Total Coliform
Mostly harmless bacteria used as an indicator. Their presence means a path exists that should not.
- Safe range
- 0 CFU/100 mL (There is no acceptable level: the band is zero. Coliforms are regulated as an INDICATOR of the integrity of a supply rather than as a hazard in themselves.)
- Measured in
- CFU/100 mL
What it is
Total coliforms are a broad family of bacteria found in soil, vegetation and surface water as well as in intestines. Most of them will not make anyone ill. They are measured not because they are dangerous but because they are easy to detect and they travel the same routes as organisms that are.
Why it matters
This is an indicator test, and understanding that is the whole point. A positive result does not mean the water is making you sick; it means the barrier between your water and the outside world has a gap in it — a cracked well cap, a failing seal, surface water reaching the aquifer, a break in a distribution main. Whatever got the coliform in could get something worse in next. That is why the acceptable amount is none, and why a positive is followed immediately by an E. coli test to find out whether the contamination is faecal.
How to measure it
A laboratory, or a home presence/absence vial. Either way the question is yes or no rather than how many, and yes means act.
Certified laboratory, mail-in kit
- How trustworthy
- Reference grade
- Cost
- Most expensive
- Effort
- Careful sampling, then days or weeks of waiting
You are posted bottles and instructions, you fill them exactly as told, and you post them back. Weeks later a report arrives with figures nobody has to interpret by eye. This is what a result you intend to act on should be built from.
The standard well test, usually bundled with nitrate. Laboratories have a short holding time for bacteria samples — often around a day — so post it the same day and avoid sampling before a weekend.
Where to find it: State-certified drinking water laboratories; your state's certification officer keeps the list, and many public health departments will point you at one.
Home presence/absence kit
- How trustworthy
- Dependable
- Cost
- Low cost
- Effort
- Five minutes, then a day or two of waiting
A sealed vial or paddle you fill with water and leave somewhere warm for a day or two. If the bacteria it is looking for are there, they grow and change its colour. It answers yes or no — never how many — so the number to record is 0 for absent and 1 for present.
The coliform vial is the common one, sold on its own or bundled with a nitrate strip. It cannot count and it is not accepted for regulatory purposes, but for a homeowner asking whether their well is sealed it answers the question.
Where to find it: Hardware stores, well-supply shops and online. Check the date: the growth medium expires.
Your utility's annual water quality report
- How trustworthy
- Reference grade
- Cost
- Free
- Effort
- A few minutes of reading
Every community water system in the US has to publish one each year, free, listing what it found and what the limits are. It is real laboratory data about the water leaving the plant and travelling the mains. What it cannot describe is your own plumbing.
Public systems test frequently and must notify customers when they exceed the standard. On mains water this is being watched for you.
Where to find it: Your water bill, your supplier's website, or the EPA's index of reports. It is also called a Consumer Confidence Report or CCR.
Taking the sample
For several of these the sample is where the error is, and no instrument recovers from a bad one.
- • Do not sterilise the tap with a flame or bleach, and do not sample from a tap with an aerator, a hose or a filter on it — remove the aerator and run the water for several minutes first.
- • Do not touch the inside of the bottle or its cap, and do not rinse it. Many bacteria bottles contain a dechlorinating agent that rinsing washes away.
- • A positive result is most often a contaminated sample rather than a contaminated well. Retest before doing anything expensive — but boil in the meantime.
Further reading: US EPA: National Primary Drinking Water Regulations, the enforceable health-based limits; US EPA: private drinking water wells, including what to test for and when; US EPA: certified drinking water laboratories, by state
If it is too low
None detected. This is the only acceptable result, and it is what a properly sealed supply gives.
- • Nothing. Retest annually on a private well, and after any work on the well, the pump or the plumbing.
If it is too high
Any detection means a route exists between the outside and your water. Retest, test for E. coli, and find the breach.
- • Test for E. coli straight away if it was not run at the same time. That is what separates a seal problem from a sewage problem.
- • Boil water for drinking and cooking until you have a clean retest. This is the one contaminant where boiling genuinely is the answer.
- • Inspect the well cap, casing and seal, and look for surface water pooling near the wellhead.
- • Shock chlorinate the well, then retest — and retest again a week later, because a single clean result after chlorination can just be the chlorine.
Common questions
Total coliform is positive but E. coli is negative. Is my water safe?
It means the contamination is probably environmental — soil or vegetation entering through a gap — rather than faecal, which is the better of the two answers. It is still not a clean result: something has a way in. Find the breach, disinfect, and retest.
Does boiling make coliform-positive water safe?
Yes, for bacteria. A rolling boil kills them, and this is the one category of contaminant where boiling is genuinely the right emergency measure. It does nothing for metals, nitrate or chemicals — for those it makes the concentration worse.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice. If a health parameter is out of range, confirm it with an accredited lab and speak to your water provider.