Water Quality Index

1
Water Quality Index

• pp.9-17

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
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

6
pH Q-Graph and Q-Table

• 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

7
E.coli Q-Graph Q-Table

• 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

8
Weighting Factors (Fig. 2.4, p.10)
9
WQI Interpretation
10
Example WQI Calculation
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
13
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.

14
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.