Eogenetic Karst Hydrology: Insights from the
2004 Hurricanes, Peninsular Florida
by Lee J. Florea1,2 and H.L. Vacher3
Abstract
Eogenetic karst lies geographically and temporally close to the depositional environment of limestone
in warm marine water at low latitude, in areas marked by midafternoon thunderstorms during a summer rainy
season. Spring hydrographs from such an environment in north-central Florida are characterized by smooth,
months-long, seasonal maxima. The passage of Hurricanes Frances and Jeanne in September 2004 over three
field locations shows how the eogenetic karst of the Upper Floridan Aquifer responds to unequivocal recharge
events. Hydrographs at wells in the High Springs area, Rainbow Springs, and at Morris, Briar, and Bat Caves all
responded promptly with a similar drawn-out rise to a maximum that extended long into the winter dry season.
The timing indicates that the typical hydrograph of eogenetic karst is not the short-term fluctuations of springs
in epigenic, telogenetic karst, or the smoothed response to all the summer thunderstorms, but rather the
protracted response of the system to rainfall that exceeds a threshold. The similarity of cave and noncave
hydrographs indicates distributed autogenic recharge and a free communication between secondary porosity and
permeable matrix—both of which differ from the hydrology of epigenic, telogenetic karst. At Briar Cave, drip
rates lagged behind the water table rise, suggesting that recharge was delivered by fractures, which control the
cave’s morphology. At High Springs, hydrographs at the Santa Fe River and a submerged conduit apparently
connected to it show sharp maxima after the storms, unlike the other cave hydrographs. Our interpretation is that
the caves, in general, are discontinuous.
Introduction
Many karst springs are well known for their rapid
response to rainfall (e.g., Birk et al. 2006). Such springs
are characterized by ‘‘fast-response hydrographs’’ (White
2005, 298), which show individual storm peaks that return
quickly to base flow levels. In contrast, other karst springs
have ‘‘slow response hydrographs showing only wet or dry
periods’’ (White 2005, 298). We have shown from a survey
1Corresponding author: Department of Geology, University of
South Florida, Tampa, FL 33620.
2Currently at U.S. Geological Survey, 3110 SW 9th Ave.,
Ft. Lauderdale, FL 33315; (954) 377-5951; fax (954) 377-5901;
[email protected]
3Department of Geology, University of South Florida, Tampa,
FL 33620.
Received March 2006, accepted January 2007.
Copyright ª 2007 The Author(s)
Journal compilation ª 2007 National Ground Water Association.
doi: 10.1111/j.1745-6584.2007.00309.x
of many large springs (Florea and Vacher 2006) that the
slow-response springs are typical of eogenetic karst,
whereas the fast-response springs typify telogenetic karst.
From the perspective of the flashy hydrographs of
telogenetic karst, the explanation of the smooth hydrographs of eogenetic karst is that the ‘‘contributions of
individual storms have been averaged out and that only
the rise and fall of discharge with seasonal wet and dry
periods is reflected in the hydrographs’’ (White 2005,
568). Similarly, we wrote, ‘‘The cumulative effects of all
the precipitation events during the wet season drive the
changes in discharge at these (eogenetic) springs’’ (Florea
and Vacher 2006, 358). We will present evidence in this
paper of a different explanation. Although seasonal rainfall is an expected feature of areas where eogenetic karst
is located (Jordan 1984), the smooth hydrographs can logically be seen to be the product of the long, drawn-out
response to a few major storms as opposed to the sum of
the near daily rainfalls of the rainy season.
Vol. 45, No. 4—GROUND WATER—July–August 2007 (pages 439–446)
439
Hurricanes Frances and Jeanne were discrete, identifiable recharge events in the Florida peninsula and
provided an opportunity to study how eogenetic karst responds to recharge-generating rainfalls. The result of the
study is clearer hydrologic distinction between eogenetic
and telogenetic karst.
