Drinking Water Quality Index

A Water Quality Index (WQI) is a means by which water quality data is summarized for reporting to the public in a consistent manner. It is similar to the UV index or an air quality index, and it tells us, in simple terms, what the quality of drinking water is from a drinking water supply.

Essentially the WQI is calculated by comparing the water quality data to "Guidelines for Canadian Drinking Water Quality". The WQI measures the scope, frequency, and amplitude of water quality exceedances and then combines the three measures into one score. This calculation produces a score between 0 and 100. The higher the score the better the quality of water. The scores are then ranked into one of the five categories described below:

Excellent: (WQI Value 95-100) - Water quality is protected with a virtual absence of impairment; conditions are very close to pristine levels. These index values can only be obtained if all measurements meet recommended guidelines virtually all of the time.

Very Good: (WQI Value 89-94) - Water quality is protected with a slight presence of impairment; conditions are close to pristine levels.

Good: (WQI Value 80-88) - Water quality is protected with only a minor degree of impairment; conditions rarely depart from desirable levels.

Fair: (WQI Value 65-79) - Water quality is usually protected but occasionally impaired; conditions sometimes depart from desirable levels.

Marginal: (WQI Value 45-64) - Water quality is frequently impaired; conditions often depart from desirable levels.

Poor: (WQI Value 0-44) - Water quality is almost always impaired; conditions usually depart from desirable levels.

WQI scores are computed for each public water supply system that has been sampled in a sampling season. The same variables are used in the computation of the WQI for all public water supply systems and only the six most recent samples are used. However if a public water supply system is on a Boil Water Order, or it has a current contaminant exceedance, or has a THMs average above the drinking water quality guideline a WQI score is not computed.

The WQI was developed by the Canadian Council of the Ministers of the Environment (CCME) in 2001 with the intent of providing a tool for simplifying the reporting of water quality data. Prior to the implementation and modification of the WQI for reporting drinking water quality in Newfoundland and Labrador, pilot level testing was carried on selected public water supply systems and a paper describing the "Modification and Application of the CCME WQI for the Communication of Drinking Water Quality Data in Newfoundland and Labrador" was published in the national journal "Water Quality Research Journal of Canada" to allow scientific scrutiny of the use of the CCME WQI for drinking water quality reporting. The methodology described in the paper has been further refined to screen and highlight current aesthetic exceedances. Click this link to see additional information on the CCME WQI and its application in NL.

The WQI is a summary tool and the Department does not intend to use the WQI to replace detailed analysis of drinking water quality data. The Department continues to closely monitor and analyze drinking water quality to protect drinking water safety on a proactive basis.

Drinking Water Quality Community Reports - Winter 2009
(Sorted by Region then Community)

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