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2B Municipal Drinking Water Quality

Description | Specific Indicators | Corresponding Mandatory Objectives | Corresponding National Indicators | Data Sources | Survey Questions | Alternative Data Sources | Analysis Check List | Method of Calculation | Basic Categories | Indicator Comments | Definitions | Cross-References to Other Sections | References



  • The number of days that any health parameter exceeded drinking water quality standards.
  • The number of days that fluoride levels were below recommended levels.

Specific Indicators
  • Drinking water quality exceedences for health parameters
  • Trihalomethanes (THMs) in drinking water
  • Coliform bacteria in drinking water
  • E.coli in drinking water
  • Fluoride in drinking water

Corresponding Mandatory Objectives

Program Standard: Infectious Diseases, Safe Water

  • To ensure that community drinking water systems meet the health-related chemical, physical, microbiological and radionuclide objectives as published in the Ontario Drinking Water Objectives (revised 1994) and the Guidelines for Canadian Drinking Water Quality (sixth edition).

Corresponding National Indicators
  • None

Data Sources

Original source:
Municipal water supply plants
Distributed by:
1. Municipal water supply plants
2. Ontario Ministry of the Environment
Suggested citation (see Data Citation Notes):
[Name of municipal water supply plant] [year]

Analysis Check List
  • Some public health units may have their own database for tracking drinking water quality exceedences.

Method of Calculation

  • Number of days that a health parameter was exceeded according to the drinking water quality standards.
    • Number of days that the level of THMs in treated drinking water > than 0.1 mg/L.
    • The number of days that total coliforms > 0 colonies per 100 ml of water.
    • The number of days that E.coli > 0 colonies per 100 ml of water.
    • The number of days that fluoride levels were < 0.5 mg/L or > 1.5 mg/L.

Basic Categories

  • Geographic areas: Geographic areas: by municipal water system.
  • By parameter.

Indicator Comments

  • Municipal water systems are part of the “regulated drinking water system”. Some sources of drinking water are not regulated, including households on wells. A regulated drinking water system may be anything from a large municipal water treatment plant to a single water well servicing a campground. All regulated systems are required by law to ensure that they are taking measures to protect the quality of their water. Regulated systems must treat their water to prevent contamination and must test their water quality regularly.1
  • The Walkerton Escherichia coli O157:H7 outbreak in May 2000 resulted in seven deaths and 2,300 people becoming ill. Recommendations from the Walkerton Commission of Inquiry2, prepared by Mr. Justice Dennis O’Connor, resulted in the Safe Drinking Water Act of 20023 as well as other drinking water regulations.
  • Recommendation 6 from Part One of the Walkerton Commission was that upon the implementation by the MOE of the Integrated Divisional System (management information system), access to it should be made available to local health units and, where appropriate, to the public. This should include access to profiles of municipal water systems, and data concerning adverse drinking water quality sample test results, as included in that database. As of March 2005, implementation of this recommendation is underway.4
  • The Drinking-Water Systems Regulation stipulates that the owner of a water treatment or distribution system is required to ensure that notice is given to the local Medical Officer of Health and to the MOE if a parameter does not meet the Maximum Acceptable Concentration (MAC) or Interim Maximum Acceptable Concentration (IMAC) of the Ontario Drinking Water Quality Standards (ODWQS), or if a test result indicates adverse water quality. The Medical Officer of Health, through the Health Protection and Promotion Act, has the authority to judge if drinking water is safe for human consumption.
  • The ODWQS5 are the provincial standards of drinking water quality, most of which have been adopted from the Canadian drinking water quality guidelines established by the Federal-Provincial-Territorial Committee on Drinking Water.6 The guidelines are derived from risk assessment based exposure limits as modified by a risk management process incorporating review of the geographic scope and prevalence of the contaminant, available technology to remove it and associated costs.
  • Since not all health parameters can be described, four of the more common parameters are discussed below: trihalomethanes (THMs), coliforms, E.coli and fluoride. Scientific summaries for each of the parameters are available as part of the Guidelines for Canadian Drinking Water Quality.6
  • Trihalomethanes are disinfection by-products and the most widely occurring volatile organics found in chlorinated drinking water. The principal source in drinking water results from the chemical interaction of chlorine with naturally occurring organics (precursors) in the raw water. The four most commonly detected THMs in drinking water are chloroform, bromodichloromethane, chlorodibromomethane, and bromoform. Some studies have linked THMs to bladder cancer and rectal cancer, and to adverse reproductive outcomes.7,8,9
  • Trihalomethanes (THMs) are monitored quarterly, expressed as a running annual average of four quarterly test results from the distribution system. The MAC is based on the risk associated with chloroform, the THM most often present and generally found in the greatest concentration in drinking water. According to the Canadian Drinking Water Guidelines and the ODWQSs, the MAC for THMs is a seasonal average of 100 µg/L.
  • Coliform are a group of indicator bacteria used to assess the bacteriological quality of drinking water. Coliforms are not necessarily harmful but their occurrence in water may indicate the presence of pathogenic organisms. Similarly the presence of E.coli in drinking water is a definite indicator of recent faecal pollution and therefore poses a potential health risk. E.coli bacteria are normally harmless inhabitants of the intestines of humans and animals. Some forms of E.coli, notably 0157:H7, can cause illness and death.
  • The April 2004 Summary of Guidelines for Canadian Drinking Water Quality6 stated the following: Guidelines for bacteria are under review. The maximum acceptable concentration (MAC) for bacteriological quality of public, semi-public, and private drinking water systems is no coliforms detectable per 100 mL. However, because coliforms are not uniformly distributed in water and are subject to considerable variation in public health significance, drinking water that fulfills the following conditions is considered to conform to this MAC:
    1. No sample should contain Escherichia coli. E. coli indicates recent faecal contamination and the possible presence of enteric pathogens that may adversely affect human health. If E. coli is confirmed, the appropriate agencies should be notified, a boil water advisory should be issued, and corrective actions taken.
    2. No consecutive samples from the same site or not more than 10% of samples from the distribution system in a given calendar month should show the presence of total coliform bacteria. The ability of total coliforms to indicate the presence of faecal pollution is less reliable than E. coli. However, this group of bacteria is a good indicator of quality control. The presence of total coliforms does not necessarily require the issuance of a boil water advisory but corrective actions should be taken.
  • Fluoride is a naturally occurring mineral found in soil, air and water. In Canada, naturally occurring fluoride concentrations in fresh untreated water varies from 0.01mg/L to 11mg/L. Fluoride helps to re-mineralize cavities when they first begin to form and increase the resistance of tooth enamel to acids that cause tooth decay. Ensuring a recommended concentration of fluoride in drinking water of 1.0 mg/l (+/-0.2mg/L) can lead to a 20-40% reduction in tooth decay in people of all ages and can result in children being free of tooth decay. Consuming water with fluoride concentration above 1.5mg/L can result in dental fluorosis or “mottled teeth”, characterized by white chalky spots in teeth, generally cosmetic in nature.
  • In 2000, the MOE reaffirmed the Ontario Drinking Water Objective for Fluoride of 1.5 mg/L and reduced the recommended concentration of fluoride in drinking water where fluoridation is practised to 0.5 - 0.8 mg/L (from 1.0-1.2 mg/L).11
  • Boards of health monitor fluoride levels for their local Municipal or Regional water supply(s) on a monthly basis according to the MOHLTC’s protocol on Monitoring the Fluoridation of Local, Municipal or Regional Water Supply. The protocol was revised August 2000 to reflect the MOE change. If the reported monthly average level of fluoride is below 0.5 ppm (mg/L) or above the Maximum Allowable Concentration (MAC) of 1.5 mg/L, the board of health shall contact the Ministry of Environment for an explanation; and institute a contingency water monitoring plan.12

