CCME Water Quality Index

An integral part of any environmental monitoring program is the reporting of results to both managers and the general public. This poses a particular problem in the case of water quality monitoring because of the complexity associated with analyzing a large number of measured variables. The traditional practice has been to produce reports describing trends and compliance with official guidelines or other objectives on a variable by variable basis. The advantage of this approach is that it provides a wealth of data and information; however, in many cases, managers and the general public have neither the inclination nor the training to study these reports in detail. Rather, they require statements concerning the general health or status of the system of concern.

One possible solution to this problem is to reduce the multivariate nature of water quality data by employing an index that will mathematically combine all water quality measures and provide a general and readily understood description of water. In this way, the index can be used to assess water quality relative to its desirable state (as defined by water quality objectives) and to provide insight into the degree to which water quality is affected by human activity. An index is a useful tool for describing the state of the water column, sediments and aquatic life and for ranking the suitability of water for use by humans, aquatic life, wildlife, etc.

An index can be used to reflect the overall and ongoing condition of the water. As with most monitoring programs, an index will not usually show the effect of spills, and other such random and transient events, unless these are relatively frequent or long lasting. Although there have been a variety of attempts to create such a water quality index, the most successful attempt to date appears to be the index developed by the British Columbia Ministry of Environment, Lands and Parks (Rocchini and Swain 1995). This index has been adopted for use by a number of provinces, including Manitoba (Manitoba Environment 1997).

The index is based on a combination of three factors:

1. the number of variables whose objectives are not met, (Scope)
2. the frequency with which the objectives are not met, (Frequency) and
3. the amount by which the objectives are not met, (Amplitude).

These are combined to produce a single value (between 0 and 100) that describes water quality.

Unlike some earlier indices, the basic BC formulation captures all key components of water quality, is easily calculated, and is sufficiently flexible that it can be applied in a variety of situations. The index can be very useful in tracking water quality changes at a given site over time and can also be used to compare directly among sites that employ the same variables and objectives. However, if the variables and objectives that feed into the index vary across sites, comparing among sites can be complicated. In these cases, it is best to compare sites only as to their ability to meet relevant objectives. For example, in calculating the index for a mountain stream and a prairie river, one might employ different nutrient objectives but the sites could still be compared as to their rank (e.g., both sites are ranked as “Good” under the index).