WQM: Water Quality Index
Water quality index is a 100 point scale that summarizes results from a total of nine different measurements when complete:
According to the book Field Manual for Water Quality Monitoring, the National Sanitation Foundation surveyed 142 people representing a wide range of positions at the local, state, and national level about 35 water quality tests for possible inclusion in an index. Nine factors were chosen and some were judged more important than others, so a weighted mean is used to combine the values.
- dissolved oxygen
- fecal coliform
- biochemical oxygen demand
- temperature change
- total phosphate
- total solids
So that field measurements could be converted to index values, respondents were asked by questionnaire to graph the level of water quality (0 through 100) corresponding to the field measurements (e.g., pH 2-12). The curves were then averaged and are thought to represent the best professional judgement of the respondents.
When test results from fewer than all nine measurements are available, we preserve the relative weights for each factor and scale the total so that the range remains 0 to 100.
Water Quality Factors and Weights
|Biochemical oxygen demand
The following two graphs show the weighting curves for fecal coliform and total solids, which are not currently measured and therefore not shown elsewhere.
Note: If the number of fecal coliform colonies is greater than 100,000,
the quality index equals 2.
Note: If total solids is greater than 500 ppm,
the quality index equals 20.
The 100 point index can be divided into several ranges corresponding to the general descriptive terms shown in the table below.
Water Quality Index Legend