The Centre St Laurent Index
Prior to the development of the CCME Water Quality Index, there were a number of jurisdictions and institutions in Canada using some type of metric to assess water quality. The Water Quality Index Technical Subcommittee was formed by the Water Quality Guidelines Committee of the Canadian Council of Ministers of the Environment in 1997 to assess different approaches to index formulation and to develop an index that could be used to simplify water quality reporting in Canada.
The Centre St Laurent Index (CSL) is an Environment Canada office responsible for reporting on the St. Lawrence River. The CSL calculated a number of different water quality indices, depending on what water uses they were considering. For example, separate index calculations were made based on guidelines for protection of aquatic life, primary contact recreation, human consumption, and for saltwater areas of the St. Lawrence estuary, for shellfish harvesting.
Uses and Limitation:
The CSL develop this index for the monitoring of St. Lawrence River water quality.
The Centre St Laurent Index is more sophisticated than the Alberta Index but is not able to account for intermediate situations e for example, a water-quality parameter being within acceptable level but very close to the limit compared to another situation wherein the parameter may be well below the limit. In the former case, the probability (or the risk) that a parameter may ‘cross over’ to an unacceptable level is much higher than the latter case, but the index is not sensitive to this intermediate situation.
Standards Required for The Centre St Laurent Index
Any national or international water quality standards could be used in this method.
Calculation of The Centre St Laurent Index:
The WQI developed at CSL was:
\begin{eqnarray*}
WQI = \frac {\sum {(A_i} * {F_i})} {n}\
\end{eqnarray*}
Where:
Ai is the mean level of exceedence for variable i for guideline i. When a variable value exceeds a guideline for that variable, the ratio “exceeding value/guideline value” is calculated. These ratios are summed and then divided by the number of times they occur.
Fi is the frequency of values that exceed a guideline for a variable relative to the total
number of values obtained for that variable (Fi = Fexceed/Ftotal for variable i).
n is the number of variables.v