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Coliform Bacteria in Well Water: What a Positive Test Means

Last updated Reviewed by Sean Mihelich, founder of Save My Well

A positive coliform test does not mean your water made anyone sick. It means your sealed water system is open to the surface somewhere, and the lab found the tracer organisms that prove it. Here is how to read the result and what to do this week.

What does a positive coliform test mean?

Labs report two things. Total coliform is a family of common environmental bacteria, mostly harmless on their own, used as an indicator: the standard for drinking water is zero, so any detection is a fail. E. coli is the subset that lives in the gut of warm-blooded animals, and its presence means fecal material is reaching your water. Total coliform alone says fix the well soon. E. coli says stop drinking the water now, switch to bottled or boiled water, and treat the well immediately.

First, find the doorway

Before any treatment, walk out and look at the wellhead. The cap should be tight, vermin-proof, and above grade, with its screened vent intact and the ground sloping away from the casing. A loose or cracked cap is the single most common answer, and insects nesting under an old cap are a classic source. Fix what you find, because treating a well that still has an open doorway buys you a clean month and another positive test.

Positive test? We size the treatment to your exact well.

Answer a few questions about your well. The kit is from $329, with a lidded mixing pail and a bacteria test before and after. For context, a professional treatment visit can cost up to $500, and it may not include a bacteria test.

Look up my well See what's in the kit

Then treat the well

The treatment is a full disinfection of the standing water, the casing walls above the water line, the pressure tank, and the household plumbing, following the steps in how to treat a well. Dose to the well's real volume rather than a guess: the number comes from depth, diameter, and static water level, and your state's well record usually has all three. Our treatment kit ships chlorine dioxide pouches at the manufacturer's published 20 ppm treatment dose, with a bacteria test before the treatment and another after the flush.

Retest, then retest again

Flush the treated water out, wait one to two weeks, and retest for coliform. A clean result is real progress; confirm it with a second clean test a few months later, because a well that reinfects slowly is telling you the doorway is still open. Keep drinking from a safe source until the retest comes back clean.

Need the numbers for your own well? The well disinfection calculator gives the gallons, the bleach amount, and the kit for it.

Straight answers

Is one coliform colony in well water a big deal?

The drinking water standard for total coliform is zero, so any detection means the well is open to surface influence and fails the test. One colony of total coliform with no E. coli is usually a fixable housekeeping problem: inspect the wellhead, treat the well, and retest. E. coli at any level means stop drinking the water until it tests clean.

Can I shower with coliform in my well water?

For total coliform without E. coli, bathing is generally considered low risk for healthy adults; keep the water out of your mouth and cover open cuts. With an E. coli positive, be more careful, especially for small children and anyone immune-compromised, and put the well on the fast track for treatment.

Is it chloroform bacteria or coliform bacteria?

The word you want is coliform. Chloroform is an unrelated industrial chemical. Coliform bacteria are a family of mostly harmless organisms that testing labs use as the canary: they live in soil and on surfaces, so finding them in a sealed water system means something on the surface is getting in.

How often should I test my well for coliform?

Once a year at minimum, plus after any pump or casing work, after a flood, and after any change in taste, smell, or clarity. Testing is the only way to know: coliform has no taste and no smell.

What if my well keeps testing positive after treating?

Repeat positives after a proper treatment and retest mean bacteria are re-entering: a cracked casing, a bad cap, surface water reaching the aquifer, or a nearby septic problem. At that point bring in a licensed well contractor to inspect the casing and cap, because more disinfectant will not fix a hole.

Sources

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