1. Torrey Lindbo, Jan Miller, SWRP Clean Water Services
Water Quality Index
Torrey Lindbo, Jan Miller,
SWRP Clean Water Services

2. Role of the Water Quality Index
WQI numerically summarizes the information from multiple water quality parameters into a single value
The single value can be used to compare data from several sites
It can be used to look at trends over time on a single site

3. Parameters used for WQI
Units
For a Q value of 90 [Oregon standards]
DO % sat.
% sat.
>82% [>90% unless spawning, then >95%] pH
pH units
6.9 or 7.9 [6.5 to 8.5]
E. coli
CFU/100 mL
<4 CFU/100 mL [406 CFU/100 mL for grab sample]
 Temp
 °C
+2.5°C or -4°C  [<17.8 deg. C]
Turbidity
NTU
4.0 NTU’s
T Phos
mg/L P
0.05 mg/L P [0.1 mg/L P, Tualatin TMDL]
NO3
mg/L N
0.5 mg/L N
BOD5
mg/L
0.8 mg/L
Student Watershed Research Project (SWRP)

4. Parts of the Water Quality Index
Q-Value - indication of water quality relative to 100 of one parameter
Weighting Factor - sets the relative importance of the parameter to overall water quality

5. Q Value
The Q-Value is an indication of how good (or bad) the water quality is relative to one parameter.
100 = Very Good
1 = Very Bad
Q = 0
Q = 100
Q = 25
Q = 75
Q = 50
Very Bad
Very Good
Medium

6. pH Q-Graph and Q-Table pH Q-Value 6.6 81 6.8 88 7.0 94 7.2 98 7.4 100

7. E.coli Q-Graph & Q-Table
E. coli Q-Value
0 100
1 99
2 94
5 89
10 84
20 80
50 74
100 69
200 65
500 27

8. Weighting Factors DO 0.18 pH 0.12 E. coli 0.17 Temp 0.11 Turbidity
0.09
T Phos
NO3
0.10
BOD
Total
1.00

9. Water Quality Index Range
WQI Interpretation
Water Quality Index Range
Water Quality Rating
90-100
Excellent
70-89
Good
50-69
Medium
25-49
Bad
0-24
Very Bad

10. Example WQI Calculation
Parameter
Units
Result
Q-Value
Weight Factor
Parameter Index
DO % Sat % 95 98
0.17
17.67 pH
pH units 8 88
0.11
10.55
E. coli
CFU/100 mL
1750 23
0.16
3.94
Temp
 deg C
100
11.00
Turbidity
NTU 10 79
0.09
7.07
T Phos
mg/L P
0.2 50
0.1
5.50
NO3
mg/L N
0.4 91
9.12
BOD
mg/L 6 36
4.34
Total
1.00
69.2
WQI =
Quality Rating =
MEDIUM

11. Adjusting for Missing Data
When a parameter is missing, the sum of the weighting factors that have data can be used to adjust the WQI
Sum of the Parametric Index Values is divided by the sum of the weighting factors that have data.

12. Calculation with Missing Parameter
Units
Result
Q-Value
Weight Factor
Parameter Index
DO % Sat % 95 98
0.17
17.67 pH
pH units 8 88
0.11
10.55
E. coli
CFU/100 mL
1750 23
0.16
3.94
Temp
 deg C
100
11.00
Turbidity
NTU 10 79
0.09
7.07
T Phos
mg/L P NM
NO3
mg/L N
0.4 91
0.1
9.12
BOD
mg/L 6 36
4.34
Total
0.89
63.7
WQI =
71.6
Quality Rating =
GOOD