Canadian Water Quality Index - A freshwater quality indicator
The purpose of the Water Quality Index (WQI) is to summarize large amounts of water quality data for a specific river into simple terms (i.e. one number and a statement such as “good”). This makes it easily understandable for communities in the watershed and for watershed management, as well as creates a consistent system for reporting throughout Canada. The WQI was created by the Canadian Council of Ministers of the Environment (CCME) based on the previous work developed by British Columbia. The WQI is now used nationally in all provinces and territories. The WQI is based on the protection of aquatic life, not on human health.
What are the steps involved in calculating the WQI?
What are the guidelines based on?
How does the program calculate the WQI?
Are variables weighed? How are values below the detection limit dealt with?
What do the index scores mean?
Why is the three year score not always equal to the average of the three applicable annual scores?
Why have some of the annual scores changed in the recent calculation?
How were the water quality sites chosen? Is the sample random?
Where can I find out more information?
What are the steps involved in calculating the WQI?
1. Choose variables
For each monitoring site, five to fifteen variables (such as acidity, lead, dissolved oxygen) are chosen that best characterizes the water quality or are a concern for the local aquatic life. These variables usually depend on the land use practices in the watershed, whether the variable has been high at any point in the past, and whether there is an increasing trend for the variable. For example, if there is agriculture in the watershed, nitrogen and phosphorus would likely be chosen as variables. This same site might have had a couple events of high cadmium ten years ago, as well as a long term increase of water temperature, therefore cadmium and water temperature would also be chosen as variables.
2. Choose guidelines
Once variables have been selected, the guidelines for each of these variables have to be chosen. The CCME has published guidelines for many water quality variables, and most often these guidelines are used to calculate the WQI. But other times, these guidelines are not applicable; for example, if the natural state of a river has aluminum levels higher than the CCME guidelines because of a glacier that feeds the river, a higher guideline higher than the CCME one would be chosen. Over the past couple of decades, variable and guidelines have been developed with respect to specific water bodies, taking into consideration the physical and biological characteristics of the water body and watershed; these are science-based and called site-specific guidelines, and should be developed for the river before the index is calculated.
3. Calculate WQI score
After the variables and guidelines are chosen, this information and the appropriate water quality data are inputted into a computer program that calculates the WQI. Some variables also require additional information (i.e. modifying factors) in order for the score to be calculated. For example, the guidelines for several metals are dependant on water hardness, therefore hardness values must also be included for the WQI to be calculated.
What are the guidelines based on?
The guidelines are intended to provide protection to aquatic life from human-caused stressors, both physical (e.g. water temperature) and chemical (e.g. nitrate). The guidelines were based on the most current toxicological data available for each variable. Note that the WQI is based only on the protection of aquatic life, not on the protection of water for drinking or agriculture or any other use.
How does the program calculate the WQI?
The program uses the inputted guidelines and data to base the calculation on three functions relating to site specific water quality guidelines:
- Scope - How many guidelines have been exceeded? - The number of water quality variables that do not meet guidelines in at least one sample during the time period under consideration, relative to the total number of variables measured.
- Frequency - How often has the data exceeded guidelines? - The number of individual measurements that do not meet guidelines, relative to the total number of measurements made in all samples for the time period of interest.
- Amplitude - How much has the data exceeded guidelines? - The amount by which measurements which do not meet the guidelines depart from those guidelines.
In the following graph can be used as an example for discussing the concepts of frequency and amplitude. In the graph, the pink line represents the guideline level of cadmium for this site. For the year of 2004, there are 25 exceedences. This leads to a high frequency of exceedences. The amplitude of the exceedences is less important in this case, since the highest points are about two times the guideline. Amplitude would be more important if there was an event of ten times guideline level. The scope cannot be seen in this graph, since there is only one variable. In 2004 at this site there are 12 variables, with cadmium and zinc having one or more exceedences. Therefore, the scope and amplitude are less important in terms of the WQI calculation than the frequency.
Are variables weighed? How are values below the detection limit dealt with?
All variables used in the WQI calculation are treated with equal importance.
The WQI uses ‘less than' values as observations which are within the site specific guideline (where the site specific guideline is higher than the level of detection), so the results are counted while circumventing all the statistical problems associated with values below detection limits.
What do the index scores mean?
The WQI score is a number between 1 and 100, with 100 being obtained when no data for any variables are above guideline values. This score can be further simplified by assigning it to one of five descriptive categories. These categories are as follows:
Excellent: (CCME WQI Value 95-100) - water quality is protected with a virtual absence of threat or impairment; conditions very close to natural or pristine levels. These index values can only be obtained if all measurements are within objectives virtually all of the time.
Good: (CCME WQI Value 80-94) - water quality is protected with only a minor degree of threat or impairment; conditions rarely depart from natural or desirable levels.
Fair: (CCME WQI Value 65-79) - water quality is usually protected but occasionally threatened or impaired; conditions sometimes depart from natural or desirable levels.
Marginal: (CCME WQI Value 45-64) - water quality is frequently threatened or impaired; conditions often depart from natural or desirable levels.
Poor: (CCME WQI Value 0-44) - water quality is almost always threatened or impaired; conditions usually depart from natural or desirable levels.
The WQI score can be used to look at trends in the water quality of a river. For example, the WQI score for a river might slowly increase from 75 to 90 over a ten year period; this would be evidence that the water quality of this river is improving. Because there has only been four years of WQI scores published, it is difficult to conduct trend analysis using the WQI at this time.
Why is the three year score not always equal to the average of the three applicable annual scores?
Sometimes the three year running score is less than the average of the three applicable annual scores, such as for the Thompson at Spences Bridge, where the annual scores for 2002-2004 are 94.2, 70.6 and 76.4, all higher than the three year score of 65.2. This is because of how the WQI is calculated, with the score being based on several functions (related to scope, frequency, and amplitude). If, for example, water temperature guidelines were exceeded in 2002, while cadmium guidelines were exceeded in 2003, the scope for the combined years would be larger, therefore the score for the combined years would be lower. Similarly, the number of exceedences for the combined years is often larger than for any individual year, thereby changing the frequency function.
Why have some of the annual scores changed in the recent calculation?
The WQI has now been calculated twice for approximately 30 rivers in BC and the Yukon. In 2005, Environment Canada calculated the annual and running scores for 2001-2003. In 2006, the annual and running scores for 2002-2004 were calculated. Sometimes the scores from these two calculations for the same year differ slightly. This is for several reasons. One reasons is that some of the data had not been confirmed (approved) at the time of the 2005 calculation. A second reason is the methods of calculating the WQI evolved slightly between 2005 and 2006. This is a new system of reporting water quality, therefore changes are likely. A third reason is that the guidelines may have changed, as the CCME updates the guidelines using more recent and accurate toxicological data. As with all science, the WQI is based on our best knowledge at present, and as this knowledge changes, there may be slight changes in the resulting WQI score.
How were the water quality sites chosen? Is the sample random?
The sites used for the WQI calculations include all the water quality monitoring sites in BC and the Yukon with enough data to calculate the index (i.e. at least four points of data for each variable for each year). The locations of water quality monitoring stations are often chosen because of potential present or future water quality problem. This has lead to the majority of stations being located in the southern part of the region, where there is a higher population and more development. In other words, the choice of the stations is not random, and therefore the WQI scores do not represent the overall water quality of the region.
Where can I find out more information?
The CCME has published a user's guide to WQI, which gives both background information on the index as well as example calculations. The CCME's website has more information about the WQI, with links to applicable downloadable documents and frequently asked questions. The CCME guidelines are also available on their website. The Province of British Columbia has published reports on the water quality of various water bodies, including site-specific guidelines for these water bodies.