FLORIDA GEOLOGICAL SURVEY


 The Florida Platform is composed of a thick sequence of variably permeable carbonate
sediments, limestone and dolostone, lying on older igneous, metamorphic and sedimentary
rocks. The carbonate sediments may exceed 4,000 ft (1,220 m) in thickness. A sequence of
sand, silt and clay with variable amounts of limestone and shell overlie the carbonate
sequence (see Scott [1992 a, b] for discussion of the Cenozoic sediment sequence). In por-
tions of the west-central and north-central peninsula and in the central panhandle, the car-
bonate rocks, predominantly limestone, occur at or very near the surface. Away from these
areas, the overlying sand, silt and clay sequence becomes thicker. The complex Floridan
aquifer system (FAS) occurs within this thick sequence of permeable carbonate sediments
(see Miller, 1986; Berndt et al., 1998 for discussion of the FAS).

 Natural recharge to the FAS by rain water, made slightly acidic by carbon dioxide from
the atmosphere and organic acids in the soil, dissolved portions of the limestone. The dis-
solution enhanced the permeability of the sediments and formed cavities and caverns.
Sinkholes formed by the collapse of overlying sediments into the cavities. Occasionally, the
collapse of the roof of a cave creates an opening to the land surface.

 Karst springs occur both onshore and offshore in Florida. Little is currently known
about the offshore springs with the exception of the Spring Creek Group of springs the
largest spring in Florida (more than one billion gallons of water discharged per day) (Lane,
2001). In order to better understand the water resources of the state, the FGS has initiat-
ed a program to investigate the occurrence, discharge and water quality of the offshore
springs.

 Florida's first magnitude springs occur in the northern two-thirds of the peninsula and
the central panhandle where carbonate rocks are at or near the land surface (Figure 5). All
of these springs produce water from the upper FAS (Berndt et al., 1998) which consists of
sediments that range in age from Late Eocene (approximately 38 36 million years old [my])
to mid-Oligocene (approximately 33 my). Miocene to Pleistocene sediments (24 my to 10,000
years) may be exposed in the springs.

 The geomorphology physiographyy) of the state, coupled with the geologic framework,
controls the distribution of springs. The springs occur in areas where karst features (for
example, sinkholes and caves) are common and the surface elevations are low enough to
allow groundwater to flow at the surface. These areas are designated karst plains, karst
hills and karst hills and valleys on Figure 6. The state's springs occur primarily within the
Ocala Karst District and the Dougherty Karst Plain District (Scott, in preparation). Three
springs, Alexander, Silver Glen and Volusia Blue occur in the Central Lakes District (Scott,
in preparation).

 Recharge to the FAS occurs over approximately 55% of the state (Berndt et al., 1998).
Recharge rates vary from less than one inch (2.54 cm) per year to more than ten inches (25.4
cm) per year. Recharge water entering the upper FAS that eventually discharges from a
spring has a variable residence time. Katz et al. (2001) found that water flowing from larg-
er springs had a groundwater residence time of more than 20 years.

 Discharge, water quality and temperature of the first order magnitude springs remain
reasonably stable over extended periods of time (Berndt et al., 1998). However, because dis-