How to read a water test report: MCLs, units and flags

To read a water test report, match each result to the right benchmark, make sure the units match, and then check any lab flags or comments. For private wells, many reports use EPA public drinking water standards as a guide, but EPA’s enforceable drinking water rules apply to public water systems, and states may use stricter standards or added guidance. (epa.gov)

Start with the benchmark, not the scary-looking number

A water test report usually has three columns that matter most: the analyte, the result, and the units. “Analyte” is what the lab tested for. “Result” is how much was found. “Units” tell you how the amount was measured. Some reports also include a comparison value or health value right on the page. (naturalresources.extension.wisc.edu)

The most important question is not “Is this a big number?” It is “Is this result above the right benchmark?” For many private well reports, that benchmark is an EPA drinking water number such as an MCL, an action level, or a secondary standard. If your report does not show the comparison value, ask the lab or your health department which benchmark to use. (epa.gov)

If you are still in the testing stage, start with a state-certified drinking water lab and a clear test list based on your location and well history. A planning tool like the Well water test planner can help you decide what to order before you ever get a report back. (cdc.gov)

What MCL, MCLG, TT and secondary standards actually mean

An MCLG is a health goal. EPA defines it as the level below which there is no known or expected risk to health, with a margin of safety. An MCL is the highest level allowed in drinking water for public water systems. EPA says MCLs are set as close to the MCLG as feasible, using the best available treatment technology and taking cost into account. (epa.gov)

A TT, or treatment technique, is different. Instead of a single allowed number, it is a required treatment process or performance standard. EPA uses treatment techniques for some contaminants, including lead. (epa.gov)

A secondary standard is usually about taste, odor, color, staining, or scaling rather than direct health effects. EPA says national secondary drinking water regulations are non-enforceable federal guidelines, although states may adopt them as enforceable rules. Examples include iron at 0.3 mg/L, manganese at 0.05 mg/L, pH at 6.5 to 8.5, chloride at 250 mg/L, sulfate at 250 mg/L, and total dissolved solids at 500 mg/L. (epa.gov)

That means a report can show a “high” result that matters mostly for laundry stains, metallic taste, or scale buildup, and not for the same reason as nitrate, arsenic, or lead. This is one of the biggest reasons people misread water reports. (epa.gov)

Make sure the units match before you compare anything

A safe result in one unit can look alarming in another. The Wisconsin Extension guide explains that reports may use ppm or mg/L, and ppb or µg/L. It also says µg/L and ppb both mean one part per billion, while mg/L and ppm both mean one part per million. Montana State University Extension likewise notes that ppm is often used interchangeably with mg/L. (naturalresources.extension.wisc.edu)

That matters because many health-based contaminants are reported in tiny amounts. Arsenic’s EPA MCL is 0.010 mg/L, which is the same as 10 µg/L. Lead action levels for public systems are shown in parts per billion, while many other chemical results appear in mg/L. If you compare a result in µg/L to a standard in mg/L without converting, you can be off by a factor of 1,000. (epa.gov)

Nitrate is another place people get tripped up. EPA’s national table lists nitrate as measured as nitrogen, with an MCL of 10 mg/L. So when you read a nitrate result, check the exact label on the report before you compare it to any standard. If nitrate is a concern in your household, especially with babies, see Nitrate in well water, infants and what to do. (epa.gov)

Common flags and notes on a lab report

Water reports often include short codes that say how confident the lab is in a result. These flags are useful, but they are not perfectly standardized from one lab to another. Read the legend on your report first. If the report has no legend, call the lab and ask. (files.nc.gov)

Here are common ones:

• ND means non-detect or no detection. Montana State says ND means the parameter was not detected in the sample. Wisconsin says ND means the level was less than the lab’s limit of detection, or LOD. (waterquality.montana.edu)
• LOD is the limit of detection, the smallest amount the lab equipment can distinguish from a blank. (naturalresources.extension.wisc.edu)
• LOQ is the limit of quantitation, the smallest amount the lab can measure accurately and precisely. (naturalresources.extension.wisc.edu)
• RL or reporting limit is, in plain terms, the smallest concentration the lab reports as a usable result. Montana State says the RL is basically the smallest concentration a test can detect. (waterquality.montana.edu)
• U often means the lab analyzed for the contaminant but did not detect it above the practical quantitation limit, or PQL. (files.nc.gov)
• J often means estimated. North Carolina DEQ says a J-qualified value is an estimated value and may not be accurate. (files.nc.gov)
• F in the Wisconsin guide means a flagged result that falls between the LOD and LOQ, so it cannot be measured accurately or precisely. (naturalresources.extension.wisc.edu)

