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Influence of partial confinement and Holocene river formation
on groundwater flow and dissolution in the Florida
carbonate platform
J. D. Gulley,
1
*
,†
J. B. Martin,
2
P. Spellman,
2
P. J. Moore
2,3,{
and E. J. Screaton
2
1
University of Texas, Institute for Geophysics, JJ Pickle Research Campus Bldg 196, Austin TX 78758, USA
2
University of Florida, Department of Geological Sciences, 241 Williamson Hall, Gainesville FL 32611, USA
3
ExxonMobil Development Company, CORP-GP4-541, 16945 Northchase Drive, Houston, TX 77060, USA
Abstract:
Much of what is known about groundwater circulation and geochemical evolution in carbonate platforms is based on platforms
that are fully confined or unconfined. Much less is known about groundwater flow paths and geochemical evolution in partially
confined platforms, particularly those supporting surface water. In north-central Florida, sea level rise and a transition to a wetter
climate during the Holocene formed rivers in unconfined portions of the Florida carbonate platform. Focusing on data from the
Santa Fe River basin, we show river formation has led to important differences in the hydrological and geochemical evolution of
the Santa Fe River basin relative to fully confined or unconfined platforms. Runoff from the siliciclastic confining layer drove
river incision and created topographic relief, reorienting the termination of local and regional groundwater flow paths from
the coast to the rivers in unconfined portions of the platform. The most chemically evolved groundwater occurs at the end of the
longest and deepest flow paths, which discharge near the center of the platform because of incision of the Santa Fe River at the
edge of the confining unit. This pattern of discharge of mineralized water differs from fully confined or unconfined platforms
where discharge of the most mineralized water occurs at the coast. Mineralized water flowing into the Santa Fe River is diluted
by less evolved water derived from shorter, shallower flow paths that discharge to the river downstream. Formation of rivers
shortens flow path lengths, thereby decreasing groundwater residence times and allowing freshwater to discharge more quickly to
the oceans in the newly formed rivers than in platforms that lack rivers. Similar dynamic changes to groundwater systems should
be expected to occur in the future as climate change and sea level rise develop surface water on other carbonate platforms and
low lying coastal aquifer systems. Copyright © 2012 John Wiley & Sons, Ltd.
KEY WORDS carbonate platform; basin hydrology; Eogenetic karst; upper Floridan aquifer; climate change; sea level rise
Received 16 April 2012; Accepted 8 October 2012
INTRODUCTION
Carbonate platforms have complex fluid circulation
patterns because of their large width to height ratios and
heterogeneous distributions of porosity and permeability
(Back and Hanshaw, 1970; Choquette and Pray, 1970;
Vacher and Quinn, 1997; Plummer and Sprinkle, 2001;
Vacher and Mylroie, 2002). Fluid circulation can be
driven by gradients in temperature, salinity or topography
(Kohout, 1967; Back and Hanshaw, 1970; Hughes et al.,
2008) and can alter distributions of post-depositional
porosity and permeability through geochemical reactions
such as dolomitization, dedolomitization, dissolution and
cementation (e.g. Wilson et al., 2001; Machel, 2004;
Whitaker and Xiao, 2010). Patterns of fluid circulation
and the associated distribution of enhanced permeability
remain poorly constrained, however, resulting in large
uncertainties in characterizations of the storativity and
transmissibility of aquifers and hydrocarbon reservoirs
that develop in carbonate platforms (White, 2002; Dong
et al., 2003). Shallow fluid circulation and the formation
of porosity and permeability in carbonate platforms are
currently best understood on platforms that are either fully
confined or fully unconfined. The presence or absence of
confining units is recognized to cause differences in
platform circulation and dissolution (Figure 1; Back and
Hanshaw, 1970).
Unconfined carbonate platforms, such as the Yucatan
Peninsula and the Bahamian archipelago, have limited
surface water due to the high matrix porosity and
permeability of the carbonate rock (i.e. eogenetic karst,
Vacher and Mylroie, 2002), although surface water can
occur where sea level raises water tables above
topographic lows (Martin and Gulley, 2010). High matrix
permeability allows rainfall to infiltrate in a diffuse manner
and recharge freshwater lenses that float buoyantly on top of
denser saline water of marine origin. The thickness of these
freshwater lenses depends on the exposed land surface area,
amount of recharge, the volume and distribution of
secondary porosity and permeability (e.g. Cant and Weech,
*Correspondence to: J. D. Gulley, University of Texas, Institute for
Geophysics, JJ Pickle Research Campus Bldg 196, Austin, TX 78758,
USA.
