FLORIDA GEOLOGICAL SURVEY


sediments. The IAS/ICU primarily correlates
with the Hawthorn Group in this region;
however, some post-Hawthorn siliciclastics may
also be included in the uppermost IAS/ICU (e.g.
reworked Hawthorn Group sediments in
southern Pasco and northern Hillsborough
Counties).

 Throughout most of Pinellas and
Hillsborough Counties, vertical hydraulic
continuity generally exists between the Tampa
Member (Arcadia Formation) and the Suwannee
Limestone, thereby placing the base of the
IAS/ICU at the top of the Tampa Member
(Broska and Barnette, 1999). In northern
Pinellas County, however, roughly a third of the
wells penetrating the Suwannee Limestone
encounter clay beds at the base of the Tampa
Member. Given the discontinuous nature of
these clays, the base of the IAS/ICU is
considered to correlate with the top of the
Tampa Member although local-scale head
differences may occur between this unit and the
Suwannee Limestone. In southern Hillsborough
County, however, lithologic data from six
adjacent wells indicate that basal Tampa
Member clays are more laterally extensive,
suggesting contiguous hydraulic separation from
the Suwannee Limestone. In this area (Figure
8), the Tampa Member is considered part of the
IAS/ICU (Green et al., 1995; Figure 8; Plates 14,
21 and 20). As additional hydrologic and
lithologic data become available, the base of the
IAS/ICU should be reassessed in these areas.

 Mixed siliciclastic-carbonate deposits of the
IAS/ICU thicken southward from -75 ft (22.8
m) in central Polk County to more than 900 ft
(274 m) in Charlotte County (Plate 57) creating
several water-bearing zones (Trommer, 1993).
In most of west central Florida the IAS/ICU is
less permeable than the underlying FAS and
restricts movement of water between the SAS
and FAS (Ryder, 1985).

 Where multiple permeable zones exist in the
IAS/ICU in the central and southern regions,
flow regimes are not fully understood. Extents
of these permeable zones need to be better
defined along with groundwater flow patterns in
order to improve management of the IAS/ICU as
a water supply source (Torres et al., 2001). The


hydrologic parameters for these zones have been
well characterized by Basso (2002) for southern
Hillsborough, Manatee and northern Sarasota
Counties. Additional details regarding depths,
thicknesses, extents and hydraulic properties of
these permeable zones are presented in Duerr
and Enos (1991), Barr (1996), Knochenmus and
Bowman, (1998), Torres et al. (2001), Basso
(2003) and Knochenmus (2006).

 Distinct differences exist among
potentiometric levels representing IAS/ICU
permeable zones (Duerr, 2001). Regional
IAS/ICU potentiometric maps, however, are
generally constructed from wells open to
multiple permeable zones. As a result, the
potentiometric surface of the IAS/ICU
represents a composite of permeable zones,
which likely contributes to its high variability
across the study area. In May 2001, the
composite head level in the IAS/ICU ranged
from more than 120 ft (36.6 m) MSL in the
"Four Corners" area (northwestern Hardee
County) to approximately 50 ft (15.2 m) MSL in
Highlands County (Duerr, 2001). Within the
study area, IAS/ICU potentiometric elevation
lows occur in southern Hillsborough County and
northern Sarasota County (Duerr, 2001).

 Selected hydrogeologic data compiled in
SWFWMD (2006; see also discussion on p.52)
are statistically summarized in Figures 24-27
(transmissivity, storativity, leakance and Kh,
respectively). Laboratory data from the FDEP-
FGS, including falling-head Kv analyses (Figure
28) and total porosity (Figure 29) are also
presented. Basso (2002) presents similar
hydrologic data subdivided based on IAS/ICU
permeable zones. A five order-of-magnitude
difference exists between Kv and Kh mean
values reported herein.

 Similar to the SAS data, sampling bias may
account for some of this variation; Kh values
predominately reflect data from IAS/ICU
permeable zones while samples from which Kv
was measured may have been somewhat biased
toward less permeable zones or more indurated
core samples. Moreover, as with all Kv data in
heterogeneous strata, the measured values are
affected by lower-permeability horizons within
the analyzed core segments.