Total Coliform & E. coli
A group of bacteria used as an indicator that disease-causing organisms may be able to enter a water supply. E. coli, a subgroup, signals fecal contamination.
Where does it come from?
Soil, vegetation, and human or animal waste entering water through failing septic systems, flooding, surface runoff, or defects in well caps, casings, and plumbing.
Health Risk
Coliforms themselves rarely cause illness, but their presence means the pathway is open for pathogens. E. coli indicates fecal contamination, which can cause gastrointestinal illness — especially dangerous for infants, the elderly, and immunocompromised people.
How to remove it
Standard carbon filters do not remove bacteria. Contamination requires disinfection (shock chlorination for wells), fixing the entry point, or continuous treatment such as UV — and boiling water until tests come back clean.
Total coliforms are a large group of bacteria found in soil, vegetation, and the intestines of warm-blooded animals. Most coliforms do not cause illness — they are measured because they are easy to detect and signal that surface contamination can reach your water. E. coli, a coliform subgroup, is the more serious finding: it indicates fecal contamination from human or animal waste.
Unlike lead or nitrate, coliform results are not reported as a concentration. A water sample is either positive (present) or negative (absent). The EPA's Maximum Contaminant Level Goal is zero, and under the Revised Total Coliform Rule a positive result triggers mandatory follow-up sampling and an assessment of how contamination got in.
What a Positive Result Means
- Total coliform positive, E. coli negative: a contamination pathway exists — cracked well cap, casing defect, backflow, or surface intrusion. Retest to confirm, then investigate the pathway.
- E. coli positive: fecal contamination is confirmed. Stop drinking the water immediately and boil water for drinking and cooking until follow-up tests come back clean.
Why Private Wells Are at Higher Risk
Public water systems are disinfected and monitored under the Revised Total Coliform Rule. A private well has none of those safeguards — no chlorination, no routine sampling, and no violation notice. Bacteria typically enter wells through:
- Septic system proximity: a failing or too-close septic field can leach bacteria into the aquifer
- Flooding and heavy rain: surface water carrying bacteria can enter the wellhead or infiltrate shallow aquifers
- Well cap and casing defects: a cracked cap, corroded casing, or poor surface seal is an open door for insects, rodents, and runoff
- Work on the well or plumbing: any opening of the system can introduce bacteria
The CDC recommends testing private well water for total coliform bacteria and nitrates at least once a year, and immediately after flooding, well repairs, or any change in taste, smell, or appearance.
How to Test and What to Do Next
Lab-based well panels include coliform and E. coli analysis, and DIY bacteria vials (48-hour incubation) are a useful quick screen between lab tests — see our guide to the best well water test kits. After a confirmed positive, the usual sequence is: boil water, shock-chlorinate the well, fix the entry point, then retest until two consecutive samples come back clean. Persistent contamination calls for continuous treatment such as UV disinfection.
Frequently Asked Questions
Is total coliform in water dangerous?
Total coliforms themselves rarely cause illness. Their presence is a warning sign: the pathway that let harmless coliforms in can also admit disease-causing organisms. A confirmed E. coli result is the serious finding — it means fecal contamination, and the water should not be consumed without boiling or treatment.
Why is my well water coliform positive after rain?
Heavy rain and flooding push surface water — and the bacteria it carries — into wellheads and shallow aquifers. A positive result after rain often points to a compromised well cap, casing, or surface seal. Retest after conditions settle and have the wellhead inspected.
Does boiling water kill coliform bacteria?
Yes. A rolling boil for one minute kills coliform bacteria and E. coli, which is why boil-water advisories are the standard response to confirmed contamination. Boiling is a stopgap, not a fix — the source of contamination still needs to be found and corrected.
How often should I test my well for bacteria?
The CDC recommends testing private wells for total coliform bacteria at least once a year, plus immediately after flooding, any work on the well or plumbing, or a change in taste, smell, or appearance.