E-mail: gulley.jason@gmail.com
†
Current Address: Michigan Technological University, Department of
Geological and Mining Engineering and Sciences, 630 Dow Environ-
mental Sciences, 1400 Townsend Drive, Houghton, MI 49931, USA.
{
Current Address: ExxonMobil Development Company, CORP-GP4-541,
16945 Northchase Drive, Houston, TX 77060, USA.
HYDROLOGICAL PROCESSES
Hydrol. Process. 28, 705–717 (2014)
Published online 20 November 2012 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/hyp.9601
Copyright © 2012 John Wiley & Sons, Ltd.
1986; Budd and Vacher, 1991; Martin and Moore; 2008;
Martin and Gulley, 2010; Martin et al., 2012). Diffuse
recharge absorbs CO
2
in the soil and vadose zone to create
carbonic acid, which in turn dissolves carbonate minerals
and increases porosity and permeability along shallow flow
paths that radiate from the center of the platform towards the
coast. Chemical compositions are controlled by carbonate
mineral dissolution and mixing of fresh and saline water
(Figure 1; Back and Hanshaw, 1970; Back et al., 1986;
Vacher et al., 1990; Reeve and Perry, 1994).
In carbonate platforms that are fully confined, such as
in central Florida, groundwater flow paths and geochemical
evolution differ from those in unconfined platforms
(Figure 1; Back and Hanshaw, 1970). Confining units
establish surface drainage networks, but rivers and
streams are largely decoupled from the underlying
carbonate aquifer system. In aquifers below the confining
layer, groundwater flow paths radiate from a central
potentiometric high toward the coast (Back and Hanshaw,
1970; Miller, 1986; Plummer and Sprinkle, 2001), and
groundwater composition evolves systematically down-
gradient. Water—rock reactions progressively increase
the total dissolved solids along these flow paths, primarily
as coupled gypsum dissolution and dedolomitization
reactions increase concentrations of Mg
2+
,andSO
4
2
(Back and Hanshaw, 1970; Jones et al., 1993; Wicks and
Herman, 1994; Plummer and Sprinkle, 2001).
Less is known about dissolution, groundwater flow
paths and geochemical evolution in platforms that are
partially confined, although their modern flow patterns
and evolution through time are likely to be more complex
than fully confined or fully unconfined platforms.
Partially confined platforms have hydrogeological bound-
aries that are defined by the transition from confinement
to non-confinement. These boundaries should affect
groundwater circulation and dissolution by establishing
the conditions necessary for widespread interactions
between surface water and groundwater (Upchurch and
Lawrence, 1984; Martin and Dean, 2001; Gulley et al.,
2011). Surface water flows off of confining units and
across adjacent unconfined portions of platforms, result-
ing in interactions of surface water and groundwater in
river channels or in subsurface conduits, before water can
discharge to the ocean along the coast (Martin and Dean,
2001; Grubbs and Crandall, 2007). Runoff from confining
layers should dilute groundwater inputs to rivers in the
unconfined regions and lead to disequilibrium of the
water and aquifer rocks. As a result, patterns of
dissolution in partially confined platforms should differ
from platforms that lack hydrogeological boundaries
(Gulley et al., 2011; Gulley et al., in review). Because
of feedbacks between dissolution and groundwater flow,
these links between surface water and groundwater
interactions with dissolution should lead to patterns
of groundwater flow in partially confined platforms
that differ from platforms that are fully confined and
fully unconfined.
In this paper, we use data from streams within the Santa
Fe River basin, in north-central Florida (Figure 2), to
investigate relationships between surface water and
groundwater in partially confined carbonate platforms.
We discuss how river formation, caused by increasing
water table elevations during sea level rise, and river
incision, caused by dissolution by runoff from confining
units, affected groundwater circulation. These new data
are compared to conceptual models of dissolution and
groundwater circulation developed by Back and Hanshaw
(1970) for the fully confined central Florida platform and
the fully unconfined Yucatan Peninsula. While our study
focuses on the Santa Fe River Basin, the results of this
study should be generally transferable to any partially
confined aquifer with high matrix permeability.
