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A commonly-used water quality index (WQI) was developed by the National Sanitation Foundation (NSF) in 1970 (Brown and others, 1970). The NSF WQI was developed to provide a standardized method for comparing the water quality of various bodies of water.
How was the NSF index developed?
142 water quality scientists were surveyed about 35 water quality tests and asked to consider which tests should be included in an index (Brown and others, 1970; Mitchell and Stapp, 2000). Nine water quality parameters were selected to include in the index. These parameters are:
The scientists were then asked to graph the level of water quality ranging from 0 (worst) to 100 (best) from the raw data (e.g, pH values 2-12). The curves drawn were then averaged to obtain a weighting curve for each parameter. Results of the nine parameters are compared to the curves and a numerical value, or "Q-value," is obtained. For example, see the curve for fecal coliform shown on the right (from http://kancrn.org/stream/img/FCchart1.jpg ).
|
test |
result |
unit |
Q-value |
weight factor |
subtotal |
|
DO |
82 |
% sat |
90 |
0.17 |
15.3 |
|
Fecal Coliform |
12 |
#/100 mL |
72 |
0.16 |
11.52 |
|
pH |
7.67 |
std units |
92 |
0.11 |
10.12 |
|
BOD |
2 |
mg/l |
80 |
0.11 |
8.8 |
|
change T |
5 |
degrees C |
72 |
0.10 |
7.2 |
|
Total phosphate |
0.5 |
mg/L PO4-P |
60 |
0.10 |
6 |
|
nitrates |
5 |
mg/L NO3 |
67 |
0.10 |
6.7 |
|
turbidity |
5 |
NTU |
85 |
0.08 |
6.8 |
|
TS |
150 |
mg/L |
78 |
0.07 |
5.46 |
|
WQI |
77.9 |
If less than 9 tests are performed, the overall WQI can be estimated by adding the results and then adjusting for the number of tests. For example, if BOD and temperature change are not available, the seven remaining subtotals are added (from the example above, 61.9), and the seven weighting factors are added (0.79). The former is then divided by the latter, to obtain a WQI of 78.4.
The WQI ranges have been defined as (Brown and others, 1970):
There is more information about the NSF’s WQI and see the charts for converting to Q-values at at the Kansas Collaborative Research Network (KanCRN) website (http://kancrn.org/stream/cp4wqi.cfm).
