Water Quality Index pp.9-17

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 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 °C+2.5°C or -4°C  [<17.8 deg. C] TurbidityNTU4.0 NTU’s 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

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

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

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

Weighting Factors (Fig. 2.4, p.10)

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

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

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 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

Note on Sampling It is important to exercise care in the way samples are collected for analysis. A collected sample should be representative of the river or lake being tested. Near-shore samples may not be representative of the river at that location. If possible, water samples should be collected from a bridge spanning the river, from a boat, or off the end of a dock. A rule of thumb for sampling is to sample midway across the river and below the surface.

Testing A simple device can be constructed from a series of metal rods that can be extended and rubber tubing attached that holds the sample bottle. This device might be extended out from shore if no bridges are available and particularly if the river is narrow or shallow. A golf ball retriever can also be adapted very easily for this purpose.