Boron in well water: limits and treatment
Ontario and Health Canada set a limit of 5 mg/L for boron. The EPA has no enforceable limit. Most wells are well below, but some deep or salty wells run higher.
- US (EPA)
- No enforceable limit.
- Ontario
- 5 mg/L.
What it is
Boron occurs naturally in some groundwater, especially in sedimentary rock, near old seabeds and in salty or deep wells.
What your number means
- Within limitsBelow 2.5 mg/L
Comfortably under the Ontario and Health Canada limit.
- Close to limit2.5 to 5 mg/L
Under the limit but more than halfway there. Retest.
- Above limitAbove 5 mg/L
Above the Ontario and Health Canada limit. Use another source for drinking and ask about treatment.
What to do
- Retest to confirm a high result.
- Reverse osmosis removes part of the boron. Special boron selective resins remove more.
- Blending with a lower boron source is sometimes the practical fix.
How often to test
With a full panel every 3 to 5 years.
Common questions
Why does Ontario have a boron limit when the EPA does not?
Health Canada and Ontario set a limit based on effects on reproduction and development seen at high doses. The EPA has studied boron but has not set an enforceable limit.
Is boron the same as borax?
Borax is a boron compound. In water, boron is measured as the element.
Sources: Ontario Drinking Water Quality Standards (O. Reg. 169/03); Health Canada guidelines summary table. Limits checked November 2026.