Coliform bacteria turn up in private wells often enough that annual testing is the standard recommendation rather than a precaution for problem wells. A positive result is common, and it is also commonly misread: total coliform is an indicator rather than a pathogen, and what it means depends entirely on which test came back positive.
How common, and why nobody tells you
The EPA notes that private wells are not federally regulated under the Safe Drinking Water Act, that around 15 percent of the US population, over 43 million people in more than 23 million households, rely on them, and that a USGS study of 2,100 private wells found about one in five carried at least one contaminant above a human health benchmark. Because no utility samples a private well, positives are found only when owners test, which is why the standard advice is annual bacteriological testing regardless of how the water looks.
Total coliform against E. coli
Total coliform bacteria are widespread in soil and vegetation and are mostly harmless in themselves. Their presence means a route exists by which surface material is reaching your water, which is why they are used as an indicator. E. coli is the subset associated with fecal contamination, and a positive there is a different and more serious finding pointing at septic, livestock or wildlife contamination. A total coliform positive with no E. coli is common and usually points at the wellhead rather than at the aquifer.
What to do about a positive
Do not fit equipment first. Retest to confirm, since sampling technique is a frequent source of false positives: an unsanitized tap, an aerator left on, or a container handled carelessly all produce them. If confirmed, inspect the wellhead: casing height above grade, ground sloping away, an intact vermin proof sanitary seal and cap, and no route for surface water into the annulus. Then shock chlorinate the well and the plumbing, flush thoroughly, and retest twice at intervals.
When permanent disinfection is justified
When contamination recurs after the wellhead has been corrected and shock chlorination has not held. When the source is shallow, dug or spring fed and therefore under surface influence. Where flooding periodically reaches the wellhead. Or where someone in the household is immunocompromised, elderly or an infant. Ultraviolet is the usual answer and requires clear water to work, so filtration ahead of it is part of the installation. Our well water filtration guide covers the whole treatment sequence.
Questions people ask about is it common to have coliform in well water
How often do wells test positive for coliform?
Often enough that annual bacteriological testing is the standard recommendation for every private well, not just problem ones.
Is total coliform dangerous?
Mostly not in itself. It indicates a route by which surface material reaches your water. E. coli is the serious finding.
What should I do about a positive result?
Retest to rule out sampling error, then inspect and correct the wellhead, shock chlorinate, flush and retest twice at intervals.
When do I need permanent disinfection?
When contamination recurs after the wellhead is corrected, on shallow or spring fed sources, or where a household member is vulnerable.