Coliform bacteria

Well Water Program

Some common reasons for Coliform bacteria in well water samples are described below.

Figure out the reasons for the sample results, fix the problem, and retest before drinking the water.

  1. Problems with the top of the well. Is the cap loose? Are there any open holes in the cap? Are there cracks or holes that you can see in the well? If you find any of these problems, this could be the source of bacteria. FIX the problem, the shock chlorinate the well, and test again before you use the water.
  2. Work was done recently on the well, pump, or plumbing system. If the water system was not disinfected after the work, this may be your source of bacteria. Shock chlorinate your water system and test the water again.
  3. Water standing next to the well. If the area around the well pipe (casing) gets wet, this water may be causing your problem. Make sure that runoff water does not reach the well. The presence of bacteria also means that there may be problems with the grout seal around the well pipe. If you can't find any other explanation for the bacteria, contact a well driller to advise you on repairing the grout seal.
  4. The sample was contaminated in the collection process. Common problems include" (a) A hose, aerator screen, filter or other attachment on the faucet, (b) Accidentally touching the inside of the lid or the top of the bottle, (c) Not running water 3-5 minutes before collecting it. Carefully collect another sample and retest. Use rubbing alcohol on a clean paper towel to wipe the faucet before letting the water run for 3-5 minutes.
  5. A movable faucet. Sometimes bacteria grow at the swivel point. This is not harmful, but still shows up in the test. If the sample was from a movable faucet, do the test again using another faucet.
  6. The well is shallow, especially if near a stream or pond. The well may be drawing water from the surface. To avoid bacteria in drinking water: (a) Use either a UV or chlorine water-treatment system or (b) Find another source of water, such as drilling a deeper well.
  7. Storage tanks or pipes that are not water tight. Make sure there is no way for surface water to enter your drinking water system.
  8. Old, unused wells. Old wells in the area may be draining bacteria into the groundwater. Check with a well driller about sealing up unused wells.
  9. Septic system or animal waste close to the well. The is a problem only if Fecal Coliform or E. coli was found. Check for other problems with the well or surface water described above.


Prepared by Gail Glick Andrew, OSU Extension Service, March 2001