Figure 1. A) Potentiometric surface map and reconstructed flow paths within the Central Florida Platform. Note that surface water (heavy grey lines) is
present, but the confining layer prevents rivers from interacting with the underlying carbonate aquifer. B) Hypothetical flow paths within the Yucatan
Peninsula. The lack of potentiometric surface contours results from low hydraulic gradients. Note, surface water is absent. C) Piper diagram for waters
sampled along flow paths depicted in panels A and B. All panels modified from Back and Hanshaw (1970)
706 J. D. GULLEY ET AL.
Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. 28, 705–717 (2014)
Study location
In north-central Florida, pre-Miocene carbonate rocks
make up the Floridan aquifer system. These rocks are
partially overlain by siliciclastic rocks of the Hawthorn
Group that confines the underlying aquifers (Scott, 1988).
The Floridan aquifer system includes a middle confining
unit that, where present, divides the system into the upper
and lower Floridan aquifers (Miller, 1986). The Hawthorn
Group is missing over a portion of north-central Florida,
and thus the Upper Floridan Aquifer is confined only in
the northern and eastern portions of the Santa Fe River
Basin (Figure 2). The boundary between the confined and
unconfined Floridan aquifer system is an erosional marine
terrace called the Cody Scarp (Scott, 1988; Figure. 2).
The Santa Fe River basin has an area of 3,558 km
2
and
is a tributary to the larger Suwannee River, which has a
drainage basin of ~25 830 km
2
. About 40% of the Santa
Fe River basin is covered by the Hawthorn Group
confining unit. Two major rivers are perched on the
confining unit, the New River and the upper Santa Fe
River, which derive their flow from surface runoff or
drainage from the thin (~3–10 m) siliciclastic surficial
aquifer system. The upper Santa Fe River flows 40 km
west from Lake Santa Fe, coalescing with the New River
before disappearing into a series of swallets, including the
River Sink, at the Cody Scarp (Figure 2). The river
reemerges at the River Rise about 7 km from the River
Sink. The River Sink and River Rise are linked by a system
of conduits delineated by cave diving surveys and chemical
and temperature tracing studies (Martin and Dean, 2001;
Screaton et al., 2004; Moore and Martin, 2005; Martin et al.,
2006; Moore et al., 2009; Ritorto et al., 2009; Bailly-Comte
et al., 2010; Moore et al., 2010). Water flowing into the
River Sink mixes with groundwater during flow through the
conduits, but the extent of mixing depends on the stage of
the river and the elevation of the groundwater (Martin and
Dean, 2001; Screaton et al., 2004; Martin et al., 2006;
Moore et al., 2009).
Downstream from the Cody Scarp, all additional inputs
of water to the lower Santa Fe River are from
groundwater discharging from the underlying carbonate
aquifers, 85–90% of which discharges from discrete springs
rather than diffusely through the river bed (Grubbs and
Crandall, 2007). Other than short spring runs, the
Ichetucknee River is the only tributary to the lower Santa
Fe River, and its headwater is the Ichetucknee Springs group
(Martin and Gordon, 2000). Dye traces from karst windows
in the Ichetucknee River Basin to the Ichetucknee Springs
group indicate rapid flow-through times and the presence of
extensive conduit networks (Martin and Gordon, 2000). The
lack of confinement below the Cody Scarp allows extensive
exchange between surface water and the Floridan aquifer
system (Grubbs, 1998; Grubbs and Crandall, 2007; Moore
et al., 2009; Moore et al., 2010; Gulley et al., 2011).
