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Water Quality Index Torrey Lindbo,Jan Miller, SWRPClean Water Services.

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Presentation on theme: "Water Quality Index Torrey Lindbo,Jan Miller, SWRPClean Water Services."— Presentation transcript:

1 Water Quality Index Torrey Lindbo,Jan Miller, SWRPClean 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 ParameterUnitsFor a Q value of 90 [Oregon standards] DO % sat.% sat.>82% [>90% unless spawning, then >95%] pHpH units6.9 or 7.9 [6.5 to 8.5] E. coliCFU/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] TurbidityNTU4.0 NTUs T Phosmg/L P0.05 mg/L P [0.1 mg/L P, Tualatin TMDL] NO 3 mg/L N0.5 mg/L N BOD 5 mg/L0.8 mg/L

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 = 0Q = 100 Q = 25Q = 75 Q = 50 Very Bad Very Good Medium

6 pH Q-Graph and Q-Table pH Q-Value

7 E.coli Q-Graph & Q-Table E. coli Q-Value

8 Weighting Factors DO0.18 pH0.12 E. coli0.17 Temp0.11 Turbidity0.09 T Phos0.11 NO BOD0.12 Total1.00

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

10 Example WQI Calculation ParameterUnitsResultQ-ValueWeight Factor Parameter Index DO % Sat% pHpH units E. coliCFU/100 mL Temp deg C TurbidityNTU T Phosmg/L P NO3mg/L N BODmg/L Total WQI =69.2 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 ParameterUnitsResultQ-ValueWeight Factor Parameter Index DO % Sat% pHpH units E. coliCFU/100 mL Temp deg C TurbidityNTU T Phosmg/L PNM NO3mg/L N BODmg/L Total WQI =71.6 Quality Rating =GOOD


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