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Calculating NSF Water Quality Index

Water quality index is a 100 point scale that summarizes results from a total of nine different measurements when complete:
  • dissolved oxygen
  • fecal coliform
  • pH
  • biochemical oxygen demand
  • temperature change
  • total phosphate
  • nitrates
  • turbidity
  • total solids
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.

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
Factor Weight
Dissolved oxygen 0.17
Fecal coliform 0.16
pH 0.11
Biochemical oxygen demand 0.11
Temperature change 0.10
Total phosphate 0.10
Nitrates 0.10
Turbidity 0.08
Total solids 0.07

Water Quality Index: Fecal Coli


Note: If the number of fecal coliform colonies is greater than 100,000,
the quality index equals 2. 

     Fecal coliform:  (colonies/100 ml)

                     

Water quality index:

Water Quality Index: Total Solids 


Note: If total solids is greater than 500 ppm,
the quality index equals 20. 

       Total solids:  (ppm)

                     

Water quality index:

Water Quality Index: DO sat (%)


Note: If dissolved oxygen is greater than 140%,
the quality index equals 50.


 Convert dissolved oxygen (%sat) to water quality index.

   Dissolved oxygen:  (%sat)

                     

Water quality index:

Water Quality Index: pH


Note: If pH is less than 2.0 or greater than 12.0,
the quality index equals 0.

Convert pH to water quality index.

                 pH:  (units)

                     

Water quality index:

Water Quality Index: Turbidity


Note: If turbidity is greater than 100 ntu,
the quality index equals 5.

Convert turbidity to water quality index.

          Turbidity:  (jtu)

                     

Water quality index:

Water Quality Index: BOD


Note: If biochemical oxygen demand is greater than 30 ppm,
the quality index equals 2.

Convert biochemical oxygen demand (ppm) to water quality index.

Biochemical oxygen demand:  (ppm)

                           

      Water quality index:

Water Quality Index: Nitrate


Note: If nitrate nitrogen is greater than 100 ppm,
the quality index equals 1.

Convert nitrates (ppm) to water quality index.

           Nitrates:  (ppm)

                     

Water quality index:

Water Quality Index: Total Phosphate


Note: If total phosphate is greater than 10 ppm,
the quality index equals 2.

Convert total phosphate (ppm) to water quality index.

    Total phosphate:  (ppm)

                     

Water quality index: 
   

Calculation of Overall Water Quality Index
 
Factor
 
Weight
 
Quality Index
Dissolved oxygen 0.17
Fecal coliform 0.16
pH 0.11
Biochemical oxygen demand 0.11
Temperature change 0.10
Total phosphate 0.10
Nitrates 0.10
Turbidity 0.08
Total solids 0.07

Based on the factors entered,
the water quality index is .

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
Range Quality
90-100 Excellent
70-90 Good
50-70 Medium
25-50 Bad
0-25 Very bad

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