Groundwater near the Cody Scarp has a wide range of
chemical compositions that indicate the presence of at least
three distinct sources of water (Moore et al., 2009). One
source has elevated Mg
2+
,Ca
2+
and SO
4
2
concentrations
that increase with decreasing spring discharge. This
composition reflects gypsum dissolution and associated
dedolomitization, and, along with its elevated temperature,
suggests the water upwells from several hundred meters
depth within the platform at rates of about 1 m/year (Moore
et al., 2009). Another source of water is shallow
groundwater that is saturated with respect to calcite but
lacks the higher temperatures and elevated Mg
2+
and SO
4
2
Georgia
Florida
Suwannee River
Santa Fe
River
New River
Ichetucknee
River
Cedar
Spring
Worthington
Springs
spring
sink
gauging station
Hildreth
Fort White
River
Sink
River
Rise
01020 40 km
Cody Scarp
Gulf of
Mexico
30 0’ 0” N
82 0’ 0” W83 0’ 0” W
Figure 2. The Santa Fe River Basin north central Florida, showing the distribution of the Hawthorn Group confining layer (grey area), Cody Scarp
(dashed line) and Suwannee River watershed (black line), rivers (grey lines), river gauging stations, springs and sinks
707INFLUENCE OF PARTIAL CONFINEMENT AND RIVER FORMATION ON GROUNDWATER
Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. 28, 705–717 (2014)
that is characteristic of the deep water source. Both the
shallow and the deep waters are saturated to supersaturated
with respect to calcite (Moore et al., 2009). A third source of
water is dilute runoff from the adjacent siliciclastic
confining unit. We refer to this runoff as ‘allogenic’runoff
(c.f. Palmer, 2001) to distinguish it from water derived from
portions of the Santa Fe River basin where the upper
Floridan Aquifer is unconfined. Allogenic runoff from the
confining layer is highly undersaturated with respect to
calcite and dominates the chemical composition of water at
the River Rise during high flow discharge conditions
(Martin and Dean, 2001).
While little has been published specifically on the
geological history of the Santa Fe or Suwannee River
basins, they appear to have formed recently. Presently,
surface water only happens where the water table exceeds
the elevation of the land surface in the portion of the basin
where the upper Floridan aquifer is unconfined. Elsewhere,
the high matrix permeability of the limestone allows all
effective precipitation to infiltrate into the ground, limiting
surface water. Several lines of evidence suggest surface
streams formed on the unconfined portion of the Floridan
aquifer only after water table elevations increased with
Holocene sea level rise. During the Last Glacial Maximum,
climate was drier in Florida, and aquifer levels were lower
than present (Watts and Stuiver, 1980; Watts and Hansen,
1988; Watts et al., 1992; Hughes et al., 2008). In
southwestern Florida, the water table elevation was a
minimum of 70m lower than present (Alvarez Zarikian
et al., 2005). Stalactites, which can only form during
subareal exposure, have been found at a depth of 21 m
below the modern water table in Warm Mineral Springs
(Clausen et al., 1975). The modern Suwannee River has its
headwaters in the Okefenokee Swamp, which was dry until
7500 cal year BP (Cohen et al., 1984). The paleochannel of
the Suwannee River, identified in seismic lines, extends
only about 15km across the continental shelf into the Gulf
of Mexico and disappears at a depth of 9 m below modern
sea level (Wright et al., 2005).
Evidence described above indicates that the Suwannee
River, and by extension, the Santa Fe River, mostly likely
formed as the water table rose above local topography late
in the Holocene (Gulley et al., in press). Prior to this
surface inundation, the modern Suwannee and Santa Fe
rivers would not have existed on the unconfined portion
of the platform because surface water would have
infiltrated into the high matrix permeability limestone.
In spite of the lack of surface drainage, there is no
evidence to suggest that runoff from the confining unit
flowed through an integrated network of conduits to
discharge to the Gulf of Mexico, as conduit morphologies
are unrelated to past or present surficial drainage features
(Gulley et al., in press). Such conduits could form by
mixing of fresh and salt water that result in carbonate
dissolution because reactant concentrations vary linearly
between mixtures of the two waters, but their saturation
state is controlled by a power law (Wigley and Plummer,
1976). Although this process has been proposed to have
dissolved conduits in the Bahamas Islands (Mylroie and
Carew, 1990) and the Yucatan Peninsula (Sanford and
Konikow, 1989), it is unlikely to cause carbonate
dissolution in the upper Floridan aquifer because increased
Ca
2+
concentration caused by gypsum dissolution results in
fresh groundwaters that are supersaturated with respect to
calcite (Wicks et al., 1995). The lack of a well-developed
conduit network, similar to those in the Yucatan (Smart
et al., 2006), and the lack of dissolution mechanisms at the
coast, suggests that that fresh water may have discharged
diffusely near the modern Suwannee River terminus during
lower sea levels, such as occurs in other non-conduit
dominated discharge points offshore of modern Florida (c.f.
Martin et al., 2007). Of the approximately ~30 submarine
springs that have been documented off the Florida coasts
(Scott et al., 2004), none are located by the Suwannee River
terminus, further indicating the lack of focused discharge in
this region.
