Selenium in well water: limits and treatment
The limit for selenium is 0.05 mg/L in both the US and Ontario. Small amounts are a nutrient, but too much over many years can cause hair and nail loss and numbness.
- US (EPA)
- 0.05 mg/L.
- Ontario
- 0.05 mg/L.
What it is
Selenium occurs naturally in some soils and shale, especially in parts of the western US and prairies. Irrigation and mining can raise levels.
What your number means
- Within limitsBelow 0.025 mg/L
Under the limit by a comfortable margin.
- Close to limit0.025 to 0.05 mg/L
Under the limit but more than halfway there. Retest within a year.
- Above limitAbove 0.05 mg/L
Use another source for drinking and cooking, contact your health department and retest to confirm.
What to do
- Retest to confirm a high result.
- Above the limit, use another source for drinking and cooking.
- Reverse osmosis removes selenium.
How often to test
With a full panel every 3 to 5 years.
Common questions
Isn't selenium good for you?
In small amounts it is an essential nutrient, mostly from food. The water limit protects against getting too much over a lifetime.
Does boiling remove selenium?
No. Boiling concentrates minerals like selenium.
Sources: EPA drinking water regulations; Ontario Drinking Water Quality Standards (O. Reg. 169/03). Limits checked November 2026.