Explanation of Indicator
Onsite sewage treatment and disposal systems are facilities constructed on individual sites used to provide wastewater disposal where municipal sewerage is not available. Such systems usually consist of a septic tank and a subsurface infiltration system. Within the septic tank, sedimentation and some anaerobic digestion of solids occur. The remaining partially treated wastewater, referred to as septic tank effluent, is then discharged to the subsurface infiltration system and percolates through the sur rounding soil. As the effluent moves through the soil, it is further treated by naturally occurring chemical, biological, and physical processes.¹

In many cases, when septic tanks are properly sited, designed, constructed, and operated, onsite systems are effective wastewater treatment systems. As the number and density of septic systems increase throughout Florida, however, there are serious conce rns that the use of onsite systems is having adverse impacts on ground and surface waters in the state. In fact, there is the potential for the contamination of ground and surface waters when onsite systems do not remove enough of the nutrients, suspended solids, toxics, and pathogens that exist in the wastewater.² This possibility is of particular concern in coastal counties, due to the fact that some areas surrounding the coast have a significant portion of the population utilizing septic tanks in soils with limited capacity for this use.³

Data Characteristics
SOURCE
Information on the number of onsite sewage treatment and disposal systems installed can be obtained from Kevin Sherman, Florida Department of Health and Rehabilitative Services (HRS), Onsite Sewage Program, 1317 Winewood Blvd., Tallahassee, FL 32399, or at (904) 488-4070.

ACQUISITION
The septic tank installation data are available in hard copy format at no cost.

COLLECTION
Information on the number of onsite sewage treatment and disposal systems installed is available on an annual basis by county since 1971.

Data Limitations
A limitation inherent in the data is the lack of consideration of soil suitability and septic tank density. This makes it difficult to definitively determine the impacts of increasing or decreasing septic tank use. For example, relatively high numbers o f systems in suitable soil may equal a low impact, while relatively low numbers in unsuitable areas may equal a high impact. Furthermore, the number of septic tanks installed does not give an accurate picture of the total number of septic tanks in use ov er the years because it does not account for septic tank abandonments. The number of septic tank abandonments in coastal counties is believed to be small; however, in future monitoring of onsite sewage system activity, septic tank abandonments may become a significant issue.

Data Analysis
The number of onsite sewage treatment and disposal systems installed in coastal counties decreased by 5,345 systems from 1980 to 1994; this was a decrease of 17.6 percent. There were, however, some fluctuations over the years, most likely due to economic factors which influence construction starts (i.e., the apparent decline in the early 1980s coincided with the economic depression).