METHODS
Data sources
River and spring discharge data were obtained from
United States Geological Survey (USGS) and Suwannee
River Water Management District (SRWMD) online
databases. The USGS data include daily river discharge
at five USGS gauging stations in the Santa Fe River Basin
(Figure 2). In the downstream direction, these stations
are New River (USGS 2321000); Worthington Springs
(USGS 2321500); Fort White (USGS 2322500);
Ichetucknee River (USGS 2322700) and Hildreth (USGS
2322800). Discharge at Hildreth can be reduced during
backflooding by the Suwannee River, which frequently has
floods that are out of phase with the Santa Fe River (Kelly
and Gore, 2008). Rainfall data from Alachua, Florida were
used for the Santa Fe River Basin and were downloaded
from the Florida Automated Weather Network (http://fawn.
ifas.ufl.edu/data/reports/).
Chemical compositions of water are collected at monthly
to quarterly intervals, measured for major element concen-
trations and available for download (http://fl-suwanneeriver.
civicplus.com/) from the SRWMD. We augmented this
legacy data with water samples that were collected quarterly
from the River Sink, Sweetwater Lake, the River Rise and
several groundwater monitoring wells in the gap between
the River Sink and River Rise. The samples were measured
for their specific conductivity (SpC), dissolved oxygen
concentrations, pH and T while in the field. Alkalinity was
titrated immediately, and major element concentrations
were measured within one week of returning from the field.
Samples were analyzed in accordance with the Environment
Protection Agency (EPA) regulations for each analyte
(EPA, 1983) by Advanced Environment Laboratories, Inc.,
in Gainesville, FL. In this paper, we focus on geochemical
data collected between 2005 and 2007, after which regular
collection of water samples at the River Rise ceased. Data
from 2005 to 2007 bracket discharges that range from highs
of 89 m
3
s
1
to lows of 16 m
3
s
1
at the Fort White gauging
station, which is similar in magnitude to discharge in other
708 J. D. GULLEY ET AL.
Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. 28, 705–717 (2014)
years when flooding occurred in late winter and early spring
(Figure 3A,B).
Most samples had charge balance errors lessthan +/5%.
Larger charge balance errors occurred during floods when
waters were dilute, often with SpC of <100 mScm
, and had
elevated total organic carbon (TOC) concentrations.
Negatively charged organic carbon molecules can contribute
to charge balance as well neutralize acid used in alkalinity
titrations, causing increasingly positive charge balance
errors as TOC concentrations increase (Hemond, 1990;
Gulley et al., in review). Small errors in alkalinity
measurements during flood events when water samples are
dilute can translate into large charge balance errors.
Compositions of each sample were used to calculate
calcite saturation indices (SI
CAL
) using PHREEQC
version 2.15.0 (Parkhurst and Appelo, 1999). The SI
CAL
is defined as the log of the ratio of the ion activity product
to calcite equilibrium constant. All saturation indices were
calculated by forcing a charge balance alternately on Ca
2+
and alkalinity concentrations to assess potential errors in
calculations of SI
CAL
contributed by positive charge
balance errors during floods. Bracketing the charge
balance between the major cation and anion in solution
should cover the full range of uncertainties, including
analytical uncertainty, in calculations involving limestone
dissolution described below.
Temperature, water-level and SpC data were measured
with automated conductivity, temperature and depth
(CTD) loggers at 10 min intervals at locations throughout
the Santa Fe River Basin during the transition from
baseflow, in late July 2008, to flooding associated with
Tropical Storm Fay, which continued through mid
September 2008. These data were collected from the
River Rise, Sweetwater Lake (a karst window along the
conduit flow path between the River Sink and the River
Rise), USGS gauging stations and at Cedar Spring in the
Ichetucknee Springs group. Although CTD loggers were
also deployed at Ichetucknee River and the River Rise,
data are missing because of vandalism at the Ichetucknee
River and because the logger was buried by sand at the
River Rise, compromising the SpC data but not discharge
records. Consequently, SpC data from Cedar Spring are
used to represent the Ichetucknee River, and SpC data
from Sweetwater Lake are used to represent River Rise
data. Water chemistry at Cedar Spring is similar to several
springs that discharge to the Ichetucknee River (Martin
and Gordon, 2000), and water at Sweetwater Lake is
chemically similar to water at the River Rise (Martin and
Dean, 2001; Moore et al., 2009). Depth data from the
River Rise were converted to discharge based on stage–
discharge relationships (Screaton et al., 2004).
