Learn More: Water Quality Index (WQI)

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.

Chapter 62-160 of the Florida Administrative Code defines the quality assurance rules under which monitoring must be performed. The Standard Operating Procedures that the Florida Department of Environmental Protection and other regional, state and local agencies use to collect and process water quality data are defined on the DEP website.

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.

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 (NTU*) and/or Secchi disk depth (meters or feet);
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).

*Nephelometric Turbidity Units (NTU)

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.

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• FAC 62-303.351 Nutrients in Freshwater Streams
• National Water-Quality Assessment Program (NAWQA)
• Nutrient Criteria Technical Guidance Manual: Rivers and Streams
• US EPA Ground Water Rule