What it is
Coliforms are a broad group of bacteria that live in soil, on plants and in the gut of animals. Laboratories test for them as indicator organisms: they are easy to detect and, if they can get into the water, so can things that are harder to find. Results are reported per 100 ml and the guideline is none.
Where it comes from
Because many coliforms live in soil and vegetation, finding them does not necessarily mean faecal contamination. Common causes are surface water or soil entering the source, a storage tank with a missing or damaged lid, biofilm in old pipework or a tank that has not been cleaned, and a treatment system that is not working properly. If E. coli or enterococci are also present, the contamination is faecal.
Why it matters
Coliforms on their own are rarely a direct cause of illness. Their importance is as a warning: the supply has a path for contamination that needs to be closed, and the water may not be safe if something more harmful comes along the same path. Until the cause is found it is sensible to boil water for drinking.
What can be done about it
The fix is usually physical: seal the source, repair or replace tank lids, clean and disinfect the tank and pipework, and check any treatment is working. A UV disinfection unit after filtration then protects against a repeat. A retest after the work confirms the result.
Information last reviewed 5 October 2026 by Pozzani Pure Water.