Nitrate in well water: what your number means
The limit is 10 mg/L of nitrate as nitrogen. Above it, don't use the water for infant formula or for anyone pregnant. Boiling makes it worse, not better.
- US (EPA)
- 10 mg/L nitrate as nitrogen (about 45 mg/L if reported as nitrate, NO3).
- Ontario
- 10.0 mg/L nitrate as nitrogen.
What it is
Nitrate is a form of nitrogen. It reaches wells from fertilizer, manure and septic systems, often more in shallow wells and after heavy rain. You can't see, smell or taste it.
What your number means
- Within limitsBelow 5 mg/L
Under the limit. Keep testing every year.
- Worth a look5 to 10 mg/L
Under the limit but more than halfway there. Retest within six months to see if it's rising.
- Act nowAbove 10 mg/L
Over the limit. Use another water source for infants and anyone pregnant, and look at treatment.
What to do
- Don't use the water to make infant formula, and use another source for anyone pregnant.
- Don't boil it to fix it: boiling concentrates nitrate.
- Retest to confirm, and check whether your lab reported nitrate as N or as NO3. Divide an NO3 number by 4.43 to compare with the 10 mg/L limit.
- Ask about treatment certified for nitrate: reverse osmosis, distillation or anion exchange.
- Look for sources near the well: fertilized fields, manure storage or a septic system.
How often to test
Every year, and every six months if your result is close to or above the limit or a baby is in the house.
Common questions
Why is nitrate a risk for babies?
Babies under six months can't process nitrate well. High levels reduce how much oxygen their blood carries, a condition sometimes called blue baby syndrome.
Does a water softener remove nitrate?
No. A standard softener removes hardness, not nitrate. You need a system certified for nitrate.
My lab says nitrate as NO3. Is that different?
It's the same nitrate, counted a different way. Divide by 4.43 to get nitrate as nitrogen, the number limits use.
Sources: EPA drinking water standards; Ontario Drinking Water Quality Standards, O. Reg. 169/03. Limits checked October 2026.