Eogenetic Karst
The terms ‘‘eogenetic’’ and ‘‘telogenetic’’ were applied
to karst (Vacher and Mylroie 2002) to draw a parallel with
the first and third of the three time-porosity stages defined
by Choquette and Pray (1970). According to the Choquette
and Pray scheme, the eogenetic and telogenetic stages are
before and after the reduction of primary depositional
porosity by burial diagenesis. Thus, the hallmark property
that distinguishes eogenetic and telogenetic karst is the
magnitude and relative significance of the continuum of
matrix (intergranular) porosity—effectively, matrix permeability. Not surprisingly, matrix permeability of karst limestone varies roughly inversely with geologic age (Florea
and Vacher 2006): the older the limestone, the more likely
it has experienced burial diagenesis. In the globally distributed examples of carbonate aquifers compiled by Budd
and Vacher (2004), for example, the matrix hydraulic conductivities of the five Cenozoic aquifers, log K(m/s) ¼
25.5 6 1.5, differ by 4 orders of magnitude from the
matrix hydraulic conductivities of the seven Paleozoic and
Mesozoic aquifers, log K(m/s) ¼ 29.5 6 1.5.
Cenozoic carbonate aquifers and eogenetic karst are
not randomly distributed around the world. Reasonably,
they occur in the same general region where carbonate
sediments are forming now—that is, broadly, siliciclasticdeprived, low-latitude (tropical to subtropical) settings
not far from warm, marine water. Indeed, according to the
original usage of Choquette and Pray (1970, e.g., 216), the
eogenetic zone is a ‘‘net depositional realm’’ and the telogenetic zone is a ‘‘net erosional realm.’’ Extension of the
terms to karst by emphasizing the magnitude of matrix
porosity blurs that distinction but does associate eogenetic karst with the geography of marginal marine carbonate deposition.
Where, then, do eogenetic meteoric diagenesis and
eogenetic karst occur? One locale is isolated, small carbonate islands (Vacher 1997), including low reef islands
of atolls and barrier reefs, low eolianite islands such as
Bermuda, low platform islands such as Tongatapu and
Grand Cayman Island, uplifted atolls such as Nauru and
Niue, and high–reef composite islands such as Barbados
and Guam. Another is the exposed portions of isolated or
attached carbonate platforms such as the Bahama Banks
and the Florida and Yucatan peninsulas, including their
associated small islands (e.g., San Salvador, the Florida
Keys, and Cozumel, respectively).
The rainfall characteristics of such places reflect
their low-latitude locations and proximity to warm
marine water. Many examples fall into the Aw (tropical,
dry winter) climate (Koeppen classification, e.g., Aguado
and Burt 2004), where the seasonality results from the
summer poleward shift of the Intertropical Convergence
Zone (ITCZ). Some occur in Cfa (humid subtropical), on
the west side of the subtropical anticlines. Generalizing,
440
L.J. Florea, H.L. Vacher GROUND WATER 45, no. 4: 439–446
then, the habitat of eogenetic karst typically includes
a rainfall regimen dominated by frequent thunderstorms,
whether they be due to the ITCZ, easterly waves entrained within the trade winds, or local convection—and,
of course, infrequent tropical cyclones (hurricanes). As
a result, the amount and character of rainfall are typically
seasonal, with convective thunderstorms during the summer and tropical cyclones during the late summer and
early fall. In Barbados (13 N), for example, the rainy
season is May through November. At Heron Island
(23 S, southern Great Barrier Reef), the rainy season is
December through June.
In our comparison of spring hydrographs of eogenetic vs. telogenetic springs (Florea and Vacher 2006),
we found a seasonality in the spring discharge in areas of
eogenetic karst. That study, which was based on longterm records of spring discharge and rainfall, indicated
a rather long lag time between the height of the rainy season and the culmination of the spring discharge—90 d in
the case of Silver Springs, Florida. We interpreted the lag
time to be a measure of the moderating effect of the large
storage in the permeable matrix pore space of the eogenetic karst of west-central Florida. We now have field
data from that eogenetic karst that allow association of
changes in spring discharge and cave water levels to particular rainfalls. The data shed light on how the seasonality of rainfall that typifies the climate of eogenetic karst
connects to the hydrology of eogenetic karst itself.
The field area, peninsular Florida, is well suited to
make the connection. Low, small, carbonate islands,
which in some ways could be considered the archetype of
eogenetic karst, are particularly unsuited. The reason is
that the whole of a small, low, permeable, eogenetic karst
island is affected, and analysis of water levels is confounded, by low-frequency (seasonal) sea level fluctuations (e.g., Bermuda, see Rowe 1984). Moreover, the
dynamics of a lagging ‘‘interface’’ come into play. Neither
of these complications affects the Florida interior.
