The Water Quality Index (WQI)1 is similar to the Trophic State Index (TSI) in that both are used for the statewide assessment of surface waters: the 305(b) Report. WQI is used for streams, black waters (natural tea and coffee-colored waters), and springs, while TSI is used for lakes and estuaries. The WQI is calculated by averaging the values of most or all of the parameters within five water quality parameter categories: 1) water clarity (measured as turbidity and-or Secchi disk depth), 2) dissolved oxygen, 3) oxygen demanding substances (measured as biochemical oxygen, chemical oxygen demand and-or total organic carbon), 4) nutrients (measured as total nitrogen, nitrite plus nitrate, and-or total phosphorus), and 5) bacteria (total coliform and-or fecal coliform).
Water Atlas presents WQIs over the last four seasons (three month intervals). The WQI "value" for a waterbody is determined by averaging the values (data) of the aforementioned parameters for each "season" (Jan-Mar, Apr-Jun, Jul-Sep, Oct-Dec). These seasonal averages are then averaged to provide an overall "rating" or WQI. The term "confidence" expresses the degree of completeness of the index; in other words, "confidence" states how many parameter categories were used to calculate the Overall Water Quality Index.
Ranges of WQI values have been established to provide a general ranking of the waterbody (Figure 1.) WQI values may also include the 'Confidence' (Figure 2) , which provides you with some relative idea as to how much information was used to calculate the WQI for that waterbody.
| 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 |
|
Also see, "Water Clarity" Learn More in 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 |
Water Quality Index is calculated according to FDEP. Integrated Water Quality Assessment for Florida: 2006 305(b) Report & 303(d) List Update., Appendix C. See also, Impaired Waters Rule.
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 (WQI) and the Trophic State Index (TSI) are included in the report.