Thallium in well water: limit and what to do
The EPA limit for thallium is 0.002 mg/L (2 µg/L). Ontario has no standard for it. It is rare in wells, and most results come back below detection.
- US (EPA)
- 0.002 mg/L.
- Ontario
- No standard.
What it is
Thallium can come from ore processing, old mines, coal ash, or some industrial sites. Natural levels in groundwater are usually very low.
What your number means
- Within limitsBelow 0.001 mg/L
Under the limit by a comfortable margin.
- Close to limit0.001 to 0.002 mg/L
Under the limit but more than halfway there. Retest within a year.
- Above limitAbove 0.002 mg/L
Use another source for drinking and cooking, contact your health department and retest to confirm.
What to do
- Retest to confirm a detection.
- Above the limit, use another source for drinking and contact your health department.
- Reverse osmosis or activated alumina removes thallium.
How often to test
With a full panel every 3 to 5 years.
Common questions
Why is thallium on my report?
Many labs include it in a standard metals panel because it has an EPA limit. A non detect is the usual result.
What does thallium do to health?
Long term exposure above the limit can cause hair loss and harm the blood, kidneys, intestines and liver.
Sources: EPA drinking water regulations. Limits checked November 2026.