Water Quality Index
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

Parameters used for WQI
Parameter Units
For a Q value of 90
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
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

Q Value

The Q-Value is an
indication of how
good (or bad) the
water quality is
relative to one Q = 25
parameter.
 100 = Very Good
Very
 1 = Very Bad Q = 0 Bad
Q = 50
Medium
Q = 75
Very
Good Q = 100
pH Q-Graph and Q-Table
Q-Value
81
88
94
98
100
98
94
88
80
72
pH Q-Values
100
90
80
70
Q-Value
pH
 6.6
 6.8
 7.0
 7.2
 7.4
 7.6
 7.8
 8.0
 8.2
 8.4
60
50
40
30
20
10
0
2
3
4
5
6
7
pH
8
9
10
11
12
E.coli Q-Graph & Q-Table
Q-Value
E. coli Q-Value
 0
100
 1
99
 2
94
 5
89
 10
84
 20
80
 50
74
 100
69
 200
65
 500
27
 1000
30
E. coli Q-Values
100
90
80
70
60
50
40
30
20
10
0
1
10
100
E. coli (CFU/100 mL)
1000
10000
Weighting Factors
Turbidity
DO
NO3
E. coli
T Phos
Temp
pH
BOD
DO
pH
E. coli
Temp
Turbidity
T Phos
NO3
BOD
Total
0.18
0.12
0.17
0.11
0.09
0.11
0.10
0.12
1.00
WQI Interpretation
Water Quality
Index Range
90-100
Water Quality
Rating
Excellent
70-89
Good
50-69
Medium
25-49
Bad
0-24
Very Bad
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
0
100
0.11
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
0.1
9.12
BOD
mg/L
6
36
0.11
4.34
1.00
69.2
Total
WQI =
Quality Rating =
69.2
MEDIUM
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.

Calculation with Missing Parameter
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
0
100
0.11
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
0.11
4.34
0.89
63.7
Total
WQI =
Quality Rating =
71.6
GOOD