Groundwater flow paths
Flow paths to rivers and springs were drawn on a
potentiometric surface map constructed using ArcMap 9.3
from data collected in May 2000 of the Upper Floridan
aquifer (shapefile provided by the Florida Geological
Survey). The flow paths were plotted assuming that
groundwater will cross potentiometric surfaces at right
angles (Freeze and Cherry, 1979). Flow path reconstruction
using basin scale potentiometric surface maps can result in
oversimplification of actual flow paths, as local-scale
features such as small conduits can cause diversions of
flow paths that may not be observable at the basin scale.
Because reconstructed flow paths were used primarily to
qualitatively assess relationships between downstream
changes in the geochemical composition of the Santa Fe
River and sources of groundwater inflow to the river, such
local scale diversions should not affect the general
conclusions of this work.
RESULTS
Flow 2005–2007
Between 2005 and 2007, Santa Fe River discharge varied
over several orders of magnitude (Figure 3). The greatest
flow and maximum discharges occurred in the upper reaches
of the watershed where the Floridan aquifer system is
confined. Maximum discharges of ~48 m
3
s
1
and
~100 m
3
s
1
occurred during this time at the New River
and Worthington Springs gauging stations, respectively.
These flood hydrographs are flashy, having steep rises to
peak discharge and rapid decreases to pre-flood conditions.
In contrast, flood hydrographs at Ft. White on the
Q m s
Rain (cm)
3-1
140
120
100
80
60
40
20
0
1/2005 7/2005 1/2006 7/2006 1/2007 7/2007 1/2008
0
5
10
15
20
T.S.
Fay
1930 1940 1950 1960 1970 1980 1990 2000 2010
0
200
300
400
500
600
100
Q m s
3-1
A
B
Figure 3. A) Discharge for the period of record at the Worthington
Springs and Ft White gauging stations on the Santa Fe River. B) River
discharge and rainfall at select gauging stations in the Santa Fe River
Basin during the period that water samples were collected for this study,
also indicated by the shaded period in panel A
709INFLUENCE OF PARTIAL CONFINEMENT AND RIVER FORMATION ON GROUNDWATER
Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. 28, 705–717 (2014)
unconfined Floridan aquifer are characterized by a steep rise
to peak discharge but longer recession curves than at New
River and Worthington Springs. No flow occurred at the
New River or Worthington Springs gauging stations during
a persistent drought that lasted from late summer in 2006
until late summer 2007. The lowest discharge (~16 m
3
s
1
)
at Fort White was recorded in December 2007 near the end
of the 2006–2007 drought.
Peak discharge at the Fort White gauging station, on
the lower Santa Fe River, occurred during April 2005
flooding and reached a maximum value of ~90 m
3
s
1
,
about 10 m
3
s
1
lower than peak discharge of 100 m
3
s
1
at Worthington Springs (Figure 3). Following a sustained
recession in flows, later flooding in December 2005
resulted in peak discharge at the Fort White gauging
station that was ~70 m
3
s
1
, approximately 21 m
3
s
1
less
than the peak discharge of ~91 m
3
s
1
measured at
Worthington Springs. This downstream decrease is nearly
twice the decrease in peak discharge observed
during April 2005 flooding, which occurred following a
period of normal rainfall and higher antecedent discharges
(Figure 3).
Ichetucknee River discharge shows only slight
variability and does not exhibit the rapid changes in
discharge observed at the other gauging stations in the
basin (Figure 3). The largest change in discharge occurred
when the Santa Fe River backflooded the Ichetucknee
River in April 2005, decreasing the discharge from
13.5 m
3
s
1
to ~4 m
3
s
1
. Peak discharge during this
period of record of 15 m
3
s
1
occurred immediately after
the April 2005 flooding. Similar to the other gauging
stations, Ichetucknee River discharge decreased through
the 2006–2007 drought, with discharge decreasing to
6.8 m
3
s
1
by December 2007.
