WQI Background and Sample Calculation

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:

•  dissolved oxygen (DO)

•  fecal coliform

•  pH

•  biochemical oxygen demand (BOD) (5-day)

•  temperature change (from 1 mile upstream)

•  total phosphate

•  nitrate

•  turbidity

•  total solids

 

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.

 

After the Q-value is obtained, it is multiplied by a "weighting factor," based on that test’s importance in water quality. The nine resulting values are then added to arrive at an overall water quality index (WQI). An example of the calculation is given below:

 

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):

•  90-100: Excellent
•  70-90: Good
•  50-70: Medium
•  25-50: Bad
•  0-25: Very Bad