Study Area
Geology
The Upper Floridan Aquifer (UFA), one of the
world’s classic regional aquifers (Back and Hanshaw
1970; Fetter 1994, 293–303), underlies all of peninsular
Florida. It is unconfined or semiconfined north of about
Tampa (Figure 1). Where it is unconfined, the UFA is
part of an active, modern karst characterized by sinkholes
(Tihansky 1999), extensive cave-scale porosity (Florea
2006a), and 33 first-magnitude springs (Scott et al. 2004).
This regional karst includes Alachua, Gilchrist, Marion,
and Citrus counties (Figure 1), the area of our study.
The carbonates in which the karst is developed are the
Ocala (Late Eocene) and Suwannee (Oligocene) limestones. They formed when what is now the Florida peninsula was an isolated bank and siliciclastic sediments from
the north were diverted into a now filled seaway (Back
and Hanshaw 1970). Porosity of the Ocala Limestone averages about 30% (Loizeaux 1995). Mean matrix permeability is on the order of 1026 m/s. (Florea 2006b).
Wakulla
Springs
Ginnie
Springs
PW-3
GILCHRIST
PW-4
PW-6
Atlantic
Ocean
Tampa
Santa Fe
River
PW-2
PW-5
ALACHUA
PW-8
PW-7
Gulf of
Mexico
TD
TD
Jeanne
Frances
TS
MARION
Rainbow
Springs
TS
TS
TS
Tampa
400
Briar
Cave
TS
1
Jeanne
1
1
2
3
2
Frances
Kilometers
Withlacoochee
River
landfall 9/26/04
2
landfall 9/4/04
3
3
CITRUS
Homosassa
Springs
40
Morris
Cave
Kilometers
Figure 1. Locations of monitoring sites and select springs in the unconfined Floridan Aquifer of peninsular Florida. Sites in
Alachua and Gilchrist counties constitute the HSWG data set. Index map at lower left illustrates the paths and the dates of
landfall for Hurricanes Frances and Jeanne. Labels on each path indicate the Safford-Simpson category at that location. TS =
tropical storm; TD = tropical depression.
Mapping of caves in the study area has revealed the
character of the elements of secondary porosity in rocks
of the UFA (Florea 2006a, 2006b). Detailed surveys of
caves such as Briar Cave (Figures 1 and 2) document an
architecture composed of laterally extensive, tabular cavities at several levels with vertical connections provided
by dissolved out fissures conforming to approximately
NE–SW and NW–SE fractures. The importance of the
fracture porosity is underscored by the ubiquity of solution-enlarged, soil-filled fractures in the epikarst exposed
in quarry walls. Significantly, the caves of the cave levels
are not interconnected. They likely do not represent an
integrated system of conduits leading to springs. Cave
Figure 2. Map of Briar Cave. The directional morphology of
passages are controlled by fractures.
passages often end in blind pockets, progressively narrowing fissures, and collapses. Sediments from younger units
commonly choke caves and fill drilled cavities in westcentral Florida. These sediments, as noted by Back and
Hanshaw (1970, 366), would ‘‘. reduce permeability
enough to retard flow and generate a higher head (water
table)..’’
Although less common, conduit flow does occur in
the karst of Florida. In one documented example, along
the Santa Fe River in our study area (Figure 3), conduit
flow between a sinking stream and a river rise some 5 km
downstream has been established by scuba diving, dye
tracing, and other means (e.g., Martin et al. 2006; Martin
and Portell 2002; Screaton et al. 2004). The spring hydrograph is marked by dramatic changes in spring discharge
that are directly linked to river stage and precipitation
events (Screaton et al. 2004). It differs from the smooth
hydrographs of typical eogenetic karst presented in Florea
and Vacher (2006). Chemographs at the rise document
rapid exchange of water between conduit and matrix
porosity (Martin and Dean 2001) and attest, in general, to
the easy communication between the three elements of
the triple-porosity system of eogenetic karst.
Another sinking stream and river rise conduit system
is documented in the Woodville Karst Plain (WKP) and
Wakulla Springs in the Florida Panhandle (Figure 1).
Cave divers have mapped approximately 60 km of partially connected underwater conduits in the WKP (Loper
et al. 2005). This conduit system is of a magnitude not
documented elsewhere in Florida.
