Coliform bacteria in well water: what a positive test means

A lab report showing coliform bacteria in your well water is alarming, but it's also one of the most common and fixable well problems. This guide explains what the result means, what to do in the first 24 hours, and how to bring the well back to safe water.

Total coliform, fecal coliform and E. coli

Labs report bacteria in several ways. CDC explains the differences:

• Total coliforms are a broad group of bacteria found in the digestive systems of people and animals, in soil, on plants and in surface water. Most won't make you sick, but they travel into wells by the same routes as germs that do, such as sewage leaks, and they're much easier to test for. A high total coliform count suggests harmful germs, including viruses, bacteria and parasites, may also be present.
• Fecal coliforms are a subset of total coliforms. Human and animal feces contain millions of them.
• E. coli is part of the fecal coliform group and can be tested separately. CDC says a positive fecal coliform or E. coli test likely means feces, and the harmful germs in them, have gotten into the well.

CDC adds that most E. coli are harmless, but some types can make you seriously sick, and a standard indicator test won't tell which kind is present.

What the result means for you

Result	What it suggests	Urgency
Total coliform absent	No indicator of contamination at the time of sampling	Keep up annual testing
Total coliform present, E. coli absent	A pathway exists for surface or soil bacteria to enter	Act promptly. Disinfect and retest
E. coli or fecal coliform present	Likely fecal contamination	Stop drinking the water now

Some labs report a count (colonies per 100 mL) and others simply report "present" or "absent." Any presence is a reason to act.

The first 24 hours

1. Stop drinking untreated well water. Use bottled water for drinking, cooking, brushing teeth, making ice and preparing infant formula. CDC recommends switching to bottled water or another safe source when tests show harmful germs.
2. If you must use well water, disinfect it following EPA's emergency guidance. Bring it to a rolling boil for at least one minute (three minutes above 5,000 feet), or treat it with unscented household bleach at the dose EPA specifies, then let it stand 30 minutes.
3. Call your local health department. CDC says they can help you understand the results and what to do next.
4. Look around the wellhead for obvious problems, such as a missing cap, standing water or animal activity.

Disinfect and retest

For a one-time contamination, such as after a flood or pump work, shock chlorination is the usual first step. It means dosing the well and plumbing with strong chlorine, letting it sit, then flushing. See shock chlorinating a well for the process and safety steps.

After the chlorine is fully flushed:

• Retest using a state-certified lab. CDC recommends certified labs for well testing.
• Retest again later to confirm the result holds.
• If bacteria return, the problem is ongoing and you'll need to find the source, install continuous disinfection, or both.

Finding the source

Bacteria get into wells in predictable ways:

• Wellhead problems: a loose, cracked or unvented cap, a casing that doesn't extend above grade, or cracks in the casing.
• Surface water: pooling around the well after rain or snowmelt. EPA advises siting wells so rainwater flows away from them.
• Septic influence: EPA explains that groundwater under a drainfield can carry contaminants toward a well in its path, and surfacing sewage can enter through an unsecured cap or casing cracks.
• Shallow wells: EPA notes dug and driven wells draw from near-surface water and are more easily contaminated.
• Recent work: pump replacement or plumbing repairs can introduce bacteria.
• Animals: insects, rodents or other small animals entering through a damaged cap.

Fixing the pathway is what makes the result stay clean.

Long-term treatment options

If the source can't be fully eliminated, continuous disinfection is common:

• Ultraviolet (UV) systems kill microorganisms as water passes a UV lamp. Water usually needs to be clear and low in iron and hardness for UV to work well, and lamps need regular replacement.
• Chlorination systems feed chlorine continuously and need contact time before the water reaches taps.
• Ozonation is less common for homes.

Whatever you install, keep testing. EPA notes that high bacteria levels can sometimes be controlled by adding disinfection such as chlorine, ozone or ultraviolet light.

Health symptoms to watch for

EPA says people who drink water containing microorganisms can experience gastrointestinal illness and infections. EPA's well-testing chart pairs recurring stomach illness with coliform testing. If anyone in the household develops vomiting, diarrhea or fever while you're dealing with a positive test, contact a health care provider and mention your well results. Infants, older adults, pregnant people and anyone with a weakened immune system are most vulnerable.

Frequently asked questions

Is it safe to shower? For most healthy people, brief bathing is generally lower risk than drinking, but avoid swallowing water. Ask your health department about your situation, especially with E. coli present or vulnerable family members.

Can a water softener or carbon filter remove bacteria? No. They aren't disinfection devices, and a neglected filter can even harbor bacterial growth.

How often should I test after the problem is fixed? At least once a year, as CDC recommends, and after any repair, flood or change in taste, color or smell.

Key takeaways

• Total coliform is a warning sign. E. coli likely means fecal contamination.
• Switch to safe water immediately, and contact your health department.
• Shock chlorinate, then retest with a certified lab.
• Find and fix the entry point, or install continuous disinfection.
• Keep testing every year, even after the problem is solved.