A positive coliform result on a well test means the water has a route in for surface contamination. Total coliform bacteria are mostly harmless in themselves; they are indicators. E. coli is different and points to fecal contamination, which is an immediate do-not-drink situation until the well is disinfected and retested clear.
Total coliform against E. coli
Labs report two things from the same sample. Total coliform is a broad family of bacteria found in soil, vegetation and surface water, and finding them says a pathway exists between the surface and your water. E. coli is a subset that lives in the intestines of warm-blooded animals, and finding it says that pathway is carrying waste, from a septic system, a barnyard or flooding. Total coliform positive with E. coli absent is a construction or a sampling problem more often than a health emergency. E. coli present means stop drinking the water, switch to bottled or boiled water and act now.
Nobody tests this for you
Private wells are not regulated under the federal Safe Drinking Water Act, which means no agency samples your well, sends you a notice or fixes a problem it finds. Around 15 percent of the US population, over 43 million people in more than 23 million households, rely on private wells, and a USGS study of 2,100 private wells found about one in five carried at least one contaminant above a human-health benchmark. Bacteriological testing is annual on most state and extension recommendations, and it is worth doing after any flooding, after work on the well and whenever the water changes in taste, smell or clarity.
Finding the way in before you disinfect
Shock chlorination without finding the entry point buys a clean test and a return positive a few months later. Walk the wellhead. The casing should stand clearly above grade with an intact sanitary seal and a vermin-proof cap: a rusted cap, a cracked seal or a casing at ground level in a low spot are the classic findings. Look at the grading, because surface water should run away from the well rather than pool at it. Consider distances to the septic field, the barn and any abandoned well nearby. In older properties, a hand-dug well or a well pit is a standing invitation and often the real answer.
Disinfection, retest, and permanent treatment
Shock chlorination means introducing a measured chlorine solution into the well, circulating it through every fixture until you smell chlorine at each one, holding it for the recommended period, then flushing to waste away from the septic field and landscaping. Retest after the chlorine has cleared, and retest again some weeks later, because a single clean result after disinfection proves the chlorine worked rather than that the problem is fixed. If repeat positives continue after the wellhead is sound, install continuous treatment: an ultraviolet purifier sized on flow, preceded by sediment filtration, since UV needs clear water to work. Our well water testing guide covers the sampling routine.
Questions people ask about coliform in well water
Can I drink water with total coliform present?
Total coliform without E. coli signals a pathway for surface contamination rather than immediate fecal contamination. Most guidance is to stop drinking it until the well is disinfected and retests clear.
Does boiling make the water safe?
Boiling kills bacteria and is the standard interim measure while you disinfect and retest. It does nothing about chemical contaminants, and it concentrates them slightly as water evaporates.
How often should a private well be tested for bacteria?
At least annually under most state and extension guidance, plus after flooding, after any work on the well or pump, and whenever the taste, smell or appearance of the water changes.
Will a UV purifier fix a coliform problem?
UV inactivates bacteria continuously and is the standard permanent treatment, but it needs clear water and a sound wellhead. It treats the symptom, so find and close the entry point as well.
Can a bad sample cause a false positive?
Yes, and it is common. Touching the bottle rim, sampling from an aerator or a hose, or delaying delivery to the lab all cause spurious results, which is why a repeat sample is the first step.