Water Quality Index (WQI)

The ‘grades’ for the health of Volusia’s surface waters are determined using a variation of the Water Quality Index (WQI) developed by the State of Florida (Hand, 2000). The WQI is a mathematical method consisting of condensing several parameters into a single numeric value to summarize the overall status of the water body.

Three categories of water quality parameters are used in the local implementation of the WQI:

• water clarity (turbidity and total suspended solids)
• nutrients (total nitrogen and total phosphorus)
• bacteria (fecal coliform)

The individual parameters are averaged to obtain an overall index value for each category.  The scores are obtained by transforming the data annual averages with the state’s formulas log function (i.e. LN function).  The index values for each of the categories is then combined in a weighted formula to obtain the final WQI rating (good, fair, or poor) for each water monitoring location in the water body.

Due to the unique natural chemical and biological conditions that make up each type of water body, the cut-off values have been adjusted to reflect these specific differences.  The original WQI was formulated for estuarine systems.  The adjustment factors were based on the water quality percentiles from the State data set.   Over the years, the Florida Department of Environmental Protection (FDEP) has modified the WQI to accommodate backwater, streams, and springs.  The WQI for streams was used in the assessment of the St. John’s River Project. 

The State of Florida’s implementation of the WQI is applied to the entire water body, but the local adaptation of this WQI provides data for individual stations within each water body.   In Volusia County, approximately one hundred water-sampling stations are currently being sampled.

Hand et al. (1996) summarizes the Florida Water Quality Index (WQI) has the following advantages compared to other indices:

• It is based on the percentile distribution of Florida stream data;
• It is tailored to Florida
• It uses the most important measurements of water quality in Florida (clarity, dissolved oxygen, and oxygen-demanding substances).
• It identifies areas of good, fair and poor water quality that correspond to professional and public opinion.
• It eliminates subjective assessments and individual biases in assessing water quality.

To be consistent with the State’s approach for the waters statewide, we take all the data and put it together; then;

• Sort it by the column with the WQI data on ascending (or descending order if preferred).
• We then determine the total number of records.
• For example, you have a total of 416 records for estuaries to date, divided by 10 equals 41.6, which is 10 percent of the observations.
• Ten percent of the observations (41.6) times two (2) equals 83.2.  Therefore, the cutoff value for the lower 20% of the observations is the first 83 observations approximately, with is the “excellent” range.
• Likewise, the following 21 percent to 50 percent range would be indicated as good, 51 percent to 79 percent as fair, and the lower 20 percent as poor.

In other words, rather than assigning a predetermined range, we base that data on each individual water body, by using as break-points the values for the best 20 percent as “excellent”, 21 to 50 percent as “good”, 51 to 79 percent as “fair”, and the lower 20 percent as “poor”.  This approach is very similar to the approach taken by Florida Department of Environmental Protection for the entire state, which is consistent with statistical practice.

However, the first 83 “excellent” observations might overstate the quality; therefore, we grouped the first 83 “excellent” observations under the following category, which entails the following 21 to 50 percent of the observations or “good” category.  We were able to justify where the cutoff values were placed for the last 15 years worth data in order to explain the particular problems facing the surface waters in the County of Volusia.

The WQI provides a means to establish a water quality data trend for the past 15 years. It provides a generalized way to interpret water quality data that is quick and useful.  It has been accomplished by evaluating the results of each monitoring station in order to establish the water quality trend for every surface water sampling year.

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