Coliform and E. coli in well water: what a positive result means

A positive coliform result does not always mean your well water will make you sick, but it does mean the sample found bacteria that can point to a contamination pathway into the well or plumbing. A positive E. coli result is more serious because it strongly suggests fecal contamination, so use bottled water or another safe source for drinking and cooking until you have retested and addressed the cause. (cdc.gov)

What the lab found

Total coliform bacteria are common in soil, vegetation, and water. In a private well sample, they are used as indicator bacteria, which means they help show whether outside contamination may be getting into your water system. Total coliform itself is usually harmless, but its presence can mean conditions are right for other germs to enter, too. (nj.gov)

E. coli is different. It is part of the fecal coliform group, and a positive result likely means human or animal feces, and the harmful germs that can come with it, have gotten into the well water. That is why an E. coli positive is treated as a bigger health concern than a total-coliform-only result. (cdc.gov)

If you are looking at a lab report and are not sure what “present,” “absent,” “positive,” or “negative” means, compare the wording with How to read a water test report: MCLs, units and flags. A certified lab can also explain the exact reporting language used on your sample. (epa.gov)

Total coliform vs. E. coli

A total coliform positive with E. coli negative is often best understood as a warning sign. It can mean surface water, a loose well cap, a cracked casing, poor drainage around the well, or even contamination introduced during sampling. It tells you to take the result seriously, but it does not by itself prove recent fecal contamination. (nj.gov)

An E. coli positive points more directly to fecal contamination. In practical terms, that means the water should be treated as unsafe to drink until follow-up testing shows the problem has been corrected. CDC says a positive test for fecal coliforms or E. coli likely means feces and harmful germs have gotten into the well water. (cdc.gov)

This is also why many well owners test for bacteria every year even when the water looks, smells, and tastes normal. New Jersey’s health department notes that total coliform is colorless, odorless, and tasteless in drinking water, so testing is the only way to know it is there. (nj.gov)

What to do right away

If your bacteria result came back positive, use this checklist:

1. Read the exact result. Check whether the sample was positive for total coliform only, or for E. coli too. Those results do not mean the same thing. (cdc.gov)
2. Switch to a safe source for drinking and cooking if E. coli is present. CDC says to consider bottled water or another safe water source when testing shows harmful germs. New Jersey says boiling drinking water for at least 1 minute will kill bacteria, although that is usually a short-term step, not the full fix. (cdc.gov)
3. Call the lab or local health department. Ask what they recommend for retesting, disinfection, and whether your sample method could have affected the result. CDC says your health department can help explain the result and next steps. (cdc.gov)
4. Inspect the well area. Look for a loose or unsealed cap, standing water, cracks, recent repairs, nearby flooding, or anything else that could let contaminated water reach the well. (nj.gov)
5. Plan a retest with a state-certified lab. EPA and CDC both say to use a certified laboratory for drinking water testing. If you need help finding one, start with the state-by-state directory or How to choose a state-certified drinking water lab. (epa.gov)

Why wells test positive

A positive bacteria result usually means there is a path for contamination. Common causes include damaged or cracked well casing, a loose or unsealed well cap, poor well construction, runoff collecting around the well, agricultural activity, wildlife, or problems with a septic system. Flooding and heavy rain can make these problems worse. (nj.gov)

The timing of the positive can give you clues. EPA and CDC say to test right away after flooding, land disturbance, major changes near the well, or repair or replacement of any part of the well system. Those events can open a temporary path for bacteria even if the well had been fine before. (epa.gov)

It is also possible to get a misleading result if the sample was not collected carefully. University of Minnesota Extension warns that false-positive results can happen if the faucet is not sanitized well enough before sampling. If you need a refresher before the next sample, see How to collect a well water sample without ruining the test. (extension.umn.edu)

Retesting the right way

Retesting matters because one positive sample does not tell the whole story. It helps confirm whether the bacteria are in the aquifer, in the well, in the plumbing, or came from a one-time problem during collection. Use the same care on the second sample that you would use on the first, and follow the lab’s bottle, timing, and handling instructions exactly. (cdc.gov)

State guidance can differ on timing, so do not assume there is one national deadline for every private well. As one official example, New Jersey says that if water is positive for total coliform and negative for E. coli, the well owner should retest again within the week and again 6 months later. (nj.gov)

If you disinfect the well or plumbing, do not skip the follow-up sample. Minnesota’s Department of Health says to retest water 2 to 4 weeks after disinfection because total coliform can regrow in the system. If coliform is still detected, the state says to repeat disinfection and consider further help, since repeated positives can point to a deeper well or plumbing problem. (health.state.mn.us)

Fixing the source, not just the symptom

A good response does more than kill bacteria once. It tries to find the route they used to get in. That may mean improving drainage, replacing a broken well cap, correcting casing problems, addressing septic issues, or cleaning and disinfecting parts of the water system where bacteria may be growing. (nj.gov)

Shock chlorination can be useful when the contamination appears tied to a one-time event. New Jersey notes that shock chlorination may be appropriate after something like an extreme flooding event or a nearby well being drilled, and describes it as using high concentrations of chlorine for at least 12 to 24 hours. If you are considering that step, Shock chlorinating a well after a bad bacteria test can help you understand the process and its limits. (nj.gov)

But chlorine is not a cure for every positive result. Minnesota says it is not unusual to disinfect a system more than once, and if disinfection does not work, the well may need cleaning as well as disinfection. In other words, repeat positives often mean there is an ongoing source or entry point that needs repair. (health.state.mn.us)

Long term, prevention is simpler than emergency cleanup. EPA and CDC both recommend testing a private well at least once every year for total coliform bacteria, nitrates, total dissolved solids, and pH, and testing sooner when there is flooding, a repair, or a change in taste, color, or smell. If you want a broader schedule, try the Well water test planner or What to test your private well water for (and how often). (epa.gov)

Because private well rules and follow-up advice can vary by state, county, lender, and the exact lab method, confirm the next step with your local health department, state lab certification office, or a licensed well professional when the situation is urgent, repeated, or tied to a real-estate transaction. (epa.gov)

Frequently asked questions

Does a positive total coliform result mean sewage is in my well?

Not necessarily. Total coliform bacteria are common in soil, plants, and water, so a positive result can reflect a contamination pathway without proving recent fecal contamination. E. coli is the stronger warning sign for fecal contamination. (nj.gov)

How soon should I retest after a positive result?

Follow your lab or local health department first, since private well guidance varies. As one official state example, New Jersey says to retest within the week after a total-coliform-positive, E. coli-negative result, and again 6 months later; after disinfection, Minnesota says to retest in 2 to 4 weeks. (nj.gov)

Should I shock chlorinate right away?

Sometimes, but not always. Shock chlorination may make sense when contamination seems tied to a one-time event such as flooding, but repeated positives can point to a structural or drainage problem that chlorine alone will not solve. (nj.gov)

Do I really need a state-certified lab for this?

Yes. EPA says to use laboratories certified to do drinking water testing, and CDC says to use a state-certified laboratory and contact your health department if you need help understanding the result or finding the right test. (epa.gov)