How to Understand the Results of Your Well Water Tests

Why should I test my water?

Drinking water contaminated with disease-causing bacteria can make you or your family seriously ill. Symptoms can range from stomach cramps, nausea, diarrhea, to vomiting and fever. Testing your well water at least four times a year will help you to correct adverse water analysis results.

What do the test results mean?

Your test results may include the following terms:

Total Coliform Bacteria:

The presence of coliforms may indicate a contaminated water supply. Coliforms occur naturally in the soil and decaying vegetation, but may also be associated with human and animal fecal contamination. Given the vulnerability of well water to external influence, such as human, environmental and agricultural impacts, it is important to test water frequently.

Escherichia coli (E. coli) Bacteria:

Escherichia coli can be isolated from the fecal material found in the intestines of humans and other warm-blooded animals. These bacteria are found only in the digestive systems of people and animals. Their presence in your well is usually the result of contamination from a nearby source.

The water sample is esthetically unclean for drinking:

Public health laboratories will not analyze a sample they deem UNSUITABLE FOR TESTING due to its appearance or odour. As well, samples are sometimes returned without analysis because they contain particles and risk damaging lab equipment.

Waters that are unsuitable for testing can contain other substances such as iron or other minerals which make water unclean for drinking. In such cases, a water filtration system needs to be installed before proceeding to a bacteriological analysis of the water. Private laboratories may assist you in identifying some of these other substances.

What’s safe and what’s not safe?


No significant evidence of bacterial contamination

Total coliforms : ≤ 5

E. coli: 0

Three consecutive samples with this designation, collected one to three weeks apart are required to determine the stability of the water supply. The water is considered safe to drink.


Total coliforms: ≥ 5

E. coli: 0

Significant evidence of bacterial contamination. May be unsafe to drink. Consult the Health Unit for information.

E. coli > 0

Unsafe to drink. Evidence of fecal contamination. Consult the Health Unit for information.

Overgrown (O/G)

The crowding of bacterial growth prevents the laboratory from accurately identifying the presence of E. coli or total coliforms. The water is unsafe to drink. Consult your Health Unit.

What do I do if the tests show the water is unsafe?

If the first test results indicate the presence of coliforms or E. coli bacteria, follow the instructions for a Boil Water Advisory and consult the Eastern Ontario Health Unit (EOHU) for more information.

Possible actions to take might include:

  • Identifying and eliminating possible source of contamination
  • Disinfecting of the well (see How to Disinfect Your Well)
  • Re-sampling the well water
  • If contaminants persist, consider installing water disinfecting equipment