Rainfall
With the highest elevation about 100 m and no point
of land as much as 100 km from warm marine water, the
Florida peninsula is ideally situated for summer thunderstorms. Maps in introductory textbooks showing the distribution of thunderstorms in the continental United
L.J. Florea, H.L. Vacher GROUND WATER 45, no. 4: 439–446
441
San
ta F
Ginnie
Springs
7.5m
eR
iver
Poe
Springs
River
Rise
9.1m
PW-2
PW-3
PW-4
GILCHRIST
GIL
PW-5 (Bradley Sink)
ALACHUA
10.7m
PW-6 (Bat Cave)
PW-8
PW-8
12.2m
4
Kilometers
Figure 3. Map of the HSWG study area in Alachua and Gilchrist counties. Black dots are the locations of individual
monitoring sites. Open circles along the Santa Fe River are
the locations of springs. Dashed lines are prehurricane water
levels in the Floridan Aquifer. The shaded region in the western portion of the study area delimits the outcrop of the
Miocene Hawthorn Group and the dark polygons in this
region are lakes.
States indicate ‘‘by far the greatest incidence is over central Florida, where thunderstorms occur on average more
than 100 days per year’’ (Aguado and Burt 2004, 334).
For our study area, the incidence is more than about 75
thunderstorm days per year according to a state map
based on the 30-year period, 1948 to 1977 (Jordan 1984).
According to the same data compilation, there are 1.2 to
1.5 thunderstorms per thunderstorm day; the duration is
1.0 to 1.8 h per thunderstorm; 70% to 80% of the thunderstorm days occur during the 4-month period of June to
September; and about 20 thunderstorm days occur during
the peak months of July and August. The peak occurrence of these thunderstorms is in the midafternoon, with
60% to 70% of the thunderstorms occurring between
2:00 p.m. and 7:00 p.m. (Jordan 1984).
The summer thunderstorms create a strong seasonality in Florida’s rainfall. Some 40% of the approximately
140 cm/year rainfall of Florida occurs in June, July, and
August (Martin and Gordon 2000). There is also a secondary maximum in the late winter that is associated with the
southern penetration of middle latitude weather systems
(Jordan 1984). This secondary maximum increases northward and particularly into the panhandle. According to
maps in Jordan (1984), the warm-season (June to September) rainfall averages about 18 cm or more per month in
442
L.J. Florea, H.L. Vacher GROUND WATER 45, no. 4: 439–446
our study area, and the cold-season (December to March)
rainfall averages 9 cm per month, for a warm-season/
cold-season rainfall ratio of about 2. For comparison, the
ratio is about 1.4 in the western panhandle.
Unlike the frequent, short-lived thunderstorms, hurricanes and tropical storms have huge rain areas, sometimes exceeding 400 km in diameter. They move slowly
and can take days to pass a given locality. The result is
that tropical systems contribute significantly to the rain
totals in the months that they occur, which in Florida is
typically in the late summer and early fall. According to
the summary in Jordan (1984), on average about 20 tropical storms and hurricanes per decade crossed the shore
or came within 200 km of it in the 90-year period ending
in 1980, and 61% of them occurred in September and
October. A map in the same reference shows that about
30% of the long-term average rainfall for the month of
September in the study area came from tropical storms
and hurricanes.
By any measure, 2004 was an unusual year for hurricanes in Florida. Four major hurricanes crossed the
state during a 6-week period. Frances made landfall on
September 5 as a Category 2 storm, and Jeanne made landfall on September 26 as a Category 3 storm (Figure 1).
They followed nearly identical paths across the peninsula
(Figure 1). They covered thousands of square kilometers,
producing flooding rainfalls that affected our entire study
area. In addition, feeder bands from Hurricane Ivan,
which made landfall in the panhandle, brushed our study
area during the time between Frances and Jeanne. These
storms provided us the opportunity to obtain hydrographs from eogenetic karst in which specific recharge
events can be identified and witness how the karst aquifer
responded.
Data Acquisition
The data are from three locations (Figure 1): Morris
Cave in the Radar Hill Quarry, Citrus County; Briar Cave,
Marion County; and a 540-km2 area of Gilchrist and Alachua counties, the High Springs Water Gap (HSWG),
south of the Santa Fe River (Figure 3). Exploratory water
level monitoring was under way at all three locations
before the hurricanes arrived and, after the hurricanes,
was extended well into the following dry season.
Pressure transducers at all sites provided hourly
water level data. At Morris Cave, precipitation data were
collected hourly with an on-site rain gauge. We used
on-line weather stations for daily rainfall amounts from
Ocala for Briar Cave (5 miles outside of town) and from
the town of High Springs for HSWG. We supplemented
the water level and precipitation data at Briar Cave with
hourly drip rate measurements from the matrix porosity
of the vadose zone using a tipping bucket rain gauge.
