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

Page 1: Water Quality Index

Water Quality Index

Torrey Lindbo, Jan Miller,

SWRP Clean Water Services

Page 2: 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

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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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