Total coliform in well water: what a positive result means
Total coliform bacteria aren't usually harmful on their own, but finding them means something from the surface or soil can reach your well. Treat a positive result as unsafe to drink until a retest comes back clean.
- US (EPA)
- Absent. Any coliform in a private well is a reason to act.
- Ontario
- Not detectable. More than 5 per 100 mL is significant evidence of contamination; 1 to 5 means retest.
What it is
Coliforms are a large family of bacteria that live in soil, on plants and in the gut of animals. Labs test for them because they're easy to detect and they travel the same way disease causing germs do. A positive result is a warning light: the well isn't sealed well enough to keep surface water out.
What your number means
- Within limitsAbsent, or 0 per 100 mL
No coliform bacteria were found in this sample. Keep testing on schedule.
- Worth a look1 to 5 per 100 mL (Ontario)
Public Health Ontario reports this as no significant evidence of contamination, but any coliform shows a path into the well. Retest to confirm.
- Act nowPresent, or more than 5 per 100 mL
Use bottled or boiled water for drinking, disinfect the well, find how water is getting in, and retest.
What to do
- Switch to bottled water, or boil water at a rolling boil for at least one minute, for drinking, cooking, ice and brushing teeth.
- Check the well cap, seal and casing for gaps, cracks or pooling water around the well.
- Disinfect the well (shock chlorination), or ask a licensed well contractor to do it.
- Retest for bacteria one to two weeks after disinfecting, once the chlorine is gone.
- If it comes back positive again, ask a well contractor or your health department to find the source.
How often to test
Test for bacteria every year in the US, and at least three times a year in Ontario (four for a dug well). Also test after flooding, well repairs or any change in taste, smell or color.
Common questions
Is water with total coliform safe to drink?
Not until you know more. Total coliform alone is rarely what makes people sick, but it means germs that do cause illness could get in. Health departments advise using bottled or boiled water until a retest is clean.
Does boiling kill coliform bacteria?
Yes. A rolling boil for at least one minute kills bacteria. It doesn't fix the well, so disinfect and retest too.
Why did coliform show up when my water looks fine?
Bacteria are invisible and have no taste or smell. Heavy rain, snowmelt, a loose well cap or nearby work often lets surface water in.
Sources: EPA drinking water standards; Ontario Drinking Water Quality Standards, O. Reg. 169/03; Public Health Ontario well water testing. Limits checked October 2026.