A flagged result is not something to ignore. It means you should read the comments carefully and often retest before making a big treatment decision. Wisconsin Extension specifically suggests considering another test to confirm results that are above health values. (naturalresources.extension.wisc.edu)

How to read a few common private well results

Bacteria: Coliform and E. coli usually are not reported in mg/L. You may see “present/absent,” a simple symbol, or a count per 100 mL depending on the method. Wisconsin’s guide says some reports show a smiley face for absent and a frowny face for present. If total coliform or E. coli is present, treat that as a real finding and follow up quickly. For a plain-English walkthrough, see Coliform and E. coli in well water: what a positive result means. (naturalresources.extension.wisc.edu)

Nitrate: EPA’s MCL for nitrate measured as nitrogen is 10 mg/L. EPA says infants below 6 months old who drink water above that level can become seriously ill and, if untreated, may die. CDC also recommends private well owners test for nitrates at least once every year. (epa.gov)

Arsenic: EPA’s MCL for arsenic is 0.010 mg/L, or 10 µg/L. If your result is at or above that level, do not guess. Confirm the result and talk with the lab or local health department about the next step and appropriate treatment options. If arsenic is on your report, this guide on Arsenic in private wells: testing and treatment options may help. (epa.gov)

Lead: Lead is tricky because EPA’s health goal for lead is zero, but for public water systems EPA uses a treatment technique and an action level of 15 ppb rather than an MCL. EPA also explains that lead in tap water often comes from corrosion of plumbing materials, not the well itself. So if lead shows up on a report, ask how the sample was collected and whether a follow-up first-draw or flushed sample is needed. (epa.gov)

A simple 6-step checklist for any report

1. Check the sample details first. Make sure the sample date, address, and tap location are correct. (naturalresources.extension.wisc.edu)
2. Find the benchmark used. Look for an MCL, health value, action level, or secondary standard. (epa.gov)
3. Match the units. Compare mg/L to mg/L, and µg/L to µg/L, unless you convert carefully. (naturalresources.extension.wisc.edu)
4. Read every flag and comment. ND, J, U, RL, LOD, and LOQ change how you interpret a number. (files.nc.gov)
5. Separate health issues from nuisance issues. Nitrate, arsenic, and lead are not the same kind of problem as iron, manganese, hardness, or pH. (epa.gov)
6. Confirm before you spend money. If a result is above a health value or heavily qualified, consider a repeat test and review your sample collection steps. This guide on how to collect a well water sample without ruining the test can help, and so can this overview of what to test your private well water for (and how often). (naturalresources.extension.wisc.edu)

What to do next if something looks off

Start with the lab report itself. Read the legend, the units, and the comments. Then contact the lab or your local health or environmental department and ask three plain questions: Is this above a health benchmark, does it need confirmation, and what type of follow-up sample should be collected? CDC says health departments can help you understand results and what to do next. (cdc.gov)

If you need a repeat test, use a certified lab and follow the sampling instructions closely. Keep the new result with the old one so you can compare dates, tap locations, and units side by side. Over time, that record matters as much as any single number. (cdc.gov)

Because state and local rules can differ, confirm any required tests, notices, home-sale requirements, or treatment approvals with your state or local health, drinking water, or environmental office. (epa.gov)

Frequently asked questions

Is “non-detect” the same as zero?

Not exactly. It usually means the lab did not detect the contaminant above its detection or reporting threshold. Wisconsin says ND means the level was less than the limit of detection, and North Carolina DEQ says a U-qualified result means the analyte was not detected above the lab’s practical quantitation limit. (naturalresources.extension.wisc.edu)

My report says iron is high, but there is no MCL. Is it unsafe?

Not necessarily. Iron is covered by EPA’s secondary standard of 0.3 mg/L, which is meant to address aesthetic problems such as taste, color, or staining rather than a primary federal health standard. That still matters for your plumbing and daily use, but it is a different kind of problem than nitrate or arsenic. (epa.gov)

Why does lead use an action level instead of an MCL?

EPA says lead in drinking water often comes from corrosion of household plumbing materials. Because of that, EPA uses a treatment technique under the Lead and Copper Rule, and public water systems must take added actions if more than 10 percent of tap samples exceed the lead action level of 15 ppb. EPA’s health goal for lead is still zero. (epa.gov)

Should I test again after a bad result?

Often, yes. CDC recommends testing private wells at least once each year for total coliform bacteria, nitrates, total dissolved solids, and pH, and also after events such as flooding, repairs, or changes in taste, color, or smell. Wisconsin Extension also recommends considering another test to confirm results above health values. (cdc.gov)