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What is a water quality index?
A water quality index is a means to summarize large amounts of water quality data into simple terms (e.g., good) for reporting to management and the public in a consistent manner. Similar to the UV index or an air quality index, it can tell us whether the overall quality of water bodies poses a potential threat to various uses of water, such as habitat for aquatic life, irrigation water for agriculture and livestock, recreation and aesthetics, and drinking water supplies.
Why do we need an index?
Traditional reports on water quality typically consist of complex variable-by-variable, and water body-by-water body statistical summaries. This type of information is of value to water quality experts, but may not be meaningful to Canadians who want to know about the state of their local water bodies and for managers and policy makers who require concise information about those water bodies. The index also allows water quality data to be compiled and reported in a consistent manner throughout Canadian jurisdictions.
What information does a water quality index convey?
Many water quality variables (e.g., acidity, fecal coliforms, dissolved oxygen) are compared to water quality guidelines or site-specific objectives. The results of those comparisons are combined to provide a water quality ranking (good, average, poor) for individual water bodies. The actual variables used are those which are important for the particular water body.
The importance of site-specific objectives and their selection are crucial to obtaining the most meaningful results from the CCME WQI. The index can incorporate CCME water quality guidelines or guidelines from other jurisdictions when site-specific water quality objectives are not available, so that comparisons can be made for different water uses (e.g. aquatic life, drinking water, and recreation).
The advantages of an index include its ability to represent measurements of a variety of variables in a single number, its ability to combine various measurements in a variety of different measurement units in a single metric and its effectiveness as a communication tool. When the same objectives and variables are used, the index can be used to convey relative differences in water quality between sites over time.
What information does the CCME WQI not convey?
It is important when using an index to keep in mind its limitations. While a stock market index is a good indicator of the overall performance of the market, anyone with a stock portfolio will be interested in the performance of individual stocks or stock sectors. An environmental index is similar; it is not intended to replace a detailed analysis of environmental monitoring data, nor should it be used as the only tool for management of water bodies. What it can do is provide a broad overview of environmental performance.
Disadvantages of using an index include the loss of information on single variables, the sensitivity of the results to the formulation of the index, and the loss of information on interactions between variables.
How did the CCME WQI come into being?
The CCME WQI is based on a water quality index developed by British Columbia in 1995. British Columbia presented its work on a water quality index to CCME during its development, and the CCME Water Quality Guidelines Task Group was asked to develop an index that could be used by other jurisdictions. The British Columbia index was modified by work undertaken in Alberta and by advice from an inter-jurisdictional committee.
Can the CCME WQI be used to report on the quality of other media?
The CCME WQI has not been specifically tested for media other than water column concentrations. However, the British Columbia water quality index does not discriminate among the data from the water column, sediments, or tissue and in some situations has included data from all three media to calculate an index value. Initial work using the CCME WQI for sediments from the Great Lakes indicates that the CCME WQI may be suitable for reporting this information.
How do you use the water quality index?
A sub-committee of the CCME Water Quality Task Group prepared a user's manual on the application of the index itself (not a status report of water quality in different water bodies across the country). This report describes a protocol on how the index is to be used.
A computer program to calculate values has also been developed.
How does the index relate important variables to each other? Are some variables more important than others in applying the index?
The index avoids the problem of weighting different variables that exceed an objective or guideline by treating all variables that are retained with equal importance. Since the relative impacts of different chemicals such as copper or PCBs are addressed during the development of water quality objectives, further weighting is not warranted.
How does the index deal with values below the analytical detection limit?
Another problem frequently encountered in reporting on water quality data is measurements that are below the detection limit for the analytical methods used. It is problematic to deal with these measurements statistically; however, the CCME WQI uses ‘less than’ values as observations which are within the objectives (where the objective is higher than the level of detection), so the results are counted while circumventing all the statistical problems associated with them.
How does the index compare variables to objectives?
This index is based on three attributes of water quality that relate to water quality objectives:- Scope - How many? - The number of water quality variables that do not meet objectives in at least one sample during the time period under consideration, relative to the total number of variables measured.
- Frequency – How often? - The number of individual measurements that do not meet objectives, relative to the total number of measurements made in all samples for the time period of interest.
- Amplitude – How much? – The amount by which measurements which do not meet the objectives depart from those objectives.
How are index results reported or what do the index scores mean?
Once the CCME WQI value, a number between 1 and 100, has been determined, the result can be further simplified by assigning it to a descriptive category. The following categories are suggested as a starting point. The sub-committee that developed the CCME WQI spent considerable time discussing calibration and put these categories forth as a suggestion. The sub-committee recognized that calibration was beyond the scope of their task and that it would have to be tested further and possibly changed across jurisdictions. The category ranges may be modified depending on the variables and objective chosen but this will depend on the water bodies and variables being dealt with through a process of comparing the index rank to expert opinion about the water body.
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.
What are the cautions with using the index?
Applying this index to water quality data sets must be done with due regard to how the index is formulated. Use of the index for purposes for which it was not designed can lead to erroneous conclusions. Therefore, the following points need to be considered:
a. Index comparisons should only be made when the same sets of objectives are being applied. The CCME WQI allows the index user to select the objectives on which to compare measured water quality. This increases the versatility of the index but allows for misuse. Different jurisdictions use different objectives for water quality, and different objectives for different water uses. If an index value is calculated on one set of objectives and compared to an index value based on a completely different set of objectives, any conclusions drawn may be erroneous and comparisons must be made carefully.
b. Index comparisons should only be made using the same sets of variables. This is common sense “apples to apples” reasoning. Comparing a site where most of the measured variables are pesticides to a site where most of the measured variables are metals will yield information of limited value.
c. Care should be taken with older data. Many data sets can go back to times when the sensitivity of analytical methodology was considerably less than with more modern methods, especially where older methods have detection limits that may be significantly above current water quality objectives. All analytical methods are capable of producing ‘false positive’ results and incorporation of these into the index can provide questionable index values and misleading conclusions.
d. The index should be run on sets of variables relevant to the water body being tested. Several jurisdictions have data sets where large suites of water quality variables were tested. The CCME WQI should only include ‘relevant’ variables in the calculation since the inclusion of many variables (for example, all pesticides in a ‘scan’) may result in unrealistically high index values (i.e. lots of irrelevant variables meeting objectives, so a better index value results.). This will be of particular concern in situations where comparisons between sites are desired.
e. Minimal data sets should not be used. The CCME WQI was not designed to replace proper evaluation of water quality conditions through thorough assessment of water quality chemicals of concern. The CCME WQI should not be run with less than four variables and four sampling visits per year. Users are encouraged to carry out sensitivity analyses to determine how sensitive the index is to the number of variables and number of sampling visits.
f. The CCME WQI should be based on either site-specific objectives or on guidelines for variables that have been ranked as the most important for the site. Only those variables for which objectives exist should be considered. The absence of an objective for a variable does not necessarily mean the variable should not be used. The ability of the index to provide insights into water quality is entirely reliant on the realism of the objectives. The use of poorly thought out objectives yields misleading index values.
g. The CCME WQI should not be used to replace trend analyses. The CCME WQI is meant as a broad tool to indicate success in protection and remediation efforts.
Where has a water quality index been used in Canada?
Alberta
British Columbia
Manitoba
Newfoundland and Labrador
Québec IQBP
Who can I contact about the CCME WQI?
Further information on using the CCME WQI can be obtained from the following individuals who were instrumental in its development:
Vincent Mercier, Environment Canada - vincent.mercier@ec.gc.ca
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