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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 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
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 (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


Calculations

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:

  1. Water clarity: turbidity (JTU*) and/or Secchi disk depth;
  2. Dissolved oxygen: Dissolved oxygen concentration (mg/l);
  3. Oxygen demand: biochemical oxygen demand (mg/l), chemical oxygen demand (mg/l) and/or total organic carbon (mg/l);
  4. Nutrients: total nitrogen (mg/l), and/or total phosphorus (mg/l);
  5. Bacteria: total coliform (# per mg/l) and/or fecal coliform (# per mg/l).

*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.


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 was once, but is no longer, used as an assessment measure. However, it is still often used as an informal measure of water quality.


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