The Water Quality Index (WQI)1 is similar to the Trophic State Index (TSI) in that both were developed for the statewide assessment of surface waters. While both the are useful as thumbnail indicators of water quality, they are non-regulatory measurements; they may be used by the Florida Department of Environmental Protection to identify water bodies that may need further study, but not to determine attainment of water quality standards under the Impaired Waters Rule of the Clean Water Act.
WQI is used for streams, black waters (natural tea- and coffee-colored waters), and springs, while TSI is used for lakes and estuaries.
The Water Atlas presents WQIs for three-month intervals (Jan–Mar, Apr–Jun, Jul–Sep, Oct–Dec). The quarterly WQI for a waterbody is determined by averaging the index values for selected water quality parameters for the interval. Ranges of WQI values have been established to provide a general ranking of the waterbody (Figure 1.) The index is accompanied by a ‘confidence value’ that expresses the degree of completeness of the index (Figure 2). For more information, see the Calculations section, below.WQI | Rating |
---|---|
0-45 | Good |
45-60 | Fair |
>60 | Poor |
WQI | Rating | Confidence | Season |
---|---|---|---|
30 | Good | 5/5 | Winter (2000) |
40 | Good | 3/5 | Fall (2000) |
30 | Good | 2/5 | Summer (2000) |
50 | Fair | 3/5 | Summer (2000) |
category/parameter | methods and more information |
---|---|
Water Clarity |
|
Secchi Depth |
Measuring Water Clarity (Florida LAKEWATCH) Also see the Water Clarity Learn More on the Water Atlas |
Turbidity |
Standard Methods 2130 A & B; ISO Method 7027 |
Dissolved Oxygen |
|
Membrane Electrode |
Standard Methods 4500-O G. and modifications |
Winkler titration |
Standard Methods 4500-O, B. and modifications |
Oxygen Demand |
|
Biochemical oxygen demand |
Standard Methods 5210-B & C and modifications |
Chemical oxygen demand |
Standard Methods 5220 and modifications |
Total organic carbon |
Standard Methods 5310 and modifications |
Nutrients |
|
Total nitrogen |
Standard Methods 4500-N, B & C. and modifications |
Nitrate + Nitrate |
Standard Methods 4500-NO3-E. and modifications |
Total Phosphorus |
Standard Methods 4500-P, E. and modifications |
Also see, "Nutrient Chemistry" Learn More in the Water Atlas |
|
Bacteria |
|
Fecal coliform |
9222-D and modifications |
Total coliform |
9222-B and modifications |
The formula used to calculate WQI for display on the Water Atlas is modified slightly from DEP’s formula, in that it does not incorporate biological diversity data (i.e., macroinvertebrate indices). A description of the formula used by the Florida Department of Environmental Protection can be found in Chapter 2 of the Integrated Water Quality Assessment for Florida: 1996 305(b) Report & 303(d) List Update. (See also, Impaired Waters Rule.)
The WQI is calculated by averaging the individual index values of some or all of the parameters within five water quality parameter categories:
*Jackson turbidity units
A ‘confidence value’ of 1–5 indicates how many of the above categories were incorporated into the index; for some waterbodies, unavailability of data may preclude including all categories. See Caveats & Limitations, below.
Index values for the water quality parameters are interpolated from Table 2-5 of the 1996 Water-Quality Assessment for the State of Florida, Section 305(b), Main Report.
When considering a WQI value, one should also consider the accompanying ‘confidence’ value, as well as the period of record (number of years and samples) used to develop that WQI.
The Clean Water Act requires each state to conduct water quality surveys to determine if its waterways are healthy and of sufficient quality to meet their designated uses. As part of this responsibility, every state is also required to submit a 305(b) Report every two years. The Water Quality Index was once, but is no longer, used as an assessment measure. However, it is still often used as an informal measure of water quality.