A positive coliform result on a well test can be unsettling, but it is a signal to act on, not a reason to panic. For an O'Fallon home on a private well, the first job is to confirm the result, find out whether E. coli was also found, and check whether something about the well or its surroundings gave bacteria a path in.
What the result is telling you
Coliform bacteria are a broad group of organisms found in soil, plants, and animal waste. Labs use them as indicators, meaning their presence suggests the water may have been exposed to surface influence. E. coli is a specific member of that group, and its presence points more directly to recent contamination from animal or human waste. The lab report should state clearly which organism was found. If you cannot tell from the report, call the lab and ask.
Confirm before you change anything
A single result can come from a dirty sample bottle, a faucet that was not properly cleaned, or a sample taken during a storm. Ask the MO State Public Health Lab or your county health department how they want the retest done. Follow the sampling instructions exactly, including sanitizing the faucet and collecting from the point the lab specifies. The retest result is what you use to decide your next steps.
Check the well for an open path
Bacteria usually enter a well through a physical gap, so start with the outside of the well. Look for:
- A well cap or vent screen that is cracked, missing, loose, or not sealed
- A casing top that sits low in the ground, or soil and debris piled against it
- Standing water or erosion near the wellhead after rain
- Cracked or damaged seals where wires and pipes enter the casing
Take photos and write down dates. Do not open the well or pump housing unless you are trained to do so, and do not enter a well pit or casing. Use a qualified well contractor for anything beyond a visual check.
Think about what is nearby
Many private wells in and around O'Fallon sit close to septic tanks, drain fields, livestock areas, or runoff from paved surfaces. Map where your well sits relative to these features. Shallow wells are more exposed to surface water than deeper ones, so the well depth on your well log matters. If the well was drilled many years ago, the records may not show how it was sealed, which makes a professional inspection worth considering.
Note recent changes
Write down any events that line up with the positive result, such as heavy rain or flooding, a power outage, a pump or pressure tank repair, nearby digging, or a change in taste, color, or odor. These details help a contractor or lab interpret the results. Work done on the system without afterward disinfection and retesting is a common reason bacteria show up again.
Disinfection and retesting
If the well is confirmed as contaminated, disinfection is usually part of the fix, but it should follow repair of the cause. Disinfecting before the entry point is sealed may not hold. A qualified well professional can disinfect the system, flush it according to the correct procedure, and advise when to retest. Plan a follow-up test after the work is finished, and keep those results with your well log.
Until the results are clear
Ask the health department or lab what to do about drinking and cooking water while you investigate. Do not wait for a long investigation to make that decision. Keep a simple written record of test dates, lab sample IDs, results, and every repair. That record is what lets you judge whether a problem has truly been resolved.