BULLETIN NO. 68


the study area, IAS/ICU Hawthorn Group
sediments provide increased confinement of the
FAS. Numerous sinkholes (Plate 3) and paleo-
sinks breach confining clayey sediments that
would otherwise justify delineation of confined
FAS (Plates 6, 7 and 8; Trommer, 1987). As a
result, characterization of certain areas along the
Brooksville Ridge as IAS/ICU and SAS is very
subjective.

 In the central, southern and eastern regions,
the zero MSL (sea-level) contour of the FAS
surface (central Hillsborough and Polk Counties;
Plate 58) represents a hinge-line, south of which
the dip of the FAS markedly increases. As
noted in Figure 8, the FAS surface generally
correlates with lithostratigraphic units. The
surfaces of these units have been affected by
depositional, erosional or structural features
(e.g., dissolution, basins, channel scouring,
fracture-controlled drainage systems, possible
faults, etc.). Other variations in the FAS surface
(Plate 58) are due to sub-regional confinement
between lithostratigraphic units. For example,
several wells in central Hillsborough County
indicate that confinement exists between the
Suwannee Limestone and the Tampa Member
(Arcadia Formation), resulting in correlation of
the FAS surface with the Suwannee Limestone.
Uncertainty in the area with regard to vertical
hydraulic connectivity between these two
lithostratigraphic units and lateral/regional
groundwater flow warrant further study.

 In the southern region, the FAS surface
generally coincides with the top of the
Oligocene Suwannee Limestone. Along western
DeSoto County, however, the FAS surface
correlates poorly with any specific
lithostratigraphic unit (Figure 8). In some
boreholes, such as W-18117 (ROMP 35,
northwest DeSoto County, Plate 2), the FAS
surface occurs approximately 90 ft (27.4 m)
below the top of the Suwannee Limestone
(LaRoche, 2004). Along the eastern margin of
the study area, the Ocala Limestone comprises
the surface of the FAS, except for a small area in
northeastern Osceola County, where the Ocala
Limestone is absent and the Avon Park
Formation is the uppermost unit (Figure 8;
Gates, 2006). Plate 34 is a representative cross
section of the FAS surface, where it occurs


nearly 100 ft (30.4 m) MSL in well W-15650
(ROMP 88; northern Polk County) and deepens
significantly toward the south where it occurs at
-861 ft (-262 m) MSL in well W-15289.

 The FAS potentiometric surface within the
study area (Figure 30) is highly variable and
contains several prominent features. Maximum
elevations (exceeding +130 ft [39.6 m] MSL)
occur in north-central Polk County, near Polk
City and within the Green Swamp. This
maximum is located in the center of a ridge-like
potentiometric high that extends north-northwest
into Marion and Sumter Counties, and south-
southeast along the axis of the Lake Wales
Ridge. Another topographically associated high
occurs along the Brooksville Ridge in Pasco and
Hernando Counties. At least two distinct
potentiometric highs with more than 20 ft (6.1
m) of relief also occur: 1) the Dunellon Gap
(Levy County), and 2) the Big Cypress Swamp
(central Pasco County). Minimum elevations
occur within a few feet below sea level in
southwestern Hillsborough County. Troughs in
the UFA potentiometric surface align with the
Withlacoochee River in Hernando and Pasco
Counties, the Hillsborough and Alafia Rivers
(Hillsborough County), indicating significant
baseflow contributions to these rivers (Figure
30). A less distinct trough is associated with
lakes in eastern Citrus County (i.e., Tsala-
Apopka Lake and Lake Panasofkee). Flow
directions vary considerably in response to these
highs/ridges and lows/troughs; however, in a
very broad sense the groundwater flow in the
UFA for the region west of the Lake Wales
Ridge is generally toward the Gulf coast.

 Seasonal perturbations in the potentiometric
surface are generally due to variable recharge
rates and pumping. Regionally, the most notable
difference between the May 2005 (Ortiz and
Blanchard, 2006) and September 2005
potentiometric surface (Ortiz, 2006) is a
broadening of the ridge-like high along the Lake
Wales Ridge in response to the rainy season.
Relative to a predicted "pre-development"
potentiometric surface, Bush and Johnston
(1988) report drawdown exceeding 20 ft (6.1 m)
in a region encompassing roughly one-third of
the District centered on northwest Hardee
County.