The Water Quality Index (WQI), like the UV index or the Air Quality Index, is a means of summarizing large amounts of water quality data. Calculating an index serves above all to simplify information in order to make it accessible to decision makers and the public. It also allows water quality data to be compiled and communicated at a national level in a consistent manner. This index takes into consideration many water quality variables and compares them to water quality guidelines or objectives, in order to subsequently rank them (excellent, good, average or poor). The index can be calculated on the basis of criteria adapted to different water uses (drinking water, recreation or, in the present case, aquatic life). Calculation of the WQI is based on three attributes of water quality:
the number of water quality variables that do not meet objectives;
the number of individual measurements that do not meet objectives; and
the amount by which the measurements depart from these objectives.
While it does not replace a detailed data analysis, the purpose of the WQI is to provide a broad overview of environmental performance.
The use of the WQI requires certain precautions. Thus, index comparisons should only be made when the same sets of guidelines are being applied and when the same sets of variables are used. The variables being considered should also be relevant to the water body being tested so the calculated index reflects the condition of this water body.
Water Quality in the Great Lakes - St. Lawrence Basin: Contamination by Toxic Substances
Urban development, industrialization and farming have unleashed a massive load of toxic substances into our waters over the last century. These toxic inputs have contributed to degrading the water quality in the Great Lakes - St. Lawrence basin, placing this unique ecosystem at risk. In the wake of recent efforts to reduce these sources of pollution, ecosystem monitoring programs have been set up by the Government of Canada and the governments of Quebec and Ontario. The purpose of these programs is to monitor certain indicators and thereby track changes in the state of health of the ecosystem. Characterizing the diverse sources of pollutants, estimating their concentrations and examining how they change over time are key components of these monitoring programs.
Contaminant concentrations in the water have been tracked at three reference stations in the Lake Ontario - St. Lawrence basin in order to assess the levels of contamination. These stations are located at the inlet and outlet of Lake Ontario (Niagara River and Wolfe Island, respectively) and at the mouth of the St. Lawrence River (Quebec City region).
Figure 1. Three reference stations with associated WQIs to track contaminant concentrations are located at Niagara-on-the-Lake, Wolfe Island at the outlet of Lake Ontario, and Quebec City at the mouth of the St. Lawrence River.
Figure 4. Quebec City sampling station at the Levis Water Treatment Plant intake.
Photo credit: Paul Klawunn
The Niagara River has a significant influence on Lake Ontario, contributing more than 83% of the total tributary inflow to the lake. The sampling conducted around Wolfe Island serves in assessing the quality of the water entering the St. Lawrence River from the Great Lakes, the river’s main source. At the Quebec City sampling station, the water quality at the mouth of the river is examined and the quantity of contaminants exported to the estuary is estimated.
For these purposes, measurements of organic and inorganic contaminants are taken at all three stations on a routine basis. Modern sampling and analytical techniques are employed to measure contaminants at trace and ultra-trace concentrations with reliability and precision. Some examples of this equipment are shown in figures 5 and 6.
Figure 5. Filter containing suspended sediment from the Quebec City sampling station.
Photo credit: Paul Klawunn
Figure 6. Goulden Large Volume Extractor at Bamford Point-Woodman sampling station.
