Pinellas County Water Atlas

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Water Quality Index (WQI)

What does this mean?

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
Figure 1. Water Quality Index (WQI) ranges and their designations.

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)
Figure 2. WQI rankings are provided with examples of Confidence values.
1The acronym WQI also stands for "Water Quality Inspection" in much of the DEP literature.

How are the data collected? (Methods)

category/parameter methods and more information

Water Clarity

Secchi Depth

Measuring Water Clarity

Also see, "Water Clarity" Learn More in the Water Atlas


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


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


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.

Caveats and Limitations

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.

Additional Information

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