Coliform Bacteria in Well Water: What a Positive Test Means

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 shocking a well that still has an open doorway buys you a clean month and another positive test.

Then shock the well

The treatment is a full shock 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 shock 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 kit ships chlorine dioxide sachets at the manufacturer's published 20 ppm shock dose, with a residual test to prove the well held the treatment.

Retest, then retest again

Flush until the residual is gone, 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.