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You sent your water sample to the lab, and now the results are in. But as you look at the report, you see a wall of abbreviations, numbers, and units that might feel more like a chemistry exam than useful information. MCL, MDL, ND, ppb, CFU/mL — what does it all mean, and should you be worried?

This guide breaks down the most common terms, units, and result types you will find on a water testing report. By the end, you will know how to read your results with confidence and understand when action is needed.

Key Terms on Your Report: MCL, Action Level, MDL, ND

Every water test report uses a set of standard terms and abbreviations. These are not just lab jargon — they carry specific, important meanings. Here are the four you will encounter most often:

MCL — Maximum Contaminant Level

The MCL is the legal maximum concentration of a contaminant allowed in public drinking water, as set by the U.S. Environmental Protection Agency (EPA). Think of it as the speed limit for a specific contaminant — set based on health research, it represents the highest level considered safe for long-term consumption.

For example, the MCL for arsenic is 10 parts per billion (ppb). If your water test shows arsenic at 12 ppb, it exceeds the MCL, and corrective action is warranted. If it shows 4 ppb, you are within the legal limit. Note that MCLs apply directly to public water systems, but they remain the best available benchmark for private well owners as well.

Action Level

An action level is similar to an MCL but applies to contaminants that enter water through plumbing rather than the source itself. The most well-known action levels are for lead and copper: 15 ppb for lead and 1,300 ppb (1.3 ppm) for copper. Exceeding an action level at your tap means you should investigate the source — often older plumbing, solder, or fixtures.

MDL — Method Detection Limit

The MDL is the smallest concentration of a substance that the laboratory can reliably detect using a given method. Think of it as the resolution of a camera — a more sensitive method has a lower MDL and can detect smaller amounts. If the MDL for lead is 1 ppb, the lab can confidently identify lead at 1 ppb or higher, but not below that threshold.

ND — Non-Detect

ND means the contaminant was not detected above the method detection limit. This is generally good news — the lab did not find a measurable amount of that substance in your sample. However, ND does not mean absolute zero. It means that if the substance is present, it is below what the method can measure. In practical terms, an ND result means you are well within safe limits.

Understanding Units: mg/L, ug/L, ppb, ppm, CFU/mL

The units on your report tell you how much of a substance is present. The concepts are straightforward once you understand the relationships.

mg/L and ppm

Milligrams per liter (mg/L) and parts per million (ppm) are effectively the same measurement. When you see 5 mg/L, that is 5 ppm. These units are used for contaminants at relatively higher concentrations, such as nitrate, fluoride, and total dissolved solids.

To put 1 ppm in perspective: it is equivalent to one drop of water in roughly 13 gallons. Small, but well within the range that matters for health.

ug/L and ppb

Micrograms per liter (ug/L) and parts per billion (ppb) are also equivalent. One ppb is one thousand times smaller than one ppm. These units are used for contaminants harmful at very low concentrations, such as lead, arsenic, and pesticides.

To visualize 1 ppb: imagine a single drop of water in an Olympic-sized swimming pool. The relationship is simple: 1 ppm = 1,000 ppb. So 0.010 mg/L equals 10 ug/L, which equals 10 ppb.

CFU/mL — Colony Forming Units per Milliliter

CFU/mL is the unit used for bacterial testing. It counts viable bacterial colonies that grow from a milliliter of your sample. A result of 5 CFU/mL means five colonies grew per milliliter tested.

For drinking water, the standard is straightforward: total coliform and E. coli should be absent. Any detection of E. coli indicates fecal contamination requiring immediate action.

What Do Flagged Results Mean and What Should You Do?

Most reports will flag results that exceed regulatory limits. Understanding these flags is critical to knowing whether and how urgently you need to act.

Results Above the MCL

This is the most serious category. For public water systems, exceeding an MCL triggers mandatory corrective action. Private well owners should treat an MCL exceedance with the same urgency.

Common contaminants that may exceed MCLs include:

• Arsenic (MCL: 10 ppb) — common in parts of Arizona and the western U.S.
• Nitrate (MCL: 10 mg/L) — often from agricultural or septic sources
• Total coliform / E. coli (MCL: presence/absence) — any detection is a concern
• Lead (Action Level: 15 ppb) — typically from plumbing, not the water source

If any of these exceed their limits, take the following steps: stop using the water for drinking and cooking (especially for infants, children, and pregnant women), contact your laboratory for guidance, and consult a water treatment professional about remediation options.

Results Between Detection and the MCL

A result between the MDL and MCL means the contaminant was detected but is within the legal limit. This warrants monitoring — test again in six months to a year to track whether levels are stable, rising, or declining.

For carcinogens like arsenic, there is no truly “safe” level — the MCL balances health protection with feasibility. If your result is close to the MCL, consider treatment even though you are technically within limits.

Non-Detect Results

ND means the contaminant was not found above the detection limit — the best possible outcome. Remember that ND does not mean absolute zero, but for all practical purposes, ND results require no action. Continue testing on your regular schedule.

Understanding Qualified Results

Some results carry data qualifiers — letter codes or footnotes providing additional context, such as an estimated result near the detection limit or a note that a holding time was exceeded. Your laboratory should explain any qualifiers in the report or in an accompanying legend.

When to Call Your Lab for Help Interpreting Results

A good laboratory does not just hand you numbers — it helps you understand what those numbers mean for your specific situation. Here are the situations where you should pick up the phone:

• Any result exceeds the MCL or action level: Your lab can help you understand the severity of the exceedance, recommend confirmation testing, and point you toward appropriate remediation resources.
• You are unsure what a parameter means: Water test reports can include dozens of parameters. If you do not recognize a contaminant or do not understand why it was tested, ask. No question is too basic.
• You need guidance on remediation: While laboratories typically do not sell or install treatment systems, experienced lab staff can help you understand what type of treatment is appropriate for your specific contaminants — whether that is reverse osmosis, activated carbon, UV disinfection, or another approach.
• Results seem inconsistent with previous tests: If a contaminant that was previously non-detect suddenly appears at elevated levels, your lab can help determine whether the change is real or reflects a sampling issue.
• You are making decisions about your home or family: Whether you are buying property, preparing for a baby, or considering a treatment system, your lab can provide the context you need.

How AATLS Helps

At AATLS, we believe that a test result is only valuable if you understand it. That is why expert support comes standard with every report we issue. Our team does not just deliver data — we provide clear, plain-language guidance on what your results mean and what steps, if any, you should consider.

We are an ISO 17025 accredited and CDC ELITE certified laboratory, which means our results meet the highest standards of accuracy and reliability. You can review our accreditations for full details on our certifications and quality systems.

Whether you are a homeowner testing a private well, a property manager overseeing multiple buildings, or a business ensuring regulatory compliance, we are here to help you make sense of your water quality data.