Finally, at Briar Cave and in the HSWG, we obtained
daily measurements of discharge from the combined
vents of Rainbow Springs (28 km from Briar Cave,
USGS Station no. 02313100) and the Santa Fe River (at
the northwest corner of High Springs study area, USGS
Station no. 02321500; Figure 3). These data are available
from the USGS on-line data archives.
Site Details and Limitations
The entrance to Morris Cave is a recent collapse feature exposed in the Radar Hill Quarry. The large cave entrance is adjacent to a water retention pond. Storm events
produce rapid runoff on the surface of the reclaimed
quarry, and on occasion, the pond overflows into the cave
causing erosion and sediment transport. More than 2 m of
sediment accumulated in the cave during the study period.
The sediment buried and eventually destroyed the in-cave
instrumentation.
Briar Cave is in a vastly different setting than Morris
Cave—no quarry and only a small, natural entrance. Briar
Cave is an air-filled cave with more that 1 km of surveyed
passage (Figure 3). Access to Briar Cave was limited to
one Sunday per month by request of the landowner. Our
power systems failed several times after December 2004,
causing periods of interrupted data. By March 2005,
mineral precipitation on the rain gauge collecting drip rate
data prevented further measurements.
The well-instrumented HSWG is an ongoing study
funded by the Florida Department of Environmental Protection to monitor ground water flow conditions in the
vicinity of the large springs on the Santa Fe River. The
study consists of seven water level monitoring sites
(PW-2 through PW-8) managed by Karst Environmental
Services Inc. The seven sites lie at various distances from
the Santa Fe River (Figure 2; Table 1). PW-6 is an aquifer-sustained pool in Bat Cave, an air-filled cave with
more than 1 km of surveyed passage. PW-5 is in Bradley
Sink, a site of discrete recharge to the Floridan Aquifer.
The other five sites are drilled wells. One of them, PW-2,
intersects a 3-m-high submerged cavity.
All but one monitoring site in the HSWG study provide a continuous record during the period of study. The
exception is PW-6, Bat Cave, where four important
months (July 18 to November 12, 2004) of data are missing because a rodent in the cave chewed apart the cable
from the piezometer.
Results
The time series data (Figure 4) capture the effects of
both localized rain events of the summer rainy season in
August of 2004 and the widespread precipitation from the
hurricanes that followed that September:
d
d
d
At Morris Cave (Figure 4A), the cave at the quarry, thunderstorm-induced, short-duration, high-amplitude spikes
with an amplitude of more than 2 m and an average duration of 4 d are superimposed on a long-term (multimonth)
variation of approximately 2 m that started with Hurricane
Frances.
At Briar Cave (Figure 4B), the water table variation contains two distinct step-ups that occur after each hurricane.
Overall, the variation is similar to the long-term variation
at Morris Cave. The amplitude is approximately 2 m and
the peak water level occurs 41 d after Frances. The discharge at nearby Rainbow Springs follows a similar pattern. The vadose drips from the matrix porosity inside the
cave follow a later schedule (Figure 4B).
Among the seven monitoring sites in the HSWG
(Figure 4C), only the well closest to Santa Fe River
(PW-2) has a storm-spiked pattern like that of the river.
PW-4 and PW-5 show two step-ups like the pattern at
Briar Cave. The other sites show a smooth rise and maximum like the main signal at Morris Cave (Table 1).
The time series data in Figure 4 also document the
contrast in the areas covered by the different types and
amounts of rainfall. The hurricanes covered the whole
area. Hurricane Frances dumped 28 cm in 2 d at Morris
Cave, 28 cm in 3 d at Ocala, and 34 cm in 3 d at High
Springs; Hurricane Jeanne followed with 18 cm in 1 d at
Morris Cave, 5 cm in 1 d at Ocala, and 16 cm in 2 d at
High Springs. In contrast, the other recorded rainfalls, in
particular the August rainfalls, illustrate the localized
nature of the normal summer rains. For example, a storm
on August 7, 2004, at High Springs produced 3.5 cm of
precipitation. This same storm did not occur at either
Morris or Briar Caves. The next day, 5 cm of rain fell during 4 h in the afternoon at Morris Cave, while no precipitation occurred at either Briar Cave or High Springs.
