BULLETIN NO. 68


 A meaningful comparison of the IAS/ICU
potentiometric surface (Duerr, 2001) with that of
the FAS is qualitative at best (see discussion
starting on page 62). A more accurate, although
site-specific method for assessing the
recharge/discharge relation between the
IAS/ICU and FAS is to compare water levels in
nested wells, many of which are located at
District ROMP sites. As noted above, Miller
(1986) subdivided the FAS into the Upper
Floridan aquifer (UFA) and the Lower Floridan
aquifer (LFA). The UFA is the principal source
of groundwater throughout the study area except
for Charlotte County, where only 20 percent of
total w ithdia% als originate from the UFA (based
on data from 2000 as compiled by Marella,
2004) due to naturally poor water quality. The
most productive units of the UFA are located
within the Avon Park Formation, Ocala
Limestone and the Suwannee Limestone (Ryder,
1985).

 A highly permeable facies of the FAS,
referred to as the "Boulder Zone" (Kohout,
1965), is characterized by cavernous, fractured
dolostones with very high transmissivities (Puri
et al., 1973). Vernon (1970) reported "Boulder
Zone" facies throughout the Florida peninsula.
More recently, however, this hydrogeologic
facies has been recognized as discontinuous and
found to be limited to the southern third of the
Florida peninsula (Miller, 1986). The facies
does not occur within the same lithostratigraphic
unit throughout its extent (Miller, 1986).
Maliva et al. (2001) report occurrences of
"Boulder Zone" facies in the Early Eocene
Oldsmar Formation in Charlotte, Lee and Collier
County injection wells. This facies has been
reported to occur at depths shallower than -1300
ft (-396.2 m) MSL in Charlotte County (Maliva
et al., 2001), which corresponds to Avon Park
Formation carbonates. In the southernmost
peninsula, the facies occurs within the Paleocene
Cedar Keys Formation as well as the Eocene
Avon Park Formation and Ocala Limestone
(Puri and Winston, 1974).

 Wolansky et al. (1980) mapped "the highly
permeable dolomite zone" of the Avon Park
Formation throughout the southern two-thirds of
the District. These well-indurated dolostones
are commonly fractured and contain large


dissolution channels. The zone occurs -100 ft
(~ 30.5 m) below the top of the Avon Park
Formation in the central part of the study area
and ~ 400 ft (~ 122 m) below the Avon Park
Formation surface in the southern region. The
SAS and IAS/ICU generally provide
confinement of the UFA (Berndt et al., 1998)
except for areas in the northern region where the
UFA may be unconfined and where karst and
paleokarst promote inter-aquifer connectivity.
Hydrogeologic conditions vary considerably
between the northern and southern regions
depending on the degree of confinement.

 The interpolated surface of the UFA ranges
from greater than 75 ft (22.9 m) MSL along the
Brooksville Ridge to less than -825 ft (-251 m)
MSL in southern Charlotte County (Plate 58).
Locally, maximum elevations exceed 130 ft
(39.6 m) along the Brooksville Ridge (W-14917
[ROMP 109], Plate 7). A map of overburden
thickness provides a different perspective on the
depth to the top of the UFA (Figure 31). This
map, developed by subtracting the UFA surface
from a 15 m (49.2 ft) resolution DEM (Arthur et
al., in review), allows comparison of the aquifer
system to geomorphic features. For example,
note the thin overburden along the Brooksville
Ridge compared to the Lake Wales Ridge, as
well as the maximum overburden thickness of
-900 ft (-274 m) in Charlotte County.

 The base of the UFA occurs within the lower
Avon Park Formation, where vertically and
laterally persistent evaporite minerals (gypsum
and anhydrite) are present in the carbonate rocks
(e.g., Ryder, 1985; Hickey, 1990; see Middle
Floridan confining unit [MFCU], p. 75, for more
information). Thickness of the UFA, calculated
as a "grid difference map," ranges from less than
300 ft (91.4 m) to more than 1500 ft (457 m;
Figure 32). Regional subjacent confinement of
the UFA is comprised of the MFCU where
present. Examples of basal UFA confinement
are represented in several cross sections (e.g.,
Plates 7, 9, 21 and 31). Note, however, that
Miller's (1986) delineation of overlapping
MFCU units by default suggests that the base of
the UFA in these areas is a complex
discontinuous surface. This aspect of the UFA
is described in more detail in the next section.