Home Abstract Introduction Hydrogeology of S. Florida > Floridan Aquifer System   -  Upper Floridan Aquifer   -  Middle Confining Unit   -  Lower Floridan Aquifer - Hydrogeology at Alligator Alley Site Ground-Water Movement Subsurface Storage Summary and Conclusions References PDF Version

Hydrogeology of Southern Florida: Floridan Aquifer System

Evaporite deposits in the Cedar Keys Formation of Paleocene age probably constitute the lower confining unit, or base of the active flow system (fig. 3). Overlying the evaporites, in ascending order, are limestone and dolostones of the Cedar Keys, Oldsmar, and Avon Park Formations and the Ocala and Suwannee Limestones that make up the Floridan aquifer system, part of which was once called the Floridan aquifer (Parker and others, 1955) and all of which was once called the Tertiary limestone aquifer system (Johnston and others, 1980). In southwest Florida, the lower part of the Tampa Limestone is also included in the Floridan aquifer system.

The Floridan aquifer system is defined in chapter B of this Professional Paper (Miller, 1986) as a vertically continuous sequence of permeable carbonate rocks of Tertiary age that are hydraulically connected in varying degrees, and whose permeability is generally several orders of magnitude greater than that of the rocks that bound the system above and below. In Florida, the Floridan aquifer system includes rocks ranging from Paleocene to early Miocene age, and locally in southeast Georgia, it includes rocks of Late Cretaceous age. Chapter B presents a detailed geologic description of the Floridan aquifer system, its component aquifers and confining units, and their relation to stratigraphic units. Previous definitions of the term "Floridan" and superseded terms are also discussed in chapter B (Miller, 1986).

Overlying the Floridan are alternating beds of sand, clay, marl, and limestone in the Tampa Limestone and Hawthorn Formation (both of Miocene age) that contain intermediate artesian aquifers and make up the upper confining unit for the Floridan aquifer system. In southeastern Florida, clay in the Tamiami Formation of Pliocene age is included in the upper confining unit. Overlying these deposits are limestones and sands of the Tamiami Formation and of undifferentiated Pleistocene deposits that make up the surficial aquifer and contain unconfined ground water.

Figure 3. Generalized hydrogeologic section A-A' through southern Florida showing isotherms and top of saltwater in the Floridan aquifer system. (Line of section shown in fig. 2.) [larger version]

Ground water in the Floridan aquifer system in southern Florida is generally too saline for most uses. The Lower Floridan aquifer contains ground water that is similar in composition to seawater and is chiefly used as a receptacle for injected liquid wastes; the Upper Floridan aquifer contains brackish water and is chiefly used as a source of limited industrial and agricultural supply and for feedwater to desalting plants. Pilot studies indicate that the upper part of the Floridan aquifer system in southern Florida can be used for seasonal storage of surplus freshwater (Merritt and others, 1983). Limestone aquifers in Miocene deposits, as parts of the upper confining unit, are important local sources of ground water for supply in parts of southwestern Florida. However, the surficial aquifer generally is the major source of potable water in southern Florida. In southeastern Florida, the surficial aquifer is called the Biscayne aquifer (Parker and others, 1955; Schroeder and others, 1958), and in southwestern Florida, it is called the "shallow aquifer" (McCoy, 1962). The hydrogeology of southern Florida, as described here, is based largely on data collected from an exploratory test well (Alligator Alley test well; fig. 2, site 10) that was drilled near the center of the Everglades and from test wells that were drilled in Collier County by the South Florida Water Management District.

In southeastern Florida, the Floridan aquifer system includes (from shallowest to deepest) all or part of the Suwannee Limestone of Oligocene age, the Ocala Limestone of late Eocene age, the Avon Park Formation of middle Eocene age, the Oldsmar Formation of early Eocene age, and the upper part of the Cedar Keys Formation of Paleocene age (fig. 3). In southwestern Florida, it locally includes the lower part of the Tampa Limestone of early Miocene age.

Some investigators place the top of the Floridan aquifer system in the lower part of the Hawthorn Formation of middle Miocene age wherever it contains permeable limestone hydraulically connected to deeper layers (Parker and others, 1955; Stringfield, 1966). Using regional criteria based largely on lithologic changes in the rocks, Miller (1986) placed the top of the Floridan aquifer system at or near the top of the Suwannee Limestone in southwestern Florida and at or near the base of the Suwannee Limestone in southeastern Florida. The top of the Floridan aquifer system, as used in this report, ranges from about 500 to 1,000 ft in depth. The base of the Floridan aquifer system (the lowest confining unit) generally coincides with the top of evaporite beds in the Cedar Keys Formation (Miller, 1986), and it ranges from about 3,500 to 4,100 ft in depth.

The rocks that make up the Floridan aquifer system vary greatly in permeability so that the system resembles a "layer cake" composed of many alternating zones of low and high permeability. Crossflow (vertical flow) between permeable zones probably occurs through sinkholes and fractures. However, the amount of crossflow is probably small compared with the amount of horizontal flow. The zones of highest permeability generally are at or near unconformities and are generally parallel to bedding planes.

The temperature of ground water in the Floridan aquifer system in areas near the southeastern coast generally decreases with increasing depth; however, anomalies frequently occur, probably owing to local upwelling through fractures and sinkholes (a phenomenon that is discussed later in the report). Ground-water temperatures are generally coolest along the southeast coast, where the temperature of seawater in the adjacent Straits of Florida is the lowest. Ground-water salinity is generally highest in coastal parts of southern Florida and in the lower part of the aquifer system owing to inland circulation of seawater.

In southern Florida, the Floridan aquifer system can generally be divided-largely on the basis of the geology, hydrochemistry, and hydraulics interpreted from data obtained at the Alligator Alley test well (fig. 2, site 10) - into three hydrogeologic units, as follows:

1. The Upper Floridan aquifer, which contains brackish ground water. The specific conductance of the ground water ranges from about 2,500 to 25,000 microsiemens per centimeter (µS/cm) at 78 °F (25 °C) and averages about 5,000 µS/cm.
2. The middle confining unit, which contains salty ground water. The specific conductance of the ground water ranges from about 35,000 to 37,000 µS/cm and averages about 36,000 µS/cm.
3. The Lower Floridan aquifer, which contains ground water that is similar in composition to seawater. The specific conductance of the ground water ranges from about 43,000 to 50,000 µS/cm and averages 49,000 µS/cm.