Water Quality Index
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Water Quality Index Torrey Lindbo, Jan Miller, SWRP Clean Water Services
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Water Quality Index. Torrey Lindbo,Jan Miller, SWRPClean 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 - PowerPoint PPT Presentation
Transcript of Water Quality Index
Water Quality Index
Torrey Lindbo, Jan Miller,
SWRP 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 parameter.
100 = Very Good 1 = Very Bad Q = 0 Q = 100
Q = 25 Q = 75
Q = 50
Very Bad
Very Good
Medium
pH Q-Graph and Q-Table
pH Q-Values
0
10
20
30
40
50
60
70
80
90
100
2 3 4 5 6 7 8 9 10 11 12
pH
Q-V
alu
e
pH Q-Value 6.6 81 6.8 88 7.0 94 7.2 98 7.4 100 7.6 98 7.8 94 8.0 88 8.2 80 8.4 72
E.coli Q-Graph & Q-TableE. coli Q-Values
0
10
20
30
40
50
60
70
80
90
100
1 10 100 1000 10000
E. coli (CFU/100 mL)
Q-V
alu
e
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
Weighting Factors
DO 0.18
pH 0.12
E. coli 0.17
Temp 0.11
Turbidity 0.09
T Phos 0.11
NO3 0.10
BOD 0.12
Total 1.00
DO
E. coli
pHBOD
Temp
T Phos
NO3
Turbidity
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
Example WQI CalculationParameter Units Result Q-Value Weight
FactorParameter
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
Total 1.00 69.2
WQI = 69.2
Quality Rating = 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 ParameterParameter Units Result Q-Value Weight
FactorParameter
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
Total 0.89 63.7
WQI = 71.6
Quality Rating = GOOD
