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Coliform and E. coli in well water

Measured in present or absent · Updated 2026-09-15

This one is about health, not staining. Confirm any result with a certified laboratory and raise it with your county or state health department. Treatment choices here depend on details a web page cannot establish.

The one result that is about the well's physical condition rather than the water's chemistry. Bacteria in a well mean something is getting in, and disinfection treats the consequence rather than the cause.

Why
A properly constructed, properly sealed well draws from groundwater that surface bacteria cannot reach. A positive test means that separation has been breached somewhere.
Watch out
Fitting ultraviolet disinfection and calling it solved leaves the opening open. It also leaves whatever else the surface carries free to follow the same path.

Federal health standard

Zero total coliform

Federal standards for public systems allow no total coliform. Private wells are not covered, so the standard is a benchmark rather than an obligation.

What treats it

  • UV disinfection for well water

    Passes water along a quartz sleeve containing an ultraviolet lamp, which scrambles the DNA of anything living in it. No chemicals, no taste change, nothing added to the water.

  • Chlorine and peroxide injection systems

    A metering pump injects chlorine or hydrogen peroxide, a retention tank gives the reaction time to finish, and a backwashing filter removes what it produced. It handles water that defeats air injection.

What a positive result tells you

A well is, in principle, a sealed straw into an aquifer. Soil and rock above that aquifer filter out bacteria over the distance and time water takes to travel down. A well drawing from properly protected groundwater does not contain coliform.

So a positive result is a statement about the well: somewhere, the separation between surface and aquifer has been breached. The usual candidates are a cracked or corroded casing, a failed or missing well cap, a grout seal that has degraded, surface water pooling around the wellhead, or a septic system too close or failing.

So the response to a coliform result starts with an inspection rather than a catalogue. Whatever path bacteria found is equally available to fertiliser, fuel, and whatever else reaches the ground nearby.

Coliform versus E. coli

Total coliform covers a wide family of bacteria that live in soil, on plants and in water generally. Most cause no illness. Their value is as an indicator: they are easy to test for, and they should not be present in properly protected groundwater, so finding them means surface material is getting in.

E. coli is a subset associated specifically with the intestinal tract of warm-blooded animals. Finding it means fecal contamination is reaching the well, which raises the possibility of organisms that do cause illness. Federal standards for public systems allow no total coliform at all, and an E. coli result is treated as an acute violation.

For a household the practical difference is urgency. Total coliform means retest, inspect and disinfect. E. coli means stop drinking the water today, use an alternative supply, and call the health department.

Why equipment is the second conversation

Ultraviolet disinfection works, and on a well where the source cannot be fully secured it is a reasonable permanent measure. Continuous chlorination does the same job differently, with the advantage of leaving a residual that keeps working through the plumbing and the water heater.

Both treat water on its way to the tap. Neither seals a well, and neither notices when the underlying problem gets worse.

There is also a practical constraint on UV specifically. Light has to reach the organism, so iron, manganese, hardness and turbidity all interfere with it. A UV lamp installed on raw well water that also carries iron is doing considerably less than its owner believes.

Fix the well first. Then decide what treatment the water still needs.

Read next

  • Contaminant

    Nitrate in well water

    A genuine health standard rather than a staining threshold, and the population at risk is infants under six months. It is also a signal about the well itself, because nitrate is not naturally abundant in deep groundwater.

  • Comparison

    UV vs chlorination for well water

    UV where the water is clear and the contamination is at the source. Chlorination where the water is not clear, where the plumbing itself is suspect, or where iron and sulfide need oxidising anyway.

  • Symptom

    Cloudy or milky well water

    Three possible causes, and a glass of water separates them. Cloudiness that clears from the bottom upward is dissolved air and needs nothing. Particles that settle are sediment. A haze that does not clear, especially with an orange tint, is precipitated iron.

  • Symptom

    Well water smells like rotten eggs

    Hydrogen sulfide. Run the hot and cold taps separately before doing anything else, because if only the hot water smells, the water heater is manufacturing it and the repair costs a fraction of a treatment system.

  • Symptom

    Slime or sludge in well water

    Iron bacteria, most likely. They are living organisms that metabolise iron and produce a slime as they go, and that slime is why this problem does not respond to the equipment that handles ordinary iron.

  • Treatment

    Activated carbon filters for well water

    Adsorbs organic compounds, chlorine and the things that make water taste and smell wrong, on a bed of carbon with an enormous internal surface area. It is the polishing stage, and it is mandatory behind any chemical injection.

Not sure what is in your water yet?See which tests to order, thenplan the treatment from your results.

Questions

What is the difference between total coliform and E. coli?
Total coliform is a broad group of bacteria, mostly harmless in themselves, found throughout soil and vegetation. Their presence is an indicator that surface material is reaching the well. E. coli is a narrower group specifically associated with the intestines of warm-blooded animals, so its presence indicates fecal contamination and is considerably more serious.
My test came back positive for E. coli. What now?
Stop drinking the water and use bottled water immediately. Boil only if nitrate has been tested and is below the standard, because boiling concentrates nitrate rather than removing it. Contact your county or state health department the same day. The well will need disinfecting, and the route the contamination took needs finding. This is not a purchase decision.
Will UV fix it permanently?
UV treats water on the way to the tap, continuously and effectively, provided the water is clear enough for light to pass through. What it does not do is seal the well. A well with a cracked casing or a failed cap will keep admitting surface water, and treating the output indefinitely while leaving the opening is managing a symptom.
How often should I test?
Annually is the common recommendation for bacteria, and additionally after any work on the well, after flooding, and any time the water changes in appearance, taste or smell. Testing is inexpensive relative to what it rules out.

Sources