Quick answer: A positive total coliform result means something is letting surface water or other contamination into your well. It is a warning sign, not usually an immediate danger. A positive E. coli result is different: it means the water has been in contact with feces and is not safe to drink until treated. In both cases, the usual fix is to find and repair the entry point, shock chlorinate the well, and retest.
Total coliform vs E. coli
| Result | What it means | Drink the water? |
|---|---|---|
| Total coliform absent | No sign of bacterial contamination | Yes |
| Total coliform present, E. coli absent | Contamination is getting in, but no sign of fecal matter | Montana State University Extension says there is no immediate health concern, but investigate and treat. To be safe, use boiled or bottled water until you get a clean retest. |
| E. coli present | Fecal contamination: an immediate health risk | No. Use bottled water or boil it. |
Coliform bacteria are a large group, mostly harmless themselves, that live in soil, plants and animal intestines. They are not naturally found in groundwater, which is why labs use them as an indicator: if coliform can get in, disease-causing germs may be able to get in too.
If E. coli is present: what to do today
Montana State University Extension advises stopping use of the water unless it is treated. Bring it to a rolling boil for at least one minute or use bottled water for:
- Drinking and cooking
- Mixing baby formula
- Washing fruits and vegetables
- Brushing teeth
- Making ice
Then contact your local health department.
Common reasons a well tests positive
MSU Extension lists these common sources:
- Sandy or gravelly soil that lets surface water reach the aquifer quickly.
- A well close to a septic system or animal waste.
- A flood-prone location.
- A missing, cracked or unscreened well cap, or a casing that ends too close to the ground.
- Contamination introduced during drilling or pump work.
- Leaking pipes between the well and the house.
- Poorly maintained treatment equipment (softeners and filters can grow bacteria).
- A contaminated sample: the bottle or faucet was not clean. A retest can rule this out.
Step-by-step: how to fix it
- Retest if the result is borderline or you suspect a sampling error. Take the sample from a clean cold tap with the aerator removed.
- Inspect the wellhead. Look for a loose or damaged cap, missing vent screen, cracks in the casing, or water pooling around the well. Fix what you find.
- Shock chlorinate the well. MSU Extension recommends about 3 pints of 6% bleach per 100 gallons of water in the well and plumbing. Our step-by-step shock chlorination guide has the full method and a dose chart.
- Retest at least 72 hours after the chlorine is gone. Testing sooner can give a false clean result.
- If it comes back again, the contamination source is still there. Call a licensed well contractor to inspect the casing, seals and surroundings.
When you need continuous treatment
If the well is structurally sound but keeps testing positive, ongoing disinfection may be needed. The EPA lists options such as chlorination, ozone and ultraviolet (UV) light. A UV system is the most common choice for homes because it adds no chemicals; see our guide to UV water treatment for well water.
Treatment is a backup, not a substitute for fixing the entry point.
Frequently asked questions
Can I shower with water that has coliform bacteria?
Showering is generally considered lower risk than drinking, but avoid swallowing water and ask your health department, especially if E. coli is present or someone in the home has a weak immune system.
Does boiling kill coliform bacteria?
Yes. A rolling boil for at least one minute kills bacteria. It does not fix the source, and it does not remove chemical contaminants such as nitrate.
How long after shocking can I test for coliform?
Wait until the chlorine smell is completely gone, then at least 72 hours more.
Will a water softener or carbon filter remove bacteria?
No. They are not designed for disinfection and can even harbor bacteria.
Related guides
- How to shock a well with bleach (with dose chart)
- Well water testing: what to test for and how often
- UV water treatment for well water
Sources
- Montana State University Extension – Total Coliform and E. coli Bacteria
- U.S. EPA – Protect Your Home’s Water
- CDC – Guidelines for Testing Well Water
This guide is for general information. If E. coli is present, follow your local health department’s instructions.
Leave a comment