Transcript Water Quality Index - Portland State University

Water Quality Index
Torrey Lindbo,
SWRP
Jan Miller,
Clean Water
Services
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 [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
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