Arsenic in well water: is your level safe?
The limit is 0.010 mg/L, the same as 10 µg/L or 10 ppb. Above it, use another source for drinking and cooking and look at certified treatment. Boiling doesn't remove arsenic.
- US (EPA)
- 0.010 mg/L (10 µg/L).
- Ontario
- 0.010 mg/L (10 µg/L).
What it is
Arsenic occurs naturally in some rock and soil, so it's common in some regions and absent in others, even between neighboring wells. It has no taste or smell. The risk comes from drinking it for years, not from a single glass.
What your number means
- Within limitsBelow 0.005 mg/L
Under the limit. Retest every few years.
- Worth a look0.005 to 0.010 mg/L
Under the limit but close. Retest within a year.
- Act nowAbove 0.010 mg/L
Over the limit. Use another source for drinking and cooking and look at treatment.
What to do
- Use another water source for drinking and cooking. Showering and washing are generally fine.
- Retest to confirm the number.
- Ask about treatment certified to remove arsenic at your level, such as reverse osmosis or arsenic removal media.
- After treatment is installed, test the treated water to make sure it works.
How often to test
Every 3 to 5 years if you're under the limit, and within a year if you're close.
Common questions
What does 10 ppb mean?
Parts per billion. 10 ppb is the same as 10 µg/L or 0.010 mg/L. Labs use different units for the same amount.
Can I cook with water that has arsenic?
No. Arsenic stays in food cooked with the water, and boiling concentrates it.
Why don't my neighbors have arsenic?
It depends on the rock your well draws from, so levels can differ a lot over short distances.
Sources: EPA drinking water standards; Ontario Drinking Water Quality Standards, O. Reg. 169/03. Limits checked October 2026.