Santa Fe River high-resolution SpC record –transition
from drought to flood
High temporal resolution measurements exhibit rapid
changes in discharge and SpC of Santa Fe River water
during the transition from drought to flooding caused by
Tropical Storm Fay between August–September 2008
(Figure 4). Over a three-day period of time, Tropical Storm
Fay delivered ~11.4 cm of rain to the basin, causing a flood
to be initiated on the confining unit and to propagate
downstream. Discharge increased from a maximum of
~145 m
3
s
1
at New River on 24 August to ~168 m
3
s
1
at
Worthington Springs less than 24h later (Figure 4A). Peak
discharge at the River Rise occurred on 27 August but was
71 m
3
s
1
lower (97 m
3
s
1
) than peak discharge at
Worthington Springs. Peak discharges of 108 m
3
s
1
and
104 m
3
s
1
occurred on 29 August at Fort White and 30
August at Hildreth. Discharge in the Ichetucknee River
increased from 7.8 m
3
s
1
to 11 m
3
s
1
on 30 August.
Similar to all floods in the basin (Figure 3), recession limbs
of flood hydrographs were longer for gauging stations in
portions of the basin where the upper Floridan aquifer was
unconfined relative to portions where the upper Floridan
aquifer was confined (Figure 4A).
During baseflow conditions before Tropical Storm Fay,
SpC was lowest at Worthington Springs and New River,
averaging <180 mScm
1
. These values varied by about
50 mScm
1
throughout the baseflow conditions (Figure 4B).
SpC was higher at Hildreth and Fort White (380 and
400 mScm
1
), but these values varied little until the storm.
The highest value in the basin was recorded at Sweetwater
Lake (~525 mScm
1
). At Ichetucknee, SpC remained
relatively constant at ~307 mScm
1
.
With the exception of Ichetucknee, SpC decreased at all
stations during the flood, but the magnitude, characteristic
shape and timing of the decrease varied downstream
(Figure 4B). In the confined region, Worthington Springs
and New River both decreased ~100 mScm
1
(from
150 mScm
1
to 50 mScm
1
) over several days. In contrast,
the SpC of water decreased sharply and sequentially
downstream from Sweetwater Lake to Fort White to
Hildreth with a lag of about one day between each station.
The decrease at Sweetwater Lake was around 340 mScm
1
(from 400 mScm
1
to 60 mScm
1
), at Fort White about
290 mScm
1
(from 390 mScm
1
to around 100 mScm
1
),
and at Hildreth about 230 mScm
1
(350 mScm
1
to ~120 mScm
1
). At upstream gauging stations,
SpC recovered to near pre-storm values in about 20days,
but at Sweetwater Lake and farther downstream, SpC did
not return to the pre-storm values within the period of
record. At Ichetucknee, SpC remained generally constant
throughout the flood at ~307 mScm
1
, though a small
increase in SpC to ~320 mScm
1
occurred on 30 August.
SpC µS/cm
Q m /s
3
Q m /s
3
8/4 8/11 8/18 8/25 9/1 9/8 9/15
600
500
400
200
100
0.0
7/28
300
100
80
60
40
20
0
0
40
80
120
160
8/4 8/11 8/18 8/25 9/1 9/8 9/15
7/28
New River
Worthington Springs
River Rise
Fort White
Ichetucknee River
Hildreth
A
B
Figure 4. A) Discharge and B) SpC at gauging stations in the Santa Fe
River Basin prior to and during flooding caused by Tropical Storm Fay in
2008. Note: the legend shown in panel A also applies to panel B. The
dashed line in panel B is the discharge at Fort White. Locations of all
gauging stations are shown in Fig. 2
710 J. D. GULLEY ET AL.
Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. 28, 705–717 (2014)
Following this increase, SpC at Ichetucknee began
gradually declining,reaching 313mScm
1
on 17 September
at the end of the period of record.
Discharge and SI
CAL
With the exception of Ichetucknee River, water at all
gauging stations becomes increasingly undersaturated with
respect to calcite as discharge increases (Figure 5). Water
flowingontheconfining unit (the New River and
Worthington Springs gauging stations) is undersaturated
with respect to calcite across the entire range of discharges,
with saturation indices approaching 5 when discharges
increase above 20 m
3
s
1
. Water at gauging stations
downstream of the Cody Scarp is saturated to supersaturated
with respect to calcite at low discharges but becomes
increasingly undersaturated with respect to calcite at higher
discharges. The discharge at which water becomes
undersaturated increases downstream from the Cody Scarp,
ranging from 3 m
3
s
1
at the River Rise, 16.5 m
3
s
1
at Fort
White, and 58 m
3
s
1
at Hildreth. For an equivalent
discharge, water on the confining unit has a calcite
saturation index that is 2to3 units lower than water at
gauging stations below the Cody Scarp (Figure 5).