The typical summer rainfalls register conspicuously on
the hydrograph at Morris Cave, where water overflows into
the cave from a pond at a quarry. These same summer rainfalls do not produce short-term changes in the water level at
Table 1
Peaks in the Water Level from the HSWG Data
Site
Santa Fe
PW-2
PW-3
PW-4
PW-5
PW-6
PW-7
PW-8
Time from
Event to Peak
Water Level in Figure 4 (d)
Distance from
the Santa Fe River (km)
Change in Water
Level, Pre-Frances
to Post-Jeanne (m)
Frances
Jeanne
0
3
5
10.8
10.3
15
22.5
18.3
—
2.2
3.7
2.2
2.0
—
3.1
2.1
6
10
12
23
19
—
—
—
5
8
41
25
18
—
46
55
L.J. Florea, H.L. Vacher GROUND WATER 45, no. 4: 439–446
443
5.00
5.00
Hurricane Frances
4.50
Hurricane Ivan feeder band
Hurricane Jeanne
4.00
Water level (m)
4.50
3.50
4.00
3.50
Aquifer peak on 10/31/04
3.00
3.00
2.50
2.50
2.00
2.00
1.50
1.50
1.00
1.00
0.50
0.50
0.00
8/1/2004
10/1/2004 12/1/2004 1/31/2005
4/2/2005
Precipitation (cm/hr)
A
0.00
6/2/2005
Water elevation (m- msl)
B
17
16
1.5
Hurricane Jeanne
discharge at Rainbow Springs
1.2
Hurricane Frances
water level
15
0.6
drip rate
14
0.9
0.3
13
12
8/1/2004
10/1/2004
0
1/31/2005
12/1/2004
Drip rate (cm/hr)
Precipitation (dm/day)
Rainbow River Discharge (m 3/sec x 20)
Time (days)
C
16.0
300
Santa Fe River
Water Level (m- msl)
PW-7
225
14.0
PW-3
12.0
PW-8
PW-6
PW-4
PW-5
Hurricane
Frances
PW-2
10.0
150
75
Hurricane
Jeanne
8.0
8/1/2004
10/1/2004
12/1/2004
0
1/31/2005
Precipitation (mm/day)
Santa Fe River Discharge (m3/sec)
Time (days)
Time (days)
Figure 4. Monitoring data from Morris Cave, Briar Cave,
and the HSWG. (A) Hourly precipitation (vertical bars) and
water level (solid line) data at the Morris Cave site. (B) Data
from the Briar Cave Site. Daily precipitation (vertical bars)
and hourly water level measurements (thick solid line). Daily
discharge measurements at Rainbow Springs are shown by
the dashed line. Hourly, in-cave, drip rate data at Briar Cave
are shown as individual data points. The thin, solid line is
a 3-d moving average though the drip-rate data. Note that
the water level and drip-rate data have incomplete records
due to problems with the instrumentation. (C) Daily precipitation (vertical bars), discharge measurements (thick
solid line) in the Santa Fe River, and hourly records of water
level for all seven sites in the HSWG.
the other sites, a result consistent with the findings of
Florea and Vacher (2006). In contrast to the summer rainfalls, the hurricanes register at the water level at all stations.
Insights
Significance of Major Storms
The summer thunderstorms in our record of Florida
rainfall apparently did not appreciably recharge the aquifer. They did not register on the water table. By contrast,
444
L.J. Florea, H.L. Vacher GROUND WATER 45, no. 4: 439–446
the hurricane rainfalls affected the water table and did so
promptly (Figure 4). Without question, they were
recharge events.
The difference in the scale of the two types of rainfall can be shown by a simple calculation. For Hurricane
Frances alone, the 30 cm of rainfall over the 8770 km2 of
the four-county study area amounts to approximately
2.5 3 109 m3 of water. In contrast, 5 cm of rainfall from
a thunderstorm affecting an area 25 km in diameter would
produce 2 orders of magnitude less water or approximately 2.5 3 107 m3. Thus, it would take about 100 of
these thunderstorms to deliver the same amount of water
to the four counties.
The hydrographs make clear that it is the hurricanes
that are responsible for the post-thunderstorm seasonal
high in the water levels of 2004. The amplitude of this
maximum is consistent with that interpretation. Assuming
no loss to evapotranspiration during the days of the
storms, the two hurricane rainfalls amounted to a potential
recharge of roughly 50 cm. At many of the sites, the
water table rise was about 2 m, suggesting a specific
yield of about 25%. This compares to the matrix porosity
of about 30% for these limestones (Loizeaux 1995).
