Coliform and E. coli in Tap Water: What a Positive Test Means

A positive coliform test in tap water is a warning sign, not proof that the water will make you sick. Most coliform bacteria are harmless, and EPA uses them as an indicator that something may be wrong with treatment or the pipes. E. coli is different. It is a more direct sign of contamination from human or animal waste, which can carry germs that cause illness. When a public water system confirms E. coli under EPA’s rules, it must notify customers within 24 hours, usually with a boil water advisory.

Here is what each result means, how utilities respond, and what to do if your water is affected.

Total coliform vs. E. coli

Total coliforms are a broad group of bacteria that, with few exceptions, are not harmful to people. EPA says total coliforms are a useful indicator of other pathogens in drinking water. Utilities use them to check whether treatment is working and whether the distribution system (the pipes, tanks and mains) is intact. A positive result can point to a problem such as a pipe break, low pressure, a storage tank defect or a treatment issue.

E. coli is one species within the coliform group. EPA describes it as a more direct indicator of fecal contamination, because it is always found in feces. According to the CDC, most E. coli are harmless and are a normal part of a healthy gut. Six types can cause diarrheal illness. One group, Shiga toxin-producing E. coli (STEC), can lead to hemolytic uremic syndrome, which can cause kidney failure.

EPA notes that E. coli in drinking water is not a health threat in itself. The concern is what it signals: that human or animal waste may have entered the water, bringing germs that can cause diarrhea, cramps, nausea, headaches or other symptoms. EPA says these germs may pose a special risk for infants, young children and people with severely weakened immune systems.

How the Revised Total Coliform Rule works

EPA published the Revised Total Coliform Rule (RTCR) on February 13, 2013. Public water systems had to comply starting April 1, 2016, replacing the 1989 Total Coliform Rule. It covers every public water system except aircraft water systems, which have their own rule.

The RTCR changed how coliform results are treated:

• E. coli has a legal limit. The health goal (MCLG) for E. coli is zero, and the rule sets an enforceable maximum contaminant level (MCL).
• Total coliform alone is no longer a violation. Under the old rule, too many total coliform-positive samples in a month was an MCL violation. Under the RTCR, EPA says there is no acute or monthly MCL violation for total coliform-positive samples alone. Instead, they trigger an investigation.
• “Find and fix.” When positives pass certain thresholds, the system must do a Level 1 or Level 2 assessment to look for sanitary defects, and it must fix any defects it finds within a required timeframe.

Routine and repeat samples

Utilities collect routine samples from their distribution systems on a set schedule. If a routine sample is total coliform-positive, the system must collect at least three repeat samples within 24 hours of learning the result, and test any positive sample for E. coli.

When it becomes an E. coli violation

Under federal rules (40 CFR 141.63), a system violates the E. coli MCL if:

1. a repeat sample is E. coli-positive after a total coliform-positive routine sample,
2. a repeat sample is total coliform-positive after an E. coli-positive routine sample,
3. it fails to take all required repeat samples after an E. coli-positive routine sample, or
4. it fails to test for E. coli when any repeat sample is total coliform-positive.

The last two are about missed steps, not confirmed bacteria. The rule still counts them as E. coli MCL violations, because without the follow-up tests no one can tell whether E. coli is present.

Level 1 and Level 2 assessments

• A Level 1 assessment is a self-check by the system. It is triggered by too many total coliform-positive samples in a period or by failing to collect all required repeat samples.
• A Level 2 assessment is more detailed. It is triggered by an E. coli MCL violation or by repeated Level 1 triggers, and it is done by a party approved by the state or EPA.

Missing an assessment or failing to fix a defect is a treatment technique violation.

What happens after an E. coli violation

An E. coli MCL violation requires a Tier 1 public notice, EPA’s most urgent category. The system must notify customers as soon as practical and no later than 24 hours after learning of the violation, and it must contact the state. EPA’s template notice for E. coli is a boil water advisory.

If you get a boil water advisory, the CDC says to:

• Bring clear water to a rolling boil for 1 minute (3 minutes above 6,500 feet), then let it cool.
• Use boiled or bottled water for drinking, cooking, brushing teeth, making ice and preparing baby formula. For formula, the CDC suggests ready-to-use formula if possible.
• Not use water or ice from refrigerators connected to your water line.
• Avoid swallowing water when bathing, and consider sponge baths for babies and young children.

Our boil water advisory guide covers dishes, pets and what to do after the advisory ends. Boiling kills germs but does not remove chemicals. If you get a “do not drink” notice instead, boiling will not make the water safe.

If you or a family member get sick with diarrhea, especially an infant, older adult, pregnant person or anyone with a weakened immune system, contact a doctor.

How common are coliform violations in large systems?

We checked the RTCR records of the 1,000 large systems on our site (each serving more than 50,000 people). Source: EPA ECHO compliance records, checked October 7, 2026; last 12 quarters.

Most coliform rule violations are monitoring and reporting problems, such as a sample that was missed, late or not reported correctly. These are not findings of bacteria.

Thirteen systems have an MCL violation recorded under the RTCR. Because the RTCR’s only MCL is for E. coli, these are E. coli MCL violations. As explained above, some may come from missed repeat sampling rather than a confirmed E. coli result. The table below lists all 13. Eight are still open in EPA’s record:

“Open” means EPA’s record does not yet show the violation as resolved. Records can lag, so check the system’s EPA report or ask the utility. For broader context, see US water systems with the most EPA violations.

Note: utilities may have resolved issues since October 7, 2026. Check each utility’s EPA ECHO report or its latest Consumer Confidence Report for current status.

Private wells are different

The RTCR applies to public water systems. If you have a private well, no one tests it for you. CDC and EPA recommend testing a private well at least once a year for total coliform bacteria, plus extra tests after floods, repairs or any change in taste, color or smell. Our well water testing guide explains how and how often.

Should you filter for bacteria?

For a public water supply, the answer to an E. coli problem is the utility’s fix plus boiling or bottled water until the advisory is lifted. A home filter is not a substitute for following a boil advisory. The CDC’s boil advisory guidance says to boil tap water even if it is filtered. If you rely on a well with recurring bacteria, talk to your state or local health department about well repair and disinfection options.

Bottom line

A total coliform positive is a signal for the utility to investigate. An E. coli violation is more serious and triggers a 24-hour notice, usually a boil water advisory. Follow the advisory until your utility lifts it, and check your system’s record on our search page.

Sources

• EPA: Revised Total Coliform Rule and Total Coliform Rule
• 40 CFR 141.63: Maximum contaminant levels for microbiological contaminants
• EPA: Public Notification Instructions and Templates for the Revised Total Coliform Rule
• EPA: Revised Total Coliform Rule Guide for Small Public Water Systems, Part A
• EPA Region 8: Addressing Total Coliform Positive or E. coli Positive Sample Results
• EPA: National Primary Drinking Water Regulations
• CDC: About E. coli
• CDC: Boil Water Advisory
• CDC: Preventing Crypto at Home During a Boil Water Advisory