The River Water Quality Index is based on the average of four sub-indices calculated annually (by Alberta Environment) for four variable groups:
- Metals (up to 22 variables measured quarterly)
- Nutrients (6 variables measured monthly)
- Bacteria (2 variables measured monthly)
- Pesticides (17 variables measured 4 times during open-water season)
Variables in the first three groups are compared to Alberta and federal water quality guidelines. Variables in the fourth group (pesticides) are evaluated based on if they can be detected in a water sample. This conservative approach was adopted because some pesticides do not yet have official guidelines and, unlike metals, nutrients and bacteria, pesticides do not occur naturally in the environment.
Read more… about the variables and thresholds used in the River Water Quality Index.
Read more… about water quality guidelines used by Alberta.
Read more… about the historical values for the River Water Quality Index (South Saskatchewan River at Medicine Hat).
The percentage of water quality variables that exceed objectives over a prescribed time period (failed variables).The River Water Quality Index
rates water quality as “Excellent”, “Good”, “Fair,” “Marginal” or “Poor”. The River WQI index formula incorporates three factors representing key aspects of surface water quality:
- Frequency: The percentage of water quality tests that do not meet the objectives (failed tests).
- Amplitude: The amount by which the failed tests exceed the water quality objectives.
The result is an index number between 0 and 100
that represents the overall water quality for a location at a specific time, with 0 the worst water quality relative to the objectives and 100 the best water quality. In general, the values are further divided into five descriptive categories: