Mercury in well water: limits and what to do
The EPA limit for mercury is 0.002 mg/L and the Ontario standard is 0.001 mg/L. Mercury is rarely found in wells, so any detection is worth a call to your health department.
- US (EPA)
- 0.002 mg/L (inorganic mercury).
- Ontario
- 0.001 mg/L.
What it is
Mercury in groundwater is rare. When it shows up, it usually comes from old landfills, industrial sites, or broken equipment like old mercury switches or gauges near the well.
What your number means
- Within limitsBelow 0.001 mg/L
Under both limits. Most wells are below detection.
- Worth a look0.001 to 0.002 mg/L
Above the Ontario standard, under the EPA limit. Retest and call your health department.
- Above limitAbove 0.002 mg/L
Use another source for drinking and contact your health department right away.
What to do
- Retest to confirm, with a new sample bottle.
- Check for old mercury equipment near the wellhead, such as pressure switches or gauges.
- Reverse osmosis or activated carbon certified for mercury can remove it.
How often to test
With a full panel every 3 to 5 years, or sooner if you live near a landfill or industrial site.
Common questions
Can mercury in well water come from fish or fillings?
No. Mercury in fish comes from lakes and rivers, not wells. Well mercury points to a local source in the ground.
Is the mercury limit for all forms?
The limits apply to inorganic mercury, the form found in water.
Sources: EPA drinking water regulations; Ontario Drinking Water Quality Standards (O. Reg. 169/03). Limits checked November 2026.