Water Test Results Explained: What Your Numbers Mean

How to read your water test results

Here are your water test results explained, line by line. Each line on a lab report is one substance the lab checked, called an analyte. Read four things on it: the result, the unit, any flag or limit beside it, and the benchmark you compare it with in the same unit. You can enter your values in the tool above: you confirm each one, and the tool compares it with the benchmarks we track for that substance. If anything is unclear, CDC says your health department can help you understand your test results and what to do next.

Units: mg/L, µg/L, ppm, ppb and ppt

Most chemical results are a concentration: an amount of a substance in a liter of water. Your report will use one of a few units, and some labs print both names for the same unit. The federal drinking water rules define them this way:

Each step down the list of concentration units is 1,000 times smaller. A milligram is 1,000 micrograms, and a microgram is 1,000 nanograms. So to turn a result in mg/L into µg/L, multiply it by 1,000, and to turn a result in µg/L into mg/L, divide it by 1,000. These are unit definitions, not limits.

This matters because a lab may report a substance in one unit while the benchmark you are reading is written in another. For example, Pennsylvania's guide shows a lead result in µg/L, while EPA's table of federal standards lists its values in mg/L unless it notes otherwise. Convert one of the two before you compare, or a small number can look large and a large number can look small.

Bacteria are counted differently. When a lab grows bacteria from your sample, it reports the number of colonies per 100 mL of water. CFU stands for colony forming units, another name for the same count. Some bacteria tests skip the count and report total coliform and E. coli only as present or absent.

pH is a scale, not a concentration, so it has no unit. Total dissolved solids is a concentration in mg/L, like most minerals.

Not Detected results, detection limits and lab flags

No lab can measure down to zero. Every test has a level below which the lab can't reliably say whether a substance is there, and your report uses a few terms for that level.

• Method detection limit (MDL). The federal definition is the lowest measured concentration that a lab can report with 99%^[8] confidence that it differs from a blank sample. The MDL varies from lab to lab, and the Pennsylvania Department of Health notes that it is not as reliable as the reporting limit.
• Reporting limit (RL). The lowest level the lab reports as a reliable and accurate number. Pennsylvania's guide says it is typically 2-5 times^[4] higher than the MDL. Federal reporting rules use a similar term, the minimum reporting level.
• Not Detected (ND). Michigan's state drinking water lab reports a substance as Not Detected when it was not found or was found below the reporting limit.
• The less-than sign. In federal reporting rules, a less-than sign means the substance was not detected, or was detected below the minimum reporting level. In Pennsylvania's guide, a lead result shown as a less-than sign, a number and the letter U means lead was not measured at or above that number.

Labs also add letter flags, called qualifiers, next to some results. The letters vary by lab, so check the key on your own report. Pennsylvania's guide explains three common ones:

• U stands for Not Detected.
• J means the value is an estimate. The true value is somewhere between the MDL and the reporting limit.
• Q means the test was run several times and the number shown is the average.

When Not Detected doesn't answer your question

A Not Detected result is only as good as the reporting limit behind it. A Not Detected result with a reporting limit above the benchmark can't show you're below it. For example, if a lab's reporting limit for a substance is higher than the benchmark you care about, the substance could still be present at a level above the benchmark and the report would still say Not Detected.

So look for the reporting limit on every Not Detected line and compare it with the benchmark. If the reporting limit is lower, Not Detected means your water is below that benchmark for that sample. If it is higher, the result can't tell you, and the next step is to ask a certified lab for a method with a reporting limit below the benchmark.

A J-flagged value works the other way. The substance was found, but at a level too low to measure exactly. Treat the number as approximate, and if it sits close to a benchmark, a retest is reasonable before you decide anything.

Water test results chart: common results and their benchmarks

There is no single normal range for a water test. For a contaminant with a health-based limit, the limit is a ceiling, not a target. For minerals that change how water looks and tastes, EPA's secondary standards are the usual yardstick. The chart covers nitrate, TDS and pH from the yearly well test CDC recommends, and other common results, with the unit and kind of each benchmark. Bacteria results have no number: they are present or absent (see "What makes a well water test fail" below).

A result past a secondary standard tells you to expect the effect listed. It is not a health limit set by EPA. If every result is within its benchmark, no treatment is needed. If a result is outside a range and the effect bothers you, confirm it with a certified lab result before you buy any equipment.

Primary and secondary standards

Federal drinking water standards come in two kinds, and they mean different things for your results.

Primary standards are the National Primary Drinking Water Regulations. EPA describes them as legally enforceable standards and treatment techniques that apply to public water systems. Two terms appear often:

• The maximum contaminant level (MCL) is the highest level of a contaminant that is allowed in drinking water.
• The maximum contaminant level goal (MCLG) is the level below which there is no known or expected risk to health. It is a non-enforceable public health goal that allows for a margin of safety.

Secondary standards are EPA's non-mandatory standards for 15^[1] contaminants. EPA does not enforce these secondary maximum contaminant levels (SMCLs). They cover three kinds of effects: aesthetic effects such as unpleasant taste or odor, cosmetic effects that don't damage the body but are still unwanted, and technical effects such as damage to water equipment.

What this means for you:

• A result above a primary standard is the one to act on. Retest with a state-certified lab to confirm it, and talk with your health department about what to do while you wait.
• A result above a secondary standard is usually about taste, color, staining or wear on pipes and fixtures.
• A private well is not covered by the public water system rules, but the same standards are the usual yardstick for well water.

CDC advises using a state-certified laboratory to test your water. Only a certified-lab result is a sound basis for a decision such as buying treatment equipment, and if every result is below its benchmark, no treatment is needed.

Results by contaminant

Each contaminant has its own benchmarks, units and next steps. These guides explain one contaminant each, including what a result at each level means:

• Lead in drinking water: the federal action level, the lower level that applies from the new compliance date, why EPA's health goal for lead is zero, and what to do at each result.
• Bacteria in well water: what total coliform and E. coli results mean, why a present result calls for a retest, and how disinfection and follow-up testing work.
• Arsenic in water: what a result means next to EPA's arsenic limit, its health effects, where arsenic in well water comes from, and when to retest.
• Nitrates in water: what a nitrate result means next to the federal limit, its health effects, where nitrate comes from, and when to retest.
• Iron in water: what a result above EPA's secondary standard means for color, taste and staining, and how to remove iron after a certified-lab result.
• Manganese in water: how EPA's secondary standard for staining and its health advisory differ, and what to do at each level.
• PFAS in well water: the federal benchmarks for the five PFAS we track, the Michigan and Pennsylvania state limits, the status of each as of a stated date, and what to do after a certified-lab test.
• Uranium in water: what a result means next to EPA's uranium limit and its health goal of zero, where uranium in well water comes from, and when to retest.

What makes a well water test fail

A private well has no federal pass mark, so a well test fails when a result misses the benchmark that applies, usually a primary standard or a local health rule. On the yearly test CDC recommends, two results fail:

• Coliform bacteria present. CDC says coliforms generally will not make you sick but get into water the same way as germs that do, and a positive E. coli test likely means feces have reached the well. See coliform in well water.
• Nitrate over its MCL of 10 mg/L^[2]. CDC says high levels of nitrates can make you, and especially babies, sick. See nitrates in well water.

Either result is a reason to retest with a state-certified lab and to call your health department before you buy any treatment.