Our finding of the ineffectiveness of the summer
thunderstorms for recharge confirms a similar finding by
Martin and Gordon (2000) from a 1-year study (1996 to
1997) of a nearby field site in Alachua County. Only 10%
of the 52-cm June to August rainfall recharged the aquifer. In contrast, 46.4 cm (75%) of the year’s recharge
occurred during a large storm in October and a second
large storm in April.
Time Lag between Rainfall and Peak Response
Hurricane Frances initiated and Hurricane Jeanne
augmented a rise in water levels that lasted through the
fall months to a seasonal maximum approximately 40 d
after the hurricanes. This 40-d lag between the rainfall
and the peak of the response is based on the identification
of the recharge-generating rainfall events.
In contrast, cross-correlation between 852 months
of rainfall and spring discharge data at Silver Springs
(Marion County) produced a time lag of 90 d (Florea and
Vacher 2006, Table 2). This statistically derived result is
undoubtedly incorrect because it is blind to the fact that
many of the rainfalls contributing to the rainy season do
not recharge the aquifer. The statistical 3-month lag from
the peak rainfalls of July and August to the culmination
of the spring discharge likely also reflects the occurrence
of numerous September and October recharge events due
to tropical storms and hurricanes over the 70-year period
of record.
Significance of Fractures
Not counting the anomalous spikes from the artificial
inflows at Morris Cave, the cave hydrographs (Morris
Cave, Briar Cave, and Bat Cave [PW-6]) consist of broad,
drawn-out responses to the recharge events characteristic
of diffuse rather than conduit flow (e.g. Hobbs and Smart
1986). They are synchronous with each other and, moreover, synchronous with the well (noncave) hydrographs,
indicating that the recharge is broadly distributed rather
than highly localized (e.g., sinking stream). Yet, the initial
responses to the recharge events are prompt, within a day
for both Frances and Jeanne at Briar Cave and PW-3, 4,
and 5, where the inflection correlating with Jeanne can be
picked out (Figures 4B and 4C). In contrast, the drip rate
measurements in Briar Cave, a gauge of the percolation
of recharge through the permeable matrix, did not
increase until 6 d after Jeanne, the second hurricane, and
they did not peak until 15 d after the maximum water
level in the cave. The likely candidate to deliver the
recharge to the aquifer so quickly is the fracture porosity,
which is known to be an important conductor because
many fractures are enlarged by solution. The geometry of
the caves themselves (e.g., Figure 2; Florea 2006b) attests
that the fractures are important to the hydrology.
This interpretation that fractures are significant conductors of the recharge agrees with the conclusions of
Jones et al. (1996) from a study of nitrate loading at Rainbow Springs, some 25 km from Briar Cave (Figure 1).
They found a strong connection between the land surface
and the spring and concluded that a network of fractures
feed into the spring. The discharge at Rainbow Springs
tracks the water level at Briar Cave (Figure 4B). The
recharge transport revealed at Briar Cave is widespread
and can explain why all the hydrographs started their rise
at the same time (Figure 4).
Character of the Saturated Zone
One of the striking features of the water table hydrographs (Figure 4) is how similar so many of them are;
another is how this similarity holds regardless of the presence or absence of caves at the monitoring site. At many
monitoring localities, including all three cave sites
(Morris Cave, Briar Cave, Bat Cave), a water-filled sinkhole
(Bradley Sink), and two of the noncave sites (PW-4 and
PW-8), the hydrographs began to rise promptly with Hurricane Frances and rose to approximately 2 m above prehurricane conditions some 40 d later. It certainly does not
appear that two storm pulses passed through a system of
connected conduits en route to springs while being effectively disconnected from low-permeability rocks enclosing
the conduits. Instead, the synchronous cave and noncave
water level variations suggest that the cave porosity communicates easily with the highly permeable matrix porosity. PW-2, on the other hand, is clearly a conduit—there is
a 3-m-high cavity in the well, and the hydrograph emulates the river. Why do the other cave hydrographs not
look more like PW-2? The answer, we believe, is that the
caves are discontinuous, that the norm for caves in this
karst is that they are not conduits (Florea 2006b). This
interpretation fits the findings from exploration and mapping of subaerial, formerly submerged caves in the area
(Florea 2006a).
