the MCL but well above EWG’s guideline means you’re in the gap between “technically legal” and “health-optimized.”

What “Not Detected” (ND) Actually Means

“Not detected” doesn’t mean absent. It means the contaminant wasn’t found above the method detection limit (MDL) for that specific test.

If the lab’s MDL for PFOA is 2 ppt and your result shows ND, any PFAS present in your water was below 2 ppt. That might be zero. Or it might be 1.9 ppt. The test can’t distinguish within the detection floor.

For ultra-low-MCL contaminants like PFAS, this matters. Look for labs that list their MDLs in the report. The lower the MDL, the more confidence you have in a “not detected” result.

Reading a Bacteria Result

Total coliform is a group of bacteria that includes E. coli and other fecal indicators. For public water systems, the MCL is essentially zero, any positive is a violation requiring customer notification.

For private wells, there’s no regulatory oversight. Any positive coliform result means you need to act before drinking the water. A positive E. coli result specifically indicates fecal contamination and requires immediate response: stop using the water for drinking and cooking, shock-chlorinate the well, retest before resuming use.

The Bottom Line

When results come back, run through this checklist for each detected contaminant.

Is it above the MCL? If yes, action is required.
Is it above the MCLG but below the MCL? Consider a point-of-use filter for drinking and cooking water.
Is it “not detected”? Check the MDL. ND with a MDL of 0.5 ppt is much stronger than ND with a MDL of 5 ppt.
Is there any E. coli? If yes, stop using the water immediately.

For more background on the MCL vs. MCLG distinction, see the EPA MCL vs. MCLG explained page. If you’re trying to decide what to test for, start with how to test water at home or go straight to the best mail-in water tests. Once you know what’s in your water, what water filter do I need matches contaminants to the right treatment type.