Coliform Bacteria in Well Water: What It Means and What to Do
Last updated: November 6, 2026 · Sources: U.S. EPA, CDC
Coliform bacteria in well water are a warning sign that surface water, soil or waste can reach your well. Most total coliform bacteria are not harmful themselves, but a positive test means disease-causing germs could get in too. If E. coli is also present, the water has fecal contamination: stop drinking it untreated, use bottled or boiled water, shock chlorinate the well, fix the source and retest.
Getting a “positive” coliform result on a well test can be scary. The good news is that it is common, it is usually fixable, and there is a clear set of steps to follow. This guide explains what coliform bacteria are, how to read your result, how bacteria get into wells and how to make your water safe again.
Coliform in well water at a glance
- Safe result: total coliform and E. coli both absent
- Total coliform present: a path for contamination exists; act soon
- E. coli present: fecal contamination; do not drink untreated water
- Usual fix: repair the source, then shock chlorinate and retest
- Keeps coming back: add UV or continuous chlorination
What Are Coliform Bacteria?
Coliforms are a large group of bacteria found in soil, on plants and in the intestines of people and animals. Labs test for them because they are easy to detect and they signal that other, more dangerous germs might be present. There are three levels:
- Total coliform: the whole group. Many come from soil and vegetation.
- Fecal coliform: a subgroup that lives in the gut of warm-blooded animals.
- E. coli: a species of fecal coliform and the best sign of sewage or animal waste contamination.
Coliform bacteria are often referred to as "indicator organisms" because they indicate the potential presence of disease-causing bacteria in water.
Is Coliform in Well Water Dangerous?
Most total coliform bacteria do not make people sick. The concern is what their presence means: there is a pathway from the surface into your well, and harmful bacteria, viruses or parasites could use the same path. E. coli is more serious. Some strains cause illness directly, and its presence means fecal contamination, which can carry germs that cause diarrhea, cramps, nausea and vomiting.
Babies, older adults, pregnant women and people with weakened immune systems are at the highest risk. For public water systems, the EPA does not allow any E. coli in drinking water, and private well owners should use the same standard.
How to Read Your Test Result
| Total coliform | E. coli | What it means | What to do |
|---|---|---|---|
| Absent | Absent | No sign of bacterial contamination | Keep testing once a year |
| Present | Absent | A pathway for contamination exists | Inspect the well, shock chlorinate, retest; consider bottled water until clear |
| Present | Present | Fecal contamination | Stop drinking untreated water now; boil or use bottled water; disinfect, fix the source and retest |
Some labs report a count (for example, colonies per 100 mL) instead of present or absent. Any detection in drinking water should be treated as a positive result. You can also enter your results in our water test results checker.
What to Do Right Away
- Use safe water for drinking and cooking. Use bottled water, or bring tap water to a rolling boil for 1 minute (3 minutes above 6,500 feet) and let it cool.
- Use safe water for baby formula, ice and brushing teeth.
- Bathing is generally fine for healthy adults, but avoid swallowing water and be careful with babies and anyone with open wounds or weak immunity.
- Do not rely on a pitcher or fridge filter to remove bacteria unless it is specifically certified to do so.
Boiling kills bacteria, but it does not remove chemicals like nitrate, which become more concentrated as water boils.
How Bacteria Get Into a Well
- A loose, cracked or non-sanitary well cap that lets in insects, rodents or rain
- A damaged casing or missing grout seal around the casing
- A shallow or dug well that draws from water near the surface
- Surface water pooling around the wellhead because the ground slopes toward it
- Flooding that reached the well
- A septic system, livestock area or manure pile too close to the well
- Recent work on the well, pump or plumbing
Walk around your wellhead. The casing should stick up at least a foot above the ground, the cap should be tight and vented with a screen, and the ground should slope away from the well.
How to Get Rid of Coliform Bacteria in Well Water
- Protect your family with bottled or boiled water.
- Find and fix the source. Replace a damaged cap, regrade the soil, or have a well contractor repair the casing or seal.
- Disinfect the well. Follow our step-by-step guide on how to shock a well.
- Retest with a certified lab after all the chlorine has been flushed out, usually 7 to 14 days later. Many experts recommend a second clean test a few weeks after the first.
- Add continuous treatment if bacteria keep returning. A UV purifier certified to NSF/ANSI 55 Class A or a chlorination system provides ongoing protection.
How Often to Test for Bacteria
- At least once a year, ideally in spring or after a wet period
- After flooding or heavy rain that reaches the well
- After any work on the well, pump or plumbing
- When the water’s taste, smell or color changes
- When someone in the home is pregnant, or a new baby arrives
- When anyone in the home has repeated stomach illness
For sampling tips and what else to test, see our well water testing guide. Home presence/absence kits can screen for coliform, but use a state-certified lab to confirm a positive result.
Frequently Asked Questions
Is it safe to shower with coliform bacteria in well water?
For most healthy adults, showering in water with total coliform is generally considered low risk, as long as you do not swallow the water. If E. coli is present, take extra care with babies, older adults and anyone with a weakened immune system or open wounds, and ask your health department for advice.
Can you drink well water with coliform bacteria?
It is best not to. Total coliform signals that harmful germs could get in, and E. coli means fecal contamination. Use bottled water or boil water for 1 minute until a follow-up test is clean.
Does boiling water kill coliform bacteria?
Yes. A rolling boil for 1 minute (3 minutes at elevations above 6,500 feet) kills coliform and E. coli. Boiling does not remove chemicals such as nitrate.
Why does coliform keep coming back in my well?
Repeated positive tests usually mean the source has not been fixed, such as a damaged cap, cracked casing, missing seal or surface water draining toward the well. Have a licensed well contractor inspect it, and consider UV or continuous chlorination.
How long does it take to get rid of coliform in a well?
Shock chlorination takes 12 to 24 hours plus flushing, and you should retest 7 to 14 days later. If the source is fixed, many wells test clean after one treatment.
Sources: EPA: Private Drinking Water Wells; CDC: Private Wells; EPA: Revised Total Coliform Rule.
