Last checked: October 21, 2026

How to read your well water test results

A lab report is a list of numbers next to limits. This guide explains the most common lines on a private-well report, which limits they're usually compared against, and where those limits come from.

First: the units and the kinds of limits

Units. Most results are in mg/L (milligrams per liter, the same as parts per million, ppm) or µg/L (micrograms per liter, the same as parts per billion, ppb). 1 mg/L = 1,000 µg/L. Bacteria results are usually reported as present/absent.⁹

Kinds of limits you'll see:

• MCL (Maximum Contaminant Level): an enforceable limit for public water systems, set for health reasons. EPA publishes the federal list.² States can set stricter limits. New Jersey's arsenic limit, for example, is stricter than the federal one (see below).⁹
• SMCL (Secondary Maximum Contaminant Level): a non-mandatory EPA guideline for "nuisance" problems like taste, odor, color, staining, and corrosion.³ New Jersey calls its versions "Recommended Upper Limits."¹⁰
• Health advisory: non-enforceable EPA guidance for some substances that have no MCL (manganese is one example).⁴

These limits were written for public water systems. Private wells aren't regulated by EPA's public-water rules, but the same numbers are commonly used as reference points for private-well results.¹⁵

Quick reference table

Iron

• Reference value: EPA's secondary standard is 0.3 mg/L.³ New Jersey uses the same number as its recommended upper limit.⁹
• What it affects: EPA lists iron among substances that affect taste and odor, cause color, and cause staining and scale.³
• Health: New Jersey's environmental agency says high iron is generally not considered a health concern.¹⁰
• On your report: iron is often listed next to manganese, because the two are frequently treated together. If you're choosing a treatment system, give the installer your full report, not just the iron number.

Manganese

• Reference value: EPA's secondary standard is 0.05 mg/L, based on staining and taste.³⁴
• Health advisory: EPA's 2004 health advisory sets a lifetime value of 0.3 mg/L to protect against possible neurological effects. For infants younger than 6 months, it advises using 0.3 mg/L even for short-term (10-day) exposure.⁴
• Infants: the Minnesota Department of Health recommends testing for manganese before a baby drinks the water.⁸
• New Jersey's recommended upper limit for manganese is 0.05 mg/L.⁹

Hardness

• Reference value: EPA has no secondary standard for hardness (it isn't on EPA's secondary list).³
• What the numbers mean: the U.S. Geological Survey classifies water (as mg/L calcium carbonate) as 0–60 soft, 61–120 moderately hard, 121–180 hard, and over 180 very hard.⁶
• What it affects: USGS describes hardness mainly as a scale and equipment problem. Heated hard water forms calcium carbonate deposits that can shorten the life of water heaters and clog pipes.⁶
• Units: some reports and softener vendors use "grains per gallon" instead of mg/L. Ask your lab which unit it used before you compare numbers.

Sulfur smell: hydrogen sulfide (H₂S) and sulfate

• Reference value: there's no federal drinking water standard for hydrogen sulfide.⁷ Sulfate has an EPA secondary standard of 250 mg/L.³
• What the numbers mean: according to the Connecticut Department of Public Health, most people can smell H₂S at about 0.5 mg/L, and 1–2 mg/L gives the "rotten egg" smell and makes water very corrosive to plumbing.⁷
• Hot water only? The Minnesota Department of Health says H₂S can be produced by sulfur bacteria or chemical reactions inside water heaters. It suggests running the hot and cold taps separately to find where the smell comes from.⁸
• Testing: Minnesota notes that the rotten-egg smell usually isn't related to the sanitary quality of the water, but it recommends testing for coliform bacteria and nitrate to be sure.⁸
• Treatment context: Minnesota lists activated carbon as effective below about 1 mg/L, and oxidizing filters, aeration, chlorination, or ozonation for higher levels.⁸ Because H₂S escapes from water quickly, ask your lab how to collect the sample.

pH

• Reference value: EPA's secondary range is 6.5–8.5.³ New Jersey uses the same optimum range.⁹
• What it affects: EPA lists low pH with a bitter metallic taste and corrosion, and high pH with a slippery feel, soda taste, and deposits.³
• Why low pH matters: Hunterdon County, NJ's well-owner guide notes that acidic water (below 6.5) can corrode plumbing and leach lead from soldered joints.¹³ If your pH is low, a lead result is worth looking at too.

Nitrate

• Reference value: EPA's MCL is 10 mg/L measured as nitrogen.² New Jersey's report table lists it as 10,000 µg/L, which is the same value.⁹
• Who is most at risk: EPA says infants below six months who drink water with nitrate above the MCL could become seriously ill, with symptoms that include shortness of breath and blue-baby syndrome.²
• Where it comes from: CDC lists animal waste, septic systems, wastewater, fertilizers, and farm runoff, among others. CDC also says high nitrate can be a sign that other contaminants may be present.¹
• How often: CDC recommends that all well owners test for nitrate at least once a year.¹

Coliform bacteria and E. coli

• How it's reported: usually present/absent. New Jersey's limit for both total coliform and E. coli is 0 (absent).⁹ EPA's health goal (MCLG) for total coliforms is zero.²
• What total coliform means: CDC explains that coliforms generally don't make people sick themselves, but they get into water the same way as germs that do, so a positive test suggests harmful germs may also be present.¹
• What E. coli means: CDC says a positive test for fecal coliforms or E. coli likely means feces, and the germs in them, have gotten into the well water.¹
• How often: CDC recommends testing for total coliform at least once a year, and again after flooding, well repairs, or a change in taste, color, or smell.¹
• If you get a positive result, contact your local health department for next steps.

Arsenic

• Reference value: EPA's MCL is 0.010 mg/L (10 µg/L).² Some states are stricter. New Jersey's MCL is 5 µg/L.⁹¹¹
• You can't detect it yourself: New Jersey's environmental agency says arsenic can't be detected by taste, smell, or sight, so testing is the only way to know.¹¹
• Health: EPA lists skin damage, circulatory problems, and increased cancer risk for long-term exposure above the MCL.²
• How often (New Jersey guidance): test every year if a previous result was above 2.5 µg/L, and at least every five years if it was below 2.5 µg/L. NJDEP recommends whole-house arsenic treatment above 5 µg/L.¹¹

Other lines you may see

State-required panels often include more than the eight parameters above. New Jersey's required list, for example, also covers lead, volatile organic compounds, gross alpha (radioactivity), certain PFAS, and, in some counties, mercury or uranium.¹² We'll add guides for these over time.

What to do with your report

1. Check that the lab was certified by your state for each parameter tested. CDC and EPA both recommend state-certified labs.¹⁵
2. Compare each result to the limit printed on the report. Note the units (mg/L vs µg/L).
3. Separate health-based results (nitrate, bacteria, arsenic, lead, and so on) from aesthetic ones (iron, manganese at the secondary level, pH, hardness, odor).
4. Call your local health department with questions about health-based results.
5. Keep a copy. Comparing results year to year shows whether anything is changing.

Where to get tested and treatment options

Related guides

• How often to test a private well
• Shock chlorinating a well