How to Read Your Well Water Test Results, Line by Line

On this page 7 sections

1. 01Start here: three lines that matter most
2. 02What the EPA numbers are, and why they’re a yardstick
3. 03Health limits vs. nuisance limits
4. 04The full table: 25 lines a lab may report
5. 05An annotated sample report and five common misreadings
6. 06When to call your state or county
7. 07From result to fix: use the decoder

A well water report lists what the lab looked for, how much it found, the unit, and usually a limit to compare against. Read three lines first: total coliform and E. coli, nitrate, and arsenic. Then check the unit on every line before you compare it with a limit, because mg/L versus µg/L and “nitrate as N” are where most misreadings start.

The limits printed on your report are almost always the EPA’s. Those are the legal limits for public water systems. No one enforces them on your private well, but they’re the yardstick labs and health departments use, and the one we use below.

Start here: three lines that matter most

Bacteria, nitrate and arsenic carry health limits and turn up in private wells. If any of them is over the line, call your county health department before you buy any equipment.

If you also tested for lead or PFAS, add those two to the list.

These lines matter because private wells aren’t a rare problem. In a USGS study of 2,100 private wells, about one in five had at least one contaminant above a health benchmark. A USGS model estimated that about 2.1 million people drink from private wells with arsenic above 10 µg/L.

Two of the three also tell you something about your septic system. The EPA lists high nitrate or coliform in a well as one of the signs of a failing septic system. If nitrate creeps up from one yearly test to the next, read signs of a failing septic system alongside nitrate in well water.

What the EPA numbers are, and why they’re a yardstick

They’re federal limits for public water systems. Your well isn’t covered, so the numbers are a reference point, not a rule that applies to you.

More than 43 million Americans drink from a private well. Private wells fall outside the Safe Drinking Water Act, and most states don’t regulate them either. Montana State’s guide to reading results puts it plainly: the standards “only apply to public water systems but the health implications are the same for private well water users.”

Not every number on a report is the same kind of limit. Four kinds turn up:

Some lines have no federal number at all: hydrogen sulfide and hardness among them. Sodium has only an EPA guidance figure, for people on a low-sodium diet. For these lines, extension services and state health departments give the guidance.

Health limits vs. nuisance limits

Primary limits are about health. Secondary limits are about stains, taste, smell and pipes. Over a secondary limit, you decide whether it bothers you. Over a primary limit, talk to your county.

The EPA’s secondary standards cover things like the rust-colored stains from iron, the black stains and bitter taste from manganese, salty taste from chloride, and corrosive water. Even public systems don’t have to meet them.

Two cases sit across both lists:

• Fluoride has a secondary standard of 2.0 mg/L (it can affect teeth) and a primary limit of 4.0 mg/L. Between those two numbers, public systems have to notify their customers. It’s the only secondary standard with that duty.
• Manganese has a secondary standard of 0.05 mg/L and an EPA health advisory of 0.3 mg/L. A result of 0.1 mg/L is over one and under the other. More on that in the sample report below.

Copper is on both lists too: an action level of 1.3 mg/L and a secondary standard of 1.0 mg/L.

The full table: 25 lines a lab may report

For each line: the EPA number (public systems), the nuisance number, what being over it usually means, where extension guides say treatment starts, and whether to call your county.

The treatment numbers come from extension services and state agencies (Penn State, Ohio State, Georgia, Virginia Tech, Maryland’s environment department). They’re guidance, not law, and states differ. Where a cell says “ask”, we haven’t found a public source we trust for a specific threshold yet; your county or a certified installer is the right place for that question.

Zinc (5 mg/L), silver (0.1 mg/L), color (15 color units), odor (3 threshold odor number) and foaming agents (0.5 mg/L) also have secondary standards. You’ll rarely see them on a home panel.

Two things about the table. First, “No” in the last column means none of the sources we use ask you to call over a nuisance line. You can still call and ask. Second, the treatment column gives the starting point, not the full setup. The order matters as much as the equipment: a softener can’t do its job on iron if the pH is too low, and UV goes last. Well water treatment: what you need and in what order puts the steps together.

