Bacteria in Drinking Water

Bacterial contamination is the most common water quality problem faced by private well owners. There are many different types of bacteria. The laboratory tests for only two types of “indicator” organisms; total coliform bacteria and E. coli. The results of these two tests tell us different things about the safety of your water supply. 

Total coliform bacteria are a group of organisms that are occur naturally in a wide range of environments including soils, surface waters as well as manure and sewage. Their presence alone in drinking does not necessarily mean there are harmful bacteria in your water. The presence of coliform bacteria with the absence of E.coli can simply be an indicator of the sanitary condition of the well or plumbing system. For example, it can indicate that the well is vulnerable to contamination, with the potential for more harmful bacteria to enter the well. 

E. coli is a group of bacteria found in the intestines of warm blooded animals. Not all strains of E. coli are harmful to humans, however the presence of E. coli in drinking water is a clear indicator of recent faecal contamination of the water supply, usually from sewage or manure. 

What are the health concerns?

The Guidelines for Canadian Drinking Water Quality recommend that drinking water contains no  total coliform (0 cfu/100 mls) or E. coli bacteria organisms. While the presence of total coliform in the absence of E. coli is not considered a serious public health threat, it can suggest that a water supply is vulnerable to contamination by more harmful pathogens.

The presence of E. coli indicates contamination of the water system by a source of faecal material. Faecal material can contain a number of harmful and infectious pathogens. 

What do I do if there is a detection?

The detection of any level of E. coli, or high levels of total coliform (more than 10 cfu/100mls)  in drinking water from a private well should result in the immediate adoption of the precautionary measures recommended under a “boil water advisory”. In addition, the water should be resampled to confirm the initial results. Depending on the results of this second sample, an investigation into the source of contamination may be initiated.

The presence of a low level (in the range of 10 cfu/100 mls) of total coliform bacteria in the absence of E. coli should be followed up by an inspection of the well and resampling of the water. If the problem persists, or if the level of bacteria in the water increases, homeowners should adopt the same precautions recommended above for the detection of E. coli. In either case, normal use of the water should not resume until the safety of the water has been verified by obtaining two consecutive samples showing no presence of either bacteria. 

What are the treatment options?

Many bacteria problems can be addressed by disinfection of the well and plumbing system provided there are not fundamental problems with the well construction, or there are nearby sources of faecal material.

It is recommended that wells be inspected and disinfected on a routine basis. More persistent or serious bacterial problems can be addressed by the installation of a Class A ultra-violet (U.V.) disinfection system. 

While the Department does not recommend specific brands of treatment devices, it is recommended to purchase a device bearing a label indicating it has been certified to the NSF Standard 55 for Ultra Violet (UV) Microbiological Water Treatment Systems.

The effectiveness of a new treatment system should be verified by sampling after installation. It is also important to ensure the device is maintained according the manufacturer’s directions, with periodic water sampling  to confirm its performance. 


Published date: 
February 5, 2024
Environment, Energy and Climate Action

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