BULLETIN NO. 68


unconfined FAS; see Plates 29 and 30). In
contrast, these same deposits may locally exceed
100 ft (30.5 m) thick where they infill karst
features (including paleo-sinks represented by
W-15075 and W-10829; Plates 5, 6 and 29).
Depending on the permeability of infilling
sediments, the karst features may provide
hydraulic connection between the SAS and the
FAS.

 Topographic inversion (i.e., differential
carbonate dissolution due to chemical buffering
and confinement) also contributes to the highly
variable thickness of the SAS in the northern
region. Along the axis of the Brooksville Ridge,
for example, more than 50 ft (15.2 m) of SAS is
locally observed. Much of the Ridge, however,
is perforated by sinkholes (Plate 3), making
delineation of the SAS problematic. Although
lithologic data from wells support presence of
the SAS along the Brooksville Ridge, extent of
the unit is even more subjective in the
surrounding region. Assessment of regional
mapping (geology [Figure 2], environmental
geology [Figure 4], and soil permeability [Figure
18]) warrants the dashed (i.e., approximate)
extent of the SAS in the region (Plate 55).
These hachured areas are considered semi-
confined to unconfined FAS.

 In addition to lithologic evidence, hydraulic
data support local delineation of the SAS in
parts of the northern region. Well W-15647
(ROMP 90; Plate 10) provides a classic example
of confinement between the SAS and the FAS in
this region. Water levels measured during
drilling rose 3 ft (0.9 m) when the clays
underlying the SAS were fully penetrated and
artesian conditions of the FAS became evident.
The IAS/ICU in this well is too thin to depict
graphically in Plate 10. Moreover, given the
proximity of the well to the IAS/ICU extent,
whether the clays are basal SAS or IAS/ICU is
subjective. Confined and semi-confined areas of
Citrus County have been identified (Lee, 1998).

 Water levels in paired monitor wells also
provide valuable information regarding the
presence of the SAS (i.e., basal confinement) in


the absence of an extensive IAS/ICU. This data,
however, should be interpreted with caution:
similar water levels may reflect "leaky"
confinement and seasonal or local pumping
conditions should be considered. The "Floridan"
water levels in W-14336 (ROMP 93, Plate 10),
are at least 5 ft (1.5 m) lower than SAS water
levels (U.S. Geological Survey, 1990) indicating
a well-defined hydraulic separation between the
two aquifer systems. Hydrologic data from W-
16644 (ROMP LP-6), located approximately 2
mi (3.2 km) north of cross section D-D' (Plate 7)
indicates that FAS water levels are typically
more than one foot (0.3 m) above the surficial
water table. At W-16644, 12 ft (3.7 m) of clay
and clayey sand provide sufficient confinement
between the FAS and SAS.

 As noted above, effectiveness of the
hydraulic separation between the SAS and the
subjacent FAS varies locally. Water levels in
the paired wells L11KD and L11KS (northeast
of W-5054, Plate 2), are nearly
indistinguishable, thus indicating leaky to
unconfined conditions between the FAS and
"water table" levels in surficial sediments (U.S.
Geological Survey, 1998). In contrast, water
levels from the monitor-well pair L11MM and
LI MS (southeast of W-5054, Plate 2) confirm
existence of SAS conditions because "water-
table" elevations differ from the FAS
potentiometric surface (U.S. Geological Survey,
1998). Paired monitor wells with inconsistent
trends in water levels suggest that there may be
some degree of confinement of the FAS, either
as low permeability horizons in the base of the
SAS or in the uppermost carbonates of the FAS
(e.g., mudstones/micrites or densely
recrystallized zones).

 A SWFWMD Technical Memorandum
(Basso, 2004) provides detail on the
hydrogeologic setting of the Hemando-Pasco
County (or "Northern Tampa Bay") region. In
the memorandum, the location of wetlands, soil
properties, lithologic data and hydrographs from
nested wells and lakes are used to delineate three
zones: unconfined UFA, locally perched water
table (generally restricted to the southern