An annotated sample report and five common misreadings

Most wrong conclusions come from five spots on a report: the unit, the nitrate form, “ND”, “present”, and manganese between 0.05 and 0.3 mg/L.

The report below is made up. It’s laid out the way many labs do, and the numbers are ours, picked to show each mistake.

Sample report (made up for this guide)

1. Reading mg/L as µg/L (or the other way around)

Arsenic at 0.012 mg/L is 12 µg/L. That’s over the 10 µg/L line, not far under it.

The arsenic limit is often written as “10 ppb” or “10 µg/L”. This lab reports in mg/L. Someone who sees 0.012 next to “10” thinks they’re nowhere near it. In fact 1 mg/L is 1,000 µg/L, so 0.012 mg/L is 12 µg/L.

The pairs to know:

• mg/L and ppm are used interchangeably for water.
• µg/L and ppb are the same thing.
• ng/L and ppt are the same thing. PFAS results come in ng/L.
• Hardness comes in mg/L as calcium carbonate or in grains per gallon (gpg). 1 gpg is 17.1 mg/L, so the 171 mg/L above is 10 gpg, which Virginia Tech’s scale calls hard (120 to 180 mg/L).

mg/L, ppm, ppb, pCi/L: the units on your water report has every unit and a converter.

2. Nitrate vs. nitrate as N

The EPA limit is 10 mg/L as nitrogen, about 44 to 45 mg/L as nitrate. 31 mg/L as nitrate is about 7 mg/L as N: under the line.

Labs report nitrate in one of two forms: as nitrate-nitrogen (NO3-N, “nitrate as N”) or as nitrate (NO3). Virginia Tech’s extension puts the same EPA limit as 10 mg/L in the first form and 45 mg/L in the second. Our own math from the atomic weights gives a factor of about 4.43, so 10 mg/L as N is about 44.3 mg/L as nitrate. Either way, divide a nitrate (NO3) result by about 4.4 to compare it with 10.

This lab printed 45 next to its NO3 result, so the report is consistent. The trouble starts when you compare with a number from somewhere else. The risky direction is the reverse of this example: a report that says “nitrate-N 8.5 mg/L”, read against a website that says 45, looks fine. Against the right limit of 10, it’s close. Penn State’s lab, for one, reports nitrate-N.

3. “ND” is not zero

ND means the lab couldn’t measure it at its reporting limit. If that limit sits above the number you care about, ND doesn’t answer your question.

New York’s health department explains its private well reports this way: ND (non-detect) means the chemical “was below the level that could be measured by the laboratory equipment.” The reporting limit (RL) is “the level that the laboratory equipment can reliably detect.” The method detection limit (MDL), if your report shows it, is lower: the least the equipment could detect at all.

In the sample, PFOS is ND with a reporting limit of 5.0 ng/L. The EPA level for PFOS is 4.0 ng/L. So this result tells you PFOS is below 5.0, not that it’s below 4.0. If you want an answer at 4.0, ask for a lab whose reporting limit is lower. In New York’s own example report, the reporting limit for PFAS is 1.8 ng/L.

Two letters you may see in a qualifier column, using New York’s definitions:

• U: not detected above the method detection limit.
• J: detected above the method detection limit but below the reporting limit. The PFOA result of 3.1 ng/L is under the level the lab can reliably measure, so treat it as a rough number, not a precise one. It still means the lab found PFOA.

4. “Present” with no number

For bacteria, “present” is the whole result. It doesn’t need a count to matter.

Coliform tests on private wells usually come back absent or present. Some labs report a count per 100 mL (MPN or CFU) instead; ask the lab how its numbers read against absent and present.

The EPA’s coliform limit for public systems is a share of positive samples per month (5.0 percent). That rule doesn’t translate to one well, so don’t go looking for a percentage. Montana State’s guide reads the results this way:

• Total coliform present: “No direct health threat, but coliforms should not be present in groundwater.” Find out how they got in: the septic system, animals, a poor well cap, flooding, leaking lines, or a contaminated sample.
• E. coli present: “Direct health threat; treat water for drinking and cooking.”

