FLORIDA GEOLOGICAL SURVEY


Floridan aquifer system

 The FAS occurs throughout Florida and is
often an artesian aquifer. Artesian conditions
may vary with seasonal rainfall and pumping
conditions. In the study area, springs flow year-
round along parts of coastal Citrus, Pasco and
Hemando Counties (Figure 6; Healy, 1974;
Scott et al., 2004). Other common areas of FAS
discharge include wetlands in the Tsala Apopka
Plain, southern Pasco County, and the Green
Swamp (northwest Polk County). The FAS
discharges into the SAS along the northern part
of the Lake Wales Ridge and also provides
baseflow to surface-water bodies (e.g.,
Hillsborough River). Along the trace of the
Peace River, the FAS discharges into permeable
zones of the IAS/ICU (Sepulveda, 2002).
Similar conditions exist within southern
Sarasota County, southwestern DeSoto County
and most of Charlotte County. Throughout the
remainder of the study area, the FAS is
recharged from overlying aquifer systems.

 The top of the FAS does not always coincide
with a specific lithostratigraphic unit. Instead, it
is defined by permeability and hydraulic
connection (Johnston and Bush, 1988).
Moreover, per the Southeastern Geological
Society (1986) definition, the top of the FAS
"generally coincides with the absence of
significant thicknesses of (silici)clastics from the
section and with the top of the vertically
persistent permeable carbonate section." In the
study area, the surface of this hydrostratigraphic
unit may include the following formations in
ascending order: Avon Park Formation, Ocala
Limestone, Suwannee Limestone, and units
within the Hawthorn Group (Trommer, 1993;
Ryder, 1985; Berndt et al., 1998; Corral and
Wolansky, 1984).

 Figure 8 summarizes the extent of
lithostratigraphic units that comprise the FAS
surface. The top of the FAS correlates
regionally with the Oligocene Suwannee
Limestone where present; however, in west-
central Florida, the Tampa Member (Arcadia
Formation) of the Hawthorn Group coincides
with the top of the FAS where it is hydraulically
connected to the underlying Suwannee


Limestone (generally Pinellas [e.g., Hickey,
1982; Broska and Barnette, 1999], southern
Pasco, Hillsborough and Manatee Counties
[Green et al., 1995]). In the absence of the
Tampa Member or Suwannee Limestone,
Eocene rocks (Avon Park Formation or Ocala
Limestone) comprise the top of the FAS (Figure
8; Miller, 1986). Based on lithostratigraphic
correlation and the FAS definition applied
herein (see also Appendix 2), the top of the unit
ranges in age from Middle Eocene to Early
Miocene.

 In the northern region, the FAS (Plates 4
through 8) includes the Avon Park Formation,
Ocala Limestone and Suwannee Limestone. In
Pasco County (Plate 11), hydrogeologic data
suggest that the Tampa Member is also
hydraulically connected to the FAS. For
example, water levels measured at well W-
16609 (TR 18-2A; Plates 11 and 29) increased
only -1.2 inches (~ 3 cm) in elevation while
coring through the Tampa Member into the
Suwannee Limestone (DeWitt, 1990). Overall,
the FAS is unconfined to semi-confined in the
region. Along parts of the northern coastal zone,
laterally extensive confining units are thin to
absent (Plate 56) and hydraulic head differences
allow local recharge to the FAS, which in turn
enhances development of secondary porosity
along fractures and bedding planes in the FAS.
These dissolution-widened channels have a
much higher hydraulic conductivity. For
example, dissolution channels in the Ocala
Limestone are highly developed in Hernando,
Citrus and Marion Counties (Trommer, 1993).
Abundant sinkholes in the region, indicated by
the region's pattern of closed topographic
depressions (Plate 3), locally breach confining
beds.

 As indicated by the hydrologic data
presented in the Surficial aquifer system section,
p. 53, local unconfined to semi-confined FAS
conditions exist in the northern region. For
example, water levels in a FAS monitor well and
a surficiall" monitor well at site W-16311
(ROMP LP-4, Plate 7) exhibit nearly identical
elevations, suggesting that confinement is leaky
or absent between surficial sediments and the
carbonates of the FAS. Along the Brooksville
Ridge and parts of the northeastern margin of