Updated 30 September 2026
A positive total coliform result calls for a second test specifically for E. coli or fecal coliform. If that comes back positive, stop drinking the water, switch to bottled or boiled water, and shock chlorinate the well. Retest after treatment to confirm the bacteria are gone before drinking the water again.
Not every positive result means the same thing. Mississippi State Extension notes that coliform bacteria are common and don’t necessarily indicate the water has come into contact with waste, but fecal coliform or E. coli in a sample definitely does. That distinction changes how urgently you need to respond.
Health authorities treat the two results very differently. New York’s health department warns that some rare strains of E. coli, particularly O157:H7, can cause serious illness, though it notes cases traced to contaminated drinking water are rare. Seeing coliform bacteria in well water on a lab report is common; understanding which type you actually have is what determines the next step.
The report itself rarely spells this out clearly. A lab result that just says “coliform: present” leaves the two very different scenarios, harmless bacteria versus a fecal contamination pathway, looking identical on the page until a second, more specific test tells them apart.
Once the fecal-specific retest comes back, reading the rest of the report the same way matters just as much. Our guide to reading well water test results covers every parameter line by line.
Total Coliform vs. E. coli: What Your Lab Report Is Actually Telling You
Total coliform bacteria live in soil, plants, and surface water, and their presence alone doesn’t confirm anything about waste contamination. A total-coliform-positive, E.-coli-negative result usually points to a minor pathway, a loose well cap, a crack in the casing, not necessarily sewage or septic contact.
This guide sits in our Water Testing section alongside the full testing pillar and the retest schedule for a positive result.
E. coli and other fecal coliform bacteria are a different signal entirely. Their presence means the water has come into contact with human or animal waste, which carries a real risk of gastrointestinal illness. Treat any E.-coli-positive result as an immediate stop-drinking situation, not a wait-and-see one.
Most residential labs report coliform as present or absent rather than a graded count, but if your report does show a CFU/100mL figure, treat any detectable amount, even a single colony, as a positive result that calls for the same fecal-specific retest.
Most coliform bacteria in a positive sample are harmless. The genuine health concern is the rare fecal strain: contact with human or animal waste through a compromised well is what actually puts a household at risk of illness.
Source: Mississippi State Extension, What to Do About Coliform Bacteria in Well Water
How Coliform Bacteria Actually Gets Into a Well
A defective well cap is the most common entry point. A missing, cracked, or poorly sealed cap lets surface water, insects, and debris drop straight into the casing, carrying bacteria with them.
Cracks in the casing itself work the same way from a different angle: water that hasn’t been filtered through soil finds a path directly into the well. Older, unsealed wells and wells located in low-lying spots prone to flooding around the wellhead carry the same risk, since standing water sits right at the entry point during wet seasons.
A nearby septic system is the fourth common source. A well too close to a drain field, or one that shares groundwater flow with a failing septic system, can pick up fecal coliform even when the well itself is structurally sound.
Shallow wells and older wells carry more of this risk than deep, modern ones. A well drilled to a shallow depth sits closer to surface runoff and seasonal water table changes, while an older well built before current casing and sealing standards existed often lacks the protective barriers a new well is required to have.
Location matters as much as construction. A well positioned downhill from a barn, a pasture, or a neighboring property’s failing septic system inherits that risk through groundwater flow, even when every part of the well itself is in good repair.
Stop drinking the water immediately if a test confirms E. coli or fecal coliform bacteria. Boil water advised for drinking, cooking, and brushing teeth, a rolling boil for at least one minute, until disinfection and a follow-up test confirm the contamination has been eliminated, or switch entirely to bottled water in the meantime. Showering and washing dishes are usually fine as long as you avoid swallowing the water; hold off on bathing infants or anyone with a weakened immune system in untreated water until the follow-up test comes back clear.
The Mistakes That Trip Up a Positive Coliform Response
Confusing the two coliform types is the most common mistake. Panicking over a total-coliform-positive result without waiting for the fecal-specific retest leads to unnecessary bottled-water spending and stress over a result that often reflects nothing more than a minor, easily fixed pathway.
Treating shock chlorination as a permanent fix is the second trap. A well that keeps testing positive after repeated treatments has a structural problem, a cracked casing or a failed seal, that bleach alone cannot solve. Shock chlorination disinfects the water present at the time; it does not seal the pathway that let bacteria in.
Skipping the retest is the third trap. A clear chlorine smell during treatment confirms the bleach reached the fixture, not that the bacteria are actually gone. Only a lab sample taken after the flush and wait period proves the well is safe again.
Order the fecal-specific retest at the same time as your first positive result comes in. Most labs can run it on a fresh sample within days, which shortens the time you spend deciding how seriously to take the original result.
Guessing at the cause instead of confirming it wastes time too. A homeowner who assumes a septic issue and skips inspecting the well cap can spend weeks chasing the wrong fix while the actual entry point, a loose cap or a hairline crack, keeps letting bacteria in.
How to Respond to a Coliform-Positive Well
Start with the retest, not the treatment. A second sample confirms whether you’re dealing with total coliform alone or a fecal-specific result, and that distinction determines whether you need to stop drinking the water immediately or simply plan a repair.
Move to shock chlorination once the source is understood. Treat the well, the casing, and every fixture in the house, then let the chlorine sit for the recommended period before flushing the system completely.
If the well keeps testing positive after repeated shock treatments, a permanent disinfection stage is worth pricing into the treatment plan. Our well water treatment guide covers where continuous disinfection fits in the sequence.
Close the loop with a follow-up lab test. A positive result that clears once, then reappears within weeks or months, points to a structural issue rather than a one-time event, and that calls for a licensed well contractor rather than another round of bleach.
Document each round of testing and treatment as you go. A dated record of every positive result, every shock chlorination, and every follow-up test gives a well contractor the pattern they need to diagnose a recurring problem quickly instead of starting from scratch.
If the contamination keeps returning after repeated treatment, continuous disinfection is worth considering as a longer-term fix. Well water testing after every round of treatment is the only way to know whether the well needs an ongoing solution or a one-time repair.
Our UV water purifier guide covers what that ongoing disinfection stage actually needs to work, including the pre-filter and the annual bulb change.
Coliform is one of four tests that belong on every private well sample. Our contaminants reference covers the other three and explains which additional tests your location calls for.
Frequently Asked Questions (FAQs)
Is coliform bacteria dangerous?
Total coliform bacteria alone are usually harmless and common in soil and surface water. The genuine danger is fecal coliform or E. coli, which means the well has come into contact with human or animal waste and carries a real risk of illness.
How do you remove coliform bacteria from well water?
Shock chlorinate the well, casing, and household plumbing, then retest to confirm the bacteria are gone. If the well keeps testing positive after repeated treatment, a well contractor needs to find and repair the structural entry point, since chlorine alone won’t fix a cracked casing or failed seal.
What causes coliform bacteria in a well?
A missing or defective well cap, cracks in the well casing, an old unsealed well, flooding around the wellhead, or a nearby septic system are the most common causes. Each creates a pathway for unfiltered surface water and bacteria to reach the well.
Can I shower in well water with coliform bacteria?
Showering and washing dishes are usually fine as long as you avoid swallowing the water, since the concern is ingestion rather than skin contact. Avoid bathing infants or anyone with a weakened immune system in untreated water until a follow-up test confirms the well is clear.
What’s the difference between total coliform and E. coli in well water?
Total coliform bacteria are common and don’t necessarily mean the water contacted waste. E. coli and other fecal coliform bacteria specifically indicate contact with human or animal waste, which is the result that calls for stopping water use right away.