In the sample, total coliform is present and E. coli is absent. The next steps are to look for the source, shock chlorinate the well and retest. Coliform or E. coli in your well has the timeline.

5. Manganese between 0.05 and 0.3 mg/L

At 0.12 mg/L, manganese is over the EPA’s stain-and-taste standard and under its health advisory.

Penn State notes both numbers: the recommended limit of 0.05 mg/L isn’t enforceable for private systems, and the EPA health advisory is 0.3 mg/L. The lab printed 0.05, so the line looks like a failed health test. By the EPA’s yardsticks it’s a black-stains problem, not a health-advisory problem. If you’re unsure what it means for your household, ask your county. Iron and manganese in well water covers which filter works at which level.

What the rest of this sample report says

Read the iron, pH and hardness together and you can see the setup take shape:

• Iron at 1.8 mg/L is six times the 0.3 mg/L stain standard. Expect rust-colored stains.
• Penn State lists three conditions for a softener to remove iron: pH above 6.7, hardness of 3 to 20 gpg, and dissolved iron under 5 mg/L. This water meets two of them. At pH 6.4, it misses the third. Birm, another iron filter, needs a pH of at least 6.8.
• So the pH comes before the iron question. Whether the same setup also handles the 0.12 mg/L manganese is a question for the installer, with the numbers in hand.
• The arsenic over 10 µg/L and the coliform come first anyway: county call, source, disinfection, retest.

One more line worth checking at the top of any report: where the sample was taken. If it came from a tap after a softener or filter, the numbers describe your treated water, not the well.

When to call your state or county

Always for coliform, E. coli, nitrate, nitrite, arsenic, lead and PFAS. Recommended for uranium, radium, fluoride above 2.0, barium and other metals, and radon (your state radon program).

County health departments know the local geology and land use, and can tell you what’s typical in your area. Ask them:

1. Is this result typical where I live, and what do you see in nearby wells?
2. Should I retest before I do anything, and at which certified lab?
3. Does anyone in my household need extra care? Bring it up if someone is pregnant, you have a baby on formula, or anyone is on a low-salt diet. Ask your doctor the same question.
4. Does my state have its own limit for this? Some do. New York, for example, sets 10 ng/L for PFOA and PFOS.
5. Is there a program that helps pay for treatment?

The EPA’s state private well programs page links to each state’s program, which is a good place to find the right office.

For sources of nitrate and coliform, the septic system is the one you control. Septic tank care covers the routine; test your well water covers how often to retest once you’ve fixed something.

From result to fix: use the decoder

Enter your numbers in the Water Test Decoder. For each line it shows where you stand against the EPA number, which treatment extension guides name at your level, where we have it, which certification to look for, and one suggested order for the whole setup. For bacteria, nitrate, nitrite, arsenic, lead and PFAS it points you to your state or county program.

Keep every report. A run of yearly results is how you spot nitrate climbing or a filter that has stopped working.

Quick answers

Do EPA drinking water limits apply to my private well?

Not by law. EPA limits are legal limits for public water systems. Private wells fall outside the Safe Drinking Water Act, and most states don't regulate them, but labs and health departments use the EPA numbers as the yardstick.

What does ND mean on a water test?

Non-detect. The lab didn't find the substance above the lowest level its equipment can measure. It doesn't prove the water is free of it. Check the reporting limit next to it; if that limit is higher than the number you care about, an ND doesn't settle the question.

Is "nitrate as N" the same as nitrate?

No. They're two ways of reporting the same nitrate. The EPA limit is 10 mg/L as nitrogen (N), which is about 44 to 45 mg/L as nitrate. Check which form your lab used before you compare.

What does "present" mean for total coliform?

The lab found coliform bacteria in your sample. There's no number because none is needed. For a private well, any positive result is a reason to find the source, disinfect and retest. If E. coli is present, don't drink the water untreated and call your county health department.

Is manganese at 0.1 mg/L a problem?

It's above the EPA's secondary standard of 0.05 mg/L, which is about stains and taste, and below the EPA health advisory of 0.3 mg/L. Expect black stains; it's a treatment question more than a health-limit question.