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Behaviour Study on the Water Quality Index of the Canadian Council of Ministers of the Environment

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In December 2005, Canada published its first national indicator for assessing surface water quality for the protection of aquatic life, the Canadian Council of Ministers of the Environment (CCME) Water Quality Index (WQI). This index reflects many aspects of water quality and at the same time facilitates presentation of data to the public at large.

The objective of the present study is to understand and explain how the index behaves, and at the same time determine its limitations to make a better use of it in the future. In order to do so, four datasets were made available to us thanks to participation of the following provinces: Newfoundland, Ontario, British Columbia and Quebec.

Below is a short summary of the main findings in our study, dealing with various issues raised during the water quality discussions:

  • The percentage of samples respecting parameter guidelines varies within provinces as well as inter-provincially. Some parameters have more difficulty in meeting the guidelines used by the experts than others.
  • There is little significant relationship between parameters taken in pairs, which seems to indicate that water quality experts are careful not to include parameters that are strongly correlated in the index calculation.
  • The majority of the chosen parameters have great importance in the index calculation.
  • The index is sensitive to the number of parameters that enter in its calculation for a given number of samples. The larger the number of parameters, the lower the intensity of extreme categories ("poor" and "excellent") in comparison with the "marginal" and "fair" categories, regardless of the departure point.
  • The index is also sensitive to the number of samples used by water quality experts for a given number of parameters. The larger the number of samples, the lower is the intensity of "good" and "excellent" categories in comparison with the "poor" and "marginal" categories, regardless of the departure point.
  • The CCME Water Quality Index consists of three terms, but the contribution of the first term is much higher than the contribution of the other two. An increase of the number of parameters leads to a slight reduction of the contribution of the first term, while an increase of the number of samples causes an indirect increase of its contribution.
  • The correlation is the highest between the last two terms of the index, and in general the correlation between the index terms depends on the number of parameters and samples.
  • Since the cost of water samples is higher for certain stations which are remote and more difficult to access, we studied and determined that the use of three samples per year instead of four does not influence index categorization results very much for all data made available to us. However, is this number satisfactory to represent the water quality?
  • There is a considerable variability of the index within certain stations, regardless of the number of samples used in the calculation. This variability tends to diminish when we increase the number of samples. This reduction can be explained by the fact that when a higher number of samples are used, the data is more representative and more homogeneous.
  • For certain parameters, the percentage of samples respecting the guidelines depends on the season. Therefore, the season would have an impact on the index categorization. Also, not including a certain season could introduce a bias to the index calculation and interpretation.

Therefore, the CCME water quality index depends on multiple components (the number of parameters and samples, the choice of parameters selected, the selected seasons and guidelines), which have an impact on calculation. Therefore, it is essential that a sampling protocol for these components be established and respected in order to make better use of the index, with the goal of making the findings comparable.

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