  • None

Cross-References to Other Sections
  • Enteric disease incidence (Section 8: Infectious Diseases)
  • Enteric disease hospitalization (Section 8: Infectious Diseases)
  • Fluorosis index (Section 6C: Child and Adolescent Health)


  1. Ministry of the Environment web site: Water. URL:
  2. Walkerton Commission of Inquiry Reports URL:
  3. Ministry of the Environment. Safe Drinking Water Act, 2002, Ontario Drinking Water Quality Standards. URL:
  4. Status of Part One Recommendations Report of the Walkerton Inquiry, February 2005 URL:
  5. Ministry of the Environment. Ontario Drinking Water Quality Standards. URL:
  6. Health Canada. Guidelines for Canadian Drinking Water Quality. URL:
  7. Morris RD, Audet A, Angelillo IF, Chalmers TC, Mosteller F. Chlorination, Chlorination By-products, and Cancer: A Meta-analysis. AJPH 1992;82(7):955-963.
  8. Waller K, Swan SH, DeLorenze G, Hopkins B. Trihalomethanes and Spontaneous Abortion. Epidemiology 1998; 9(2):134-140.
  9. Mills CJ, Bull RJ, Cantor KP, Reif J, Hrudey SE, Huston P, Expert Working Group. Health Risks of Drinking Water Chlorination By-Products: Report of an Expert Working Group. Chron Dis Can 1998;19(3). URL:
  10. Locker D. Benefits and Risks of Water Fluoridation: An Update of the 1996 Federal-Provincial Sub-committee Report. Prepared for: Public Health Branch, Ontario Ministry of Health First Nations and Inuit Health Branch, Health Canada, November 15, 1999. URL:
  11. Ministry of the Environment. Notice of Decision for Policy. Proposal to reaffirm the Ontario Drinking Water Objective for Fluoride and to reduce the concentration of fluoride in drinking water where fluoridation is practiced, September 20, 2000. URL:
  12. Ministry of Health, Monitoring the Fluoridation of Local, Municipal or Regional Water Supply Protocol, Child Health Program. January 1, 1998.

Date of Last Revision: April 4, 2006 .

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