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How to read well water test results: what each number and symbol means

To read your well water test results, start with the results table on your report. Each row is one test: what was tested, your result, the unit, and often a limit to compare it with. Read across one row at a time, check that the units match, and watch for letters like ND or a < sign.

If your report shows E. coli or coliform Present, or nitrate above 10 mg/L as N, read Step 6 or Step 4 first.

An example report you can follow

Sample: EXAMPLE-A. Collection point: Example kitchen faucet.

EXAMPLE ONLY. This is an invented teaching report. All results and lab reporting limits are made up, not measurements from a well. The EPA references are real figures from the sources linked below.
Test Result Unit Lab reporting limit EPA reference
Arsenic 7 µg/L 2 MCL: 10 µg/L
Nitrate, as N 2.6 mg/L 0.10 MCL: 10 mg/L as N
Lead ND µg/L 5 Action level: see Step 5
Chloride <5 mg/L 5 Secondary: 250 mg/L
Total coliform Present P/A Not stated See Step 6
E. coli Absent P/A Not stated See Step 6

In this example, the total coliform Present result is the one to act on first. See Step 6.

The last column uses EPA's primary standards and secondary guidelines. Source check dates are below. For this example, ND means not detected at the lab reporting limit; P/A means present or absent. Each lab reporting limit uses the unit on its row.

Step 1: find your result and its unit

Look at the arsenic row. The result is 7 µg/L. The 2 beside it is the lab's reporting limit, not your result or the EPA limit.

On your own report, check the sample name, collection point and dates first. The date collected, date received by the lab and date tested are different. Check any notes about the sample's condition, too.

The Nevada Analytical Services sample report puts sample details above the results, with units and reporting limits beside the results and definitions below. Your lab may arrange them differently. Its legend explains its letters and symbols; the method column names the test it used.

Keep the whole row when you enter a supported result in the checker, including its unit and any letters or symbols.

Step 2: match mg/L and µg/L before comparing numbers

The arsenic row uses µg/L. A reference table might use mg/L instead.

For water, NDSU Extension describes mg/L as approximately the same as parts per million, or ppm. EPA's monitoring data uses µg/L, or parts per billion, ppb.

mg/Lmultiply by
1,000 →µg/L
mg/Ldivide by
← 1,000µg/L
On water reports: mg/L is approximately ppm; µg/L is approximately ppb.
The amount in the water stays the same. Only the unit changes.
Sources: NDSU Extension and EPA, linked above; dates in Sources.

EPA's arsenic MCL is 0.010 mg/L, which is 10 µg/L. That is why the example shows 10 beside an arsenic result reported in µg/L. These are two ways to write one reference value.

This conversion is for concentration units, not pH or a Present/Absent bacteria result.

Step 3: ND and < don't mean zero

Look at lead. Instead of a number, the result is ND.

The Nevada lab's legend defines ND as "Not Detected at RL." RL means reporting limit: the level used to report that test. Our example uses that definition, with a lead reporting limit of 5 µg/L. Other labs may define ND differently, so read their legend.

Now look at chloride. <5 mg/L means the result is reported as less than 5 mg/L. It is not an exact result of 5 or zero. Keep the < sign.

A lab reporting limit is about the test's ability to report a result. It is not a drinking water limit. EPA makes that distinction for the minimum reporting levels in its monitoring program.

If ND has no explanation or reporting limit, ask the lab. The checker has separate entries for a number, ND and a less-than result; use the one your report actually gives.

Step 4: nitrate "as N" and "as NO3" are different

Look at nitrate: 2.6 mg/L as N. The words "as N" belong with the number.

"As N" expresses the amount using just the nitrogen part of nitrate. "As NO3" uses the whole nitrate, nitrogen and oxygen together. NDSU Extension distinguishes these ways of reporting nitrate. Changing mg/L to µg/L does not change one form into the other.

EPA's nitrate MCL is 10 mg/L measured as nitrogen. If your report says "as NO3," ask the lab for the matching value before comparing it. If it says only "nitrate," ask which form it means. Nitrate, nitrite and a combined nitrate/nitrite test also have different labels; copy yours in full.

EPA gives this warning, with its condition attached:

"Infants below the age of six months who drink water containing nitrate in excess of the MCL could become seriously ill and, if untreated, may die."

EPA’s advice for private wells and infants is: "DON'T mix well water containing more than 10 milligrams per liter of nitrate with infant formula or food." The same page says bottled water "can be used to mix powdered infant formula."

If your result is above the limit and a baby under six months drinks the water, or formula made with it, contact your health department today.

Step 5: an MCL, action level and health goal are not the same

The example's last column mixes different kinds of reference. Read the label as well as the number.

If your report just says "limit," ask the lab which kind it means and where it came from.

Step 6: a bacteria result says Present or Absent, not mg/L

The example says total coliform Present and E. coli Absent. Those are two separate tests. The E. coli entry does not erase the total coliform entry.

In NDSU Extension's report example, Present means the bacteria tested for were detected; Absent means they were not detected. On your report, keep the organism's name, test method and any sample-volume information together.

If your own report says E. coli or coliform Present, contact your local health or environmental department today with the full test name and result. CDC's testing guidance says: "Contact your local health or environmental department."

For possible contamination, CDC's emergency advice is:

"If your well is or may be contaminated with harmful germs or chemicals, do not drink the water. Contact your health department for specific advice. Drink bottled water or water from another safe source until you have treated and tested your well water."

Read the department the exact row rather than describing it as just a "failed test." EPA's state well-program directory can help you find your state's contact route.

Questions to ask your lab

Copy this into an email and attach the report:

Hello, my sample ID is ______. Please help me read these rows: ______.

Thank you.

About this guide

Built from EPA and CDC guidance, NDSU Extension and a published lab report. We have not tested water. EPA's drinking water standards and coliform rules cover public water systems; CDC explains that federal public-system rules do not apply to private wells. This guide explains report fields and references, not whether your household should use the water. Standards, local geology and state requirements all vary. Confirm anything on this page with your state or local health department before acting on it.

Sources and check dates