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Nitrate in well water: what 'as N' means and why EPA mentions infants

General information, not medical advice. This page explains what the EPA, CDC and state health departments say. It cannot tell you whether your water is safe. For that, ask your certified lab or local health department. Read the full disclaimer.

The EPA's maximum contaminant level (MCL) for nitrate is 10 mg/L measured as nitrogen, the legal limit for public water systems. For nitrate in well water, this is a federal reference, not a household safety rating. The EPA National Primary Drinking Water Regulations page warns:

"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."

If an infant in your home drinks your well water or formula made with it, contact your certified lab or your state or local health department and ask them to help you read your nitrate result.

Standards, local geology and state requirements all vary. Confirm anything on this page with your state or local health department before acting on it.

"As N" means nitrate measured as nitrogen. A lab can report nitrate as nitrogen or as nitrate; the numbers are not interchangeable. The EPA's saved IRIS nitrate assessment states: "1 mg nitrate-nitrogen = 4.4 mg nitrate." Ask your lab which basis your report uses.

What "as N" tells you about your nitrate number

"N" stands for nitrogen. "As N" tells you how the nitrate amount is expressed. The EPA's archived nitrate-monitoring guidance uses the wording "nitrate nitrogen (NO3-N)." Ask your lab to confirm the wording and basis on your own report.

This matters because a nitrate number only means something next to its label. The basis and the unit stay attached to the number. If you need help with units in general, see the explanation of report units.

How the EPA labels its nitrate entry

Source-label guide, not a sample report. This table shows how the EPA labels its own nitrate entry, so you can see which words belong with the number.

EPA entry What stays with the number
Nitrate (measured as Nitrogen) The reporting basis. The EPA reference applies only to nitrate measured as nitrogen.
MCL The type of reference. It is the legal limit for public water systems.
mg/L The unit, from the EPA table header "MCL or TT (mg/L)."

Table source: EPA National Primary Drinking Water Regulations, saved capture checked September 29, 2026.

The EPA National Primary Drinking Water Regulations list an MCL of 10 mg/L for nitrate measured as nitrogen. That value belongs to that label only.

The federal nitrate limit applies to public water systems

The EPA describes these regulations as "legally enforceable primary standards and treatment techniques that apply to public water systems."

Private wells are different. On its Private Drinking Water Wells page, the EPA says the quality and safety of private-well water "are not regulated by the Federal Government under the Safe Drinking Water Act nor by most state governments and laws." It also says: "Private well owners are responsible for delivering safe drinking water to their households."

So the public-system MCL is a federal reference point, not a federal requirement for your well. Your state may have its own rules. Your state or local health department can tell you what applies where you live.

What the EPA infant page says, and the mistake to avoid

The EPA page Private Wells and Drinking Water Safety and Infants says:

"DON'T mix well water containing more than 10 milligrams per liter of nitrate with infant formula or food."

That sentence says "nitrate" without stating a reporting basis. It does not say "as N" or "measured as nitrogen."

The mistake to avoid: matching your number to this warning based on "mg/L" alone. Two results can share a unit and still use different reporting bases. Ask your lab or health department to confirm the basis instead of guessing.

The same EPA page adds: "Contact your local health organization for help interpreting the results of well water testing."

What to ask your lab or health department

The CDC Guidelines for Testing Well Water say: "Use a state-certified laboratory to test your water." They also say: "Your health department can help you understand your test results and what to do next."

A plain question to bring:

"Is my nitrate result reported as nitrogen, and how should I read it against the EPA references and my state's guidance?"

Sources and limits: This page is built from saved EPA and CDC guidance pages listed below. WellTestAdvisor does not test water and cannot assess whether any household's water is safe.

Sources