In contrast to changes in the main stem of the Santa Fe
River, relationships between SI
CAL
and discharge remain
constant across the entire range of recorded discharges in the
Ichetucknee River (Figure 5). Water in the Ichetucknee
River was saturated to supersaturated with respect to calcite
across nearly the entire range of discharges, although brief
periods of slight undersaturation (<0.4) were recorded
during periods when the Santa Fe River backflooded the
Ichetucknee River (Figure 5).
Major element chemistry
Water in the Santa Fe Basin consists of several end
members based on its major element compositions, and
these end members show various mixing trends (Figure 6).
In the region where the upper Floridan aquifer is
unconfined (Fort White, Hildreth and the Ichetucknee
SI
CAL
Q m s
3-1
Hildreth
Fort White
New River
Worthington Springs
River Rise
Ichetucknee River
Figure 5. Calcite saturation index plotted against the log of the discharge
for each gauging stations within the Santa Fe River Basin. Error bars
represent estimates for SI
CAL
based on charge balance of samples forced
on either Ca
2+
concentrations or alkalinity
Na+ + K+
0
10
20
30
40
50
60
70
80
90
100
Mg2+
0
10
20
30
40
50
60
70
80
90
100
Ca2+
0102030405060708090100
Cl-
0 102030405060708090100
SO42-
0
10
20
30
40
50
60
70
80
90
100
HCO3- + CO32-
0
10
20
30
40
50
60
70
80
90
100
100
90
80
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
Ca2+ + Mg2+
SO42- + Cl-
100
90
80
Hildreth
Worthington Springs
Fort White
Well 4
River Rise
New River
Ichetucknee River
Well 2
AA
B
C
Figure 6. Piper diagram of water samples collected from gauging stations in the Suwannee River basin in addition to Wells 2 and 4 at Oleno State Park.
Line A shows the mixing trend of water discharging from the River Rise with deep water from Well 2 during low flow conditions. Line B shows the
mixing trend of water from the River Rise with flood waters flowing off of the confining layer at high discharges. Line C shows the evolution of runoff
from the confining layer from low conditions (start of the line) and high flow (near the arrow head). Modified from Moore et al. (2009)
711INFLUENCE OF PARTIAL CONFINEMENT AND RIVER FORMATION ON GROUNDWATER
Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. 28, 705–717 (2014)
River) and at Well 4 (near the River Rise), water
composition is largely dominated by Ca
2+
and HCO
3
.
While waters from Hildreth and Fort White are tightly
clustered, their water compositions trend towards the
Well 4 water compositions at low discharge and towards
compositions of flood waters on the confining unit at high
discharges. River waters on the confining unit are
dominated by Ca
2+
,Na
+
,Mg
2+
and HCO
3
with
increasing relative amounts of Na
+
and Cl
during high
flow. During floods, these Na
+
and Cl-rich waters form a
mixing line with the Ca-HCO
3
-type water found in
regions of the basin where the upper Floridan aquifer is
unconfined. The third water type is dominated by Ca
2+
,
Mg
2+
and SO
4
2
, and is found in Well 2 near the Santa Fe
River Sink–Rise. During low flow conditions, water
from the River Rise falls on the mixing line between the
Ca
2+
-HCO
3
and the Ca
2+
,Mg
2+
and SO
4
2
waters.
During higher discharge conditions, composition of water
at the River Rise trends toward the Na
+
and Cl-dominant
waters derived from runoff from the confining unit.
DISCUSSION
We discuss below how the discharge and chemical
composition of water flowing through the Santa Fe River
basin changes through space and time during transitions
from baseflow to floods. In the modern basin, these
changes reflect contributions to the river of varying
fractions of different sources of groundwater and runoff
from the confining layer. The Santa Fe River did not
exist, however, prior to an increase in water table
elevation caused by Holocene sea level rise and climate
change. Consequently, all groundwater would have
discharged at the coast. Our data suggest that the
formation and incision of rivers caused changes in the
direction of groundwater flow paths, reorienting
them from the coast toward the river in unconfined
portions of the platform, ultimately producing the modern
flow system.
Influence of Santa Fe River on circulation –groundwater
mixing
During baseflow, when inputs of allogenic runoff are
minimal, variations in the chemistry of water in the lower
Santa Fe River basin results from variations in the lengths
and