E. coli
Direct evidence of faecal contamination. The one result on a water report that is an emergency.
- Safe range
- 0 CFU/100 mL (There is no acceptable level: the band is zero. E. coli is regulated as direct evidence of faecal contamination rather than as a dose-dependent contaminant.)
- Measured in
- CFU/100 mL
What it is
Escherichia coli lives in the intestines of warm-blooded animals and people. Unlike the wider coliform family it does not persist in soil and vegetation, so finding it in water means faecal material has reached the supply — from a septic system, a sewer, livestock or wildlife.
Why it matters
This is the most urgent result a drinking water test can return. Most strains of E. coli are themselves harmless, but its presence proves a faecal pathway, and the organisms that travel that pathway include the ones that cause serious gastrointestinal illness. There is no threshold to discuss and no trend to watch: any detection means stop drinking the water today.
How to measure it
The same sample and the same test as total coliform — laboratories run E. coli on any coliform-positive sample as a matter of course. Never test for coliform without asking for this.
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.
Almost always bundled with total coliform, and it should be: coliform tells you a route exists and this tells you whether it runs to a sewer. Confirm the panel includes it rather than assuming.
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 kits that distinguish E. coli from the other coliforms use a second indicator that fluoresces under ultraviolet light, so read it in the dark with a UV torch as well as in daylight. Treat any positive as a laboratory job rather than as a conclusion.
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.
A public system exceeding this must issue a boil-water notice, which reaches you directly rather than waiting for the annual report.
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.
- • Same discipline as coliform: aerator off, tap run for several minutes, bottle unrinsed and untouched inside, posted the same day.
- • After flooding, wait for the water to recede and the well to settle before sampling, then test — sampling during a flood tells you about the flood rather than about the well.
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
If it is too low
None detected. This is the only acceptable result.
- • Nothing. Retest annually on a private well, and after any flooding, well work or plumbing work.
If it is too high
Faecal contamination is confirmed. Stop drinking the water immediately and boil for all consumption until repeated tests come back clean.
- • Stop drinking it now. Boil for drinking, cooking, ice, brushing teeth and washing produce until you have consecutive clean tests.
- • Tell your local health department. On a public supply they will already be acting; on a well they can advise and often help with testing.
- • Find the source: septic system, sewer line, livestock, or surface water reaching the wellhead after flooding.
- • Shock chlorinate, then retest at least twice with a gap between. One clean result immediately after chlorination is not evidence the problem is gone.
Common questions
Is E. coli in water always dangerous?
The organism found by the test is usually a harmless strain. What makes it an emergency is what it proves: faecal material is reaching your water, and the same route carries pathogens that are not harmless. Treat the finding as the warning it is rather than judging the strain.
How long should I boil water after an E. coli detection?
Until you have clean results, and more than one of them, with a gap between. A single negative straight after shock chlorination can simply reflect the chlorine still in the well rather than a fixed problem, which is why repeat testing matters more here than anywhere else.
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.