One of the reasons that the caves do not serve as conduits is that they become choked with sediment eroded
from overlying Hawthorn confining beds that confine the
aquifer (Back and Hanshaw 1970). An erosional remnant
of the Hawthorn is within the HSWG; it contains the
water table and is marked by a chain of lakes (Figure 3).
The two wells in the HSWG that experienced a 3-m rise
rather than a 2-m rise are either in the erosional remnant
(PW-7) or next to it (PW-3). The lower specific yield of
the siliciclastic sediments in the uneroded unit (PW-7) or
reworked into the cavernous porosity (PW-3) could
account for the different response and explain the occurrence of the steep gradient in the neighborhood of known
open conduits explored by cave divers at Ginnie Springs
along the Santa Fe River (Figure 3).
Discussion and Conclusions
Our findings have some broad implications to karst
hydrology that, we believe, warrant further discussion.
First, they support the concept of a threshold for rainfalls
that produce recharge in eogenetic karst. Second, they
suggest that it is relevant to draw a distinction between
caves and conduits in eogenetic karst.
The summer thunderstorms that characterize the rainy
season in our study area did not noticeably recharge the
aquifer, whereas the hurricanes certainly did. Martin and
Gordon (2000) similarly found at a nearby location that
summer rainfalls were inconsequential in comparison to
two rainfalls in the dry season. Farther away in Barbados,
Jones and Banner (2003) found from comparison of
oxygen isotope concentrations in rainfall and ground water
that the rainfalls of the rainy season did not recharge the
aquifer unless they exceeded 19.5 cm in a month, which
they called a threshold. Applying the technique to Puerto
Rico and Guam, they obtained uncannily similar thresholds
of 19 and 20 cm/month, respectively. If an even approximately similar threshold applies to our field area, where
summer rainfalls average 18 cm/month (Jordan 1984),
many summer months will not have recharging rainfalls. In
general in eogenetic karst, it cannot be assumed that
a rainy season, which is a common feature to these areas,
equates to a recharge season. If seasonal variations of temperature and thus potential evapotranspiration are also significant, the rainy season will coincide with the high
potential evaporation season. Both derive from the highsun season.
The distinction between caves and conduits is relevant
because the prevailing view of caves in karst hydrology is
heavily colored by the perspective of epigenic karst in telogenetic limestones. Thus (White 1988), ‘‘Seen in true
context, the theory of cave origins is nothing more than the
theory for the development of the conduit permeability of
a carbonate aquifer.’’ More recently, Klimchouk and Ford
(2000, 46) define karst as ‘‘. an integrated mass-transfer
system in soluble rocks with a permeability structure dominated by conduits dissolved from the rock and organized to
facilitate the circulation of fluid’’ (following Huntoon
1995). These views are appropriate for epigenic telogenetic
karst. In eogenetic karst, however, it is not appropriate
to assume that water courses through the limestone via
a system of integrated conduits among a diffuse continuum
of fracture permeability in otherwise uninvolved, lowpermeability rock. Rather, eogenetic karst features permeable rock. This permeable matrix contains fractures and
caves. Some of the caves are connected and form conduits.
Some are former conduits that are now choked with sediment that they are disconnected. Many formed as and
remain simply chambers (Mylroie and Carew 2000). All
L.J. Florea, H.L. Vacher GROUND WATER 45, no. 4: 439–446
445
permeability elements—matrix, fractures, caves that are
conduits, caves that are not conduits—come into play in
the hydrology of eogenetic karst.
Acknowledgments
We would like to thank our funding and equipment
donors for this project including the Southwest Florida
Water Management District, the Florida Department of
Environmental Protection, the National Speleological
Society, the Geological Society of America, the Society
of Sedimentary Petrology, the Gulf Coast Association of
Geological Sciences, and the Florida Studies Center at
the University of South Florida (USF). Additionally, we
wish to acknowledge the Citrus Tract of the Withlacoochee State Forest for access to the Radar Hill Quarry,
Karst Environmental Services and the Santa Fe Community College for the Bat Cave and the HSWG data, and
the Red Oak Farm in Ocala for access to Briar Cave.
Finally, we thank all of the cavers and individuals in the
USF Karst Research Group that have helped install and
maintain the field equipment. Jon Martin and an anonymous reviewer provided helpful guidance in earlier versions of this manuscript.
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