Coliform Bacteria
Fact Sheet

What are coliform bacteria?
Coliform bacteria make up a large group of bacteria that are found in soils, on plants, and in surface water.  Certain coliform bacteria live in the intestines of humans and animals. 

Coliforms are not harmful themselves, but when present in drinking water, disease-causing microorganisms such as bacteria, viruses, and parasites may have gotten into the water supply by the same route as the coliforms.  The Maine Health and Environmental Testing Laboratory (HETL) tests for coliforms as an indicator for possible contamination. 

What do I do if my drinking water tests positive for coliform bacteria? 
When a drinking water sample is positive for coliform bacteria, the water should then be tested for Eschericia coli (E. coli) or fecal coliforms.  Drinking water samples analyzed by HETL for total coliform bacteria are automatically checked for E. coli.   Public water systems are required to collect additional samples to confirm the positive water tests. Private well owners should consider collecting additional samples to insure the samples were collected properly.

If a water test for E. coli or fecal coliforms is positive, the water is not safe to drink!  The water should first be boiled for at least one minute or disinfected.  See “How can drinking water be disinfected?”

What are the health risks of drinking water positive for E. coli and fecal coliforms?
Consuming water containing E. coli and fecal coliforms can cause intestinal upset as well as diseases such as dysentery, hepatitis, and giardiasis. IMPORTANT:  If your water was positive for coliforms and the water was not tested by the lab for E. coli or fecal coliforms, you cannot be sure they are not there and you should take precautions by boiling the water or disinfecting.  See “How can drinking water be disinfected?”

What are the possible sources of contamination?
Most bacteria in wells or springs come from surface water directly entering the well. The water may be contaminated by bacteria that are naturally in the soils, decayed animal waste or human activities. As surface water seeps downward through the soil to the water table, these bacteria may be naturally removed by the soils. The extent of removal depends on the depth and character of the soil. In general, shallow wells and springs are more likely to be contaminated than deep wells. Wells and springs must be properly located, constructed and maintained in order to prevent surface water from entering the well or spring.

Presence of coliform bacteria in wells or springs can result from:

  1. Well or spring covers that allow dust, rain, bird droppings, insects, etc. to enter.
  2. Wells or springs located in areas where surface water covers the source during the wet periods of the year.
  3. Defective steel well-casing seals.
  4. Shallow wells or rock lined springs/wells.
  5. Recent changes or repairs to the well or spring, pumps, piping, etc.
  6. Improper well location and/or construction.

If you have one or more of the problems described above, it should be corrected. If it is not corrected, the problem may reoccur, even after repeated disinfection.

How can drinking water be disinfected?
Water containing coliform bacteria should be disinfected, as a precaution, by applying chlorine bleach as described below.  If you have a water treatment system, check with the installer or manufacturer of your system to be certain the disinfection process will not harm your system. 

The water system may be disinfected by mixing chlorine bleach (Clorox, etc.) with the water in the well or spring in the following dosages:   

Dug Well


5 ft

10 ft

15 ft

20 ft


1/2 gal

1 gal

1 1/2 gal


2 gal

*Approximate amount of water in the bottom of a well and not the total depth of the well. For a surface SPRING, use 2 gallons.

Drilled Well


50 ft

100 ft

150 ft

200 ft

250 ft


300 ft


2-1/2 c

1 1/2 qt

2 qt

2 1/2 qt

3 qt


3 1/2 qt

*Note: A greater amount of chlorine may be needed to disinfect the water depending on the degree of contamination.

Once the chlorine has been mixed with the water, open all faucets, sillcocks and similar outlets individually until you smell chlorine in each outlet. Allow the mixture to stand in the system overnight, then flush the chlorine mixture from the system using an outside faucet and garden hose. Do not flush the mixture into your septic system.You may resume using the toilet facilities as the septic system is designed, but the septic system cannot handle the large amount of water needed to flush the chlorine from the well. Since the chlorine will kill the grass, be careful where you run the water outside.

After disinfection, the water should be tested again to insure the disinfection worked. You should wait at least 3 or 4 days after the chlorine odor has disappeared before collecting another test. A sample kit for "bacteria only" is available at a lower cost.

Where can I find more information?

Maine Drinking Water Program:                       
Health and Environmental Testing Lab:                      www.maine.govstandard.htm
Environmental & Occupational Health Program: