Description, use and calculation of the indicator

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Description of the indicator
How the WQI for aquatic life is used
How the indicator is calculated

Description of the indicator

The freshwater quality indicator provides an overall measure of the suitability of water bodies to support aquatic life at selected monitoring sites in Canada. The indicator is based on applications of the Water Quality Index (WQI) endorsed by the Canadian Council of Ministers of the Environment (CCME) in 2001 (CCME, 2001). Given that aquatic life can be influenced by the presence of hundreds of both natural and anthropogenic substances in water, the WQI provides a useful tool that allows experts to translate vast amounts of water quality monitoring information into a simple overall rating.

At present, the freshwater quality indicator is presented as southern Canada and northern Canada histograms, and a Great Lakes map of the WQI results from individual water quality monitoring sites across the country. The histogram groups WQI values into five categories: poor, marginal, fair, good, and excellent.

The WQI measures the frequency and extent to which selected parameters exceed water quality guidelines at individual monitoring sites. Water quality guidelines are numerical values for physical, chemical, radiological, or biological characteristics of water that indicate that adverse effects may be occurring when exceeded.¹ The water quality guidelines used in the calculations are those defined for the protection of aquatic life. They include national guidelines developed by the CCME, as well as provincial and site-specific guidelines developed by federal, provincial, and territorial partners. If a guideline value is exceeded at a given site, there is an increased probability of an adverse effect on aquatic life at that site.

The WQI reflects the potential for substances to impact aquatic life based on existing knowledge of toxicity and predicted fate and behaviour of chemical substances. It is not a direct measure of changes to aquatic communities, such as changes in the composition or abundance of benthic invertebrates or fish.

In aquatic ecosystems, water quality naturally varies seasonally and annually due to fluctuations in weather such as the timing and amount of precipitation, which affects erosion in the drainage area and water levels and flows. Thus, the WQI is calculated for a period of three years (2002 to 2004) to dampen the effect of seasonal variability on the WQI score.

How the WQI for aquatic life is used

The Canadian Environmental Sustainability Indicators (CESI) 2006 report provides policy analysts and decision makers with national and regional pictures of the status of water quality for the protection of aquatic life.

On a regional level, the CCME WQI has been used by many organizations and jurisdictions, such as watershed conservation groups and territorial, provincial, and federal government agencies, to inform the public, decision makers, and relevant stakeholders on the status and trends of local water bodies (British Columbia Ministry of Environment (BCMOE), 1996; Alberta Environment, 2003; Grand River Conservation Authority, 2004; Khan et al., 2004; CCME, 2005a; Environment Canada, 2005a; and Lumb et al., 2006. It has also been used to track the effectiveness of remedial measures on local water quality (Glozier et al., 2004 and Wright et al., 1999) and to report on the effectiveness of government programs and policies (Alberta Environment, 2002).

Although the CCME provides general guidance on using the index (www.ccme.ca), practitioners are responsible for deciding which parameters, guidelines, time periods, and number of samples to include in a given application of the index. As a result of this flexibility, different approaches have been used to apply the index to achieve different objectives. For example, the British Columbia Ministry of the Environment (1996) used site-specific guidelines to evaluate the suitability of water quality to support different beneficial uses, using the most recent three years of data. Glozier et al. (2004) applied the index using background concentration²  values from reference sites³  to assess change in status and trends for downstream sites. In this work, trends were calculated as rolling values based on blocks of five years of samples (e.g. 1983–1987 and 1984–1988), while status was assessed for a 20-year period. In contrast, Wright et al. (1999) used background concentration values from a given time period (rather than reference sites) as benchmarks for the index to assess changes in water quality over time. Site-specific guidelines are developed because of the differences that exist between different aquatic ecosystems in terms of natural background, chemical interactions between water quality parameters, etc.

As a result of this flexibility in applying the index, a protocol for calculating the WQI ratings across Canada for this initiative was developed (Environment Canada, 2005b). For 2006, however, there remains variation in the applications of the WQI across Canada (see section Caveats and limitations of the indicator and data quality).

How the indicator is calculated

The freshwater quality indicator is based on the application of the CCME WQI across Canada at 370 monitoring sites (streams, rivers, and lakes) and 7 basins from the Great Lakes using ambient water quality monitoring data for the 2002 to 2004 period, and relevant water quality guidelines for the protection of aquatic life. Of the 370 sites, 30 are located in northern Canada and 340 in southern Canada. In addition, water quality was assessed separately for 7 basins of the Great Lakes from surveys conducted in April 2004 and 2005. The resulting ratings are presented in five categories (poor, marginal, fair, good, and excellent) in one national histogram.

Changes from previous period (2005 report)

A number of changes to the 2005 freshwater quality indicator were undertaken for 2006. The following list provides an overview of these changes, most of which are described in more detail in the subsequent sections:

• Separation of the indicator into north and south portions of Canada, due to differences in intensity of data collection. In 2005, eight sites were located in the area that is defined as the North for 2006.

• In the South, a subset of water quality values from 7 representative lakes was selected from a larger dataset of 62 highly clustered lakes inNew Brunswick and Nova Scotia. In 2005, only 18 lakes had met the minimum requirement for sample numbers and were reported as individual sites.

• The addition of 22 new sites in the North and 25 in the South. In the South, there were also 11 fewer sites due to the use of a subset of lakes (see previous bullet) and 11 fewer due to reduced or discontinued monitoring.

• All jurisdictions calculated WQI values using the same F₁ formula. Previously, a slightly different formula was used for sites in the province of Quebec.

• Stations for Quebec’s Réseau-Rivières included year-round data in calculations, rather than April to October. This resulted in different ratings for 22 of 115 stations.

• Minimum sampling frequency data requirements were reduced from four times to three times per year for northern locations, based on the results of a sensitivity analysis. Also, an exception was made for including three rivers in New Brunswick that had one sample less than the requirement and three rivers in Manitoba that were sampled three times a year, rather than four, but had a very long data record and local expert opinion confirming the reliability of the WQI scores for this period.

• The Great Lakes assessment included three parameters measured in sediment, as these contaminants are known to endanger aquatic life.

Formulation of the CCME Water Quality Index

The CCME WQI relates water quality data to the various beneficial uses of water⁴ using relevant water quality guidelines as benchmarks. Each index is calculated for an individual monitoring site during a chosen reference period. Water samples collected over this period of time are analyzed for a suite of water quality parameters. Each parameter value is evaluated against the appropriate water quality guideline (Appendix 1). These are called tests. The percentage of parameters, samples, and tests that fail to meet the guidelines, and the deviation (excursion) from the guideline, are captured in three factors — scope, frequency, and amplitude of excursions from water quality guidelines — used to calculate the index (CCME, 2001). The index yields a number between 0 and 100. A higher number indicates better water quality. 

CCME WQI formula

Scope (F₁)

The scope factor represents the percentage of the total number of parameters that fail to meet the water quality guidelines at any time during the reference period.

Frequency (F₂)

The frequency factor represents the percentage of individual tests that fail to meet the water quality guidelines.

A failed test occurs when an individual parameter value within a sample exceeds the guideline. The total number of failed tests represents the total number of failed parameter values in every sample during the reference period. The total number of tests for an individual site is calculated by multiplying the average number of parameters per sample by the total number of samples during the reference period.

Amplitude (F₃)

The amplitude factor represents the average deviation of failed test values from their respective guidelines. The relative deviation of a failed test from the guideline is termed an excursion and is calculated as follows:

1. When the test value must not exceed the guideline:
   
   
   
   
   
2. When the test value must not fall below the guideline:
   
   

The collective amount by which individual tests are out of compliance is calculated as follows:

          

where nse is the normalized sum of the excursions from the guidelines. The F₃ factor is then calculated by a formula that scales the nse to yield a range between 0 and 100.

           

The rating system of index values

The WQI yields a number between 0 and 100 that is indicative of the overall water quality for a particular use (Text table 1).

Data preparation and presentation

The data used to calculate the freshwater quality indicator were derived from water samples collected at sites across the country from 2002 to 2004. Data were combined to calculate a single index value for each site using the equations described in the section on How the indicator is calculated. The steps below, which are described in more detail in section Data sources: review and selection, were followed in carrying out the calculations:

1. Selection step:
   a. Selection of sites
   b. Selection of parameters
   c. Selection of relevant national, regional, or site-specific guidelines
   d. Number of samples, timing, and collection period
   
   
2. Calculation step:
   a. Extraction of data
   b. Validation of data
   c. Calculation of index

The index values for each site were then classified into the five quality categories of the WQI and presented in a histogram as the freshwater quality indicator for northern Canada and southern Canada. The line delineating the North is based on a series of climatic, biotic, and socio-economic aspects (McNiven and Puderer, 2000). WQI values were also derived for seven basins of the Great Lakes and reported separately (see Site selection).

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Notes

1. Guidelines are specific to particular water uses, such as protection of aquatic life, crop irrigation, livestock watering, drinking water, and recreation.

2. The concentration of a naturally occurring water quality constituent, not influenced by human activity.

3. An area considered to be relatively unaffected by human activity.

4. These uses are: protection of aquatic life, drinking water, livestock watering, crop irrigation, and recreational use (CCME, 1999).