Interpreting Drinking Water Quality Results

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In Prince Edward Island we use the “Guidelines for Canadian Drinking Water Quality” to assess the safety of drinking water and its suitability for domestic use. The most important values are the health-based guidelines expressed as “maximum acceptable concentrations” often referred to as “MAC”. When the concentration of a constituent exceeds the MAC, there is an increased risk of adverse health outcomes associated with drinking this water. 

For most parameters, the guideline is based on long-term consumption; short-term exposure to water slightly exceeding the guideline is unlikely to cause illness. However for some parameters such as E. coli it is important to immediately stop consuming the water until corrective actions have been taken and the safety of the water has been confirmed with follow-up sampling. There are health-based drinking water guidelines for over 70 different parameters, however a great number of them are not common in most drinking water supplies. Some of the more common ones in PEI are listed in the table below:

Health Based Drinking Water Guidelines
Parameter Maximum Acceptable Concentration
total coliform bacteria* 0 cfu/100 mls
E. coli bacteria 0 cfu/100 mls
Antimony 0.006 mg/L
Arsenic 0.01 mg/L
Barium 1 mg/L
Cadmium 0.005 mg/L
Chromium 0.05 mg/L
Lead 0.01 mg/L
Nitrate Nitrogen 10 mg/L
Selenium 0.05 mg/L
Uranium 0.02 mg/L

* The maximum acceptable concentration for total coliform bacteria is 0 cfu/100 mls, however provided no E. coli is detected in the water, low concentrations (10 cfu/100mls or less) in the occasional sample does not pose sufficient health risks to require boiling the water, but should be followed up with re-sampling. The on-going presence of low total coliform counts suggests a problem with the water supply and should be addressed. 

Are there parameters with aesthetic objectives?

In addition to parameters with health based guidelines, there are other constituents in water that are not harmful, but may affect the general use of water for domestic purposes. For these parameters no health based guideline has been established, but “aesthetic objectives” have been developed to provide general guidance on the suitability of the water for normal domestic use.

Aesthetic Objectives
Parameter Aesthetic Objective
Chloride 250 mg/L


Hardness* (as CaC03) 200 mg/L
Iron 0.3 mg/L
Manganese 0.05 mg/L
pH 6.5 - 8.5
Sodium** 200 mg/L
Sulphate 500 mg/L
Zinc 5mg/L

* Hardness of water is not a health threat and public acceptance of hardness varies considerably. Generally hardness levels of 80 to 100 mg/L are considered acceptable and levels greater than 200mg/L are considered poor but can be tolerated; those in excess of 500 mg/L are generally considered un-acceptable. Groundwater, including that found in PEI is generally quite hard with values frequently in the range of 100 to 250 mg/L.

** There is not health based drinking water guideline for sodium and aesthetic objective for sodium is based on taste considerations. The concentration of 200 mg/L is considered safe for the general population however for individuals on a sodium restricted diet it is recommended they consult their physician for advice on their sodium intake from drinking water.

Are there parameters without guidelines?

There are a number of chemical present in natural waters as a result of normal geological processes such as weathering of rocks and soils, and for which no guideline is needed.  The following list includes chemical parameters that are measured by the laboratory, but which have no health based guideline or aesthetic objective. They are included because they provide additional information on the overall characteristics of the water.  

  • Alkalinity
  • Calcium
  • Magnesium
  • Phosphorus
  • Potassium

Who can I contact for more information?

Paul Baker (Safety Standards Officer)
J. Elmer Blanchard Building
31 Gordon Drive
Charlottetown, PEI   C1A 6B8
Telephone: (902) 368-5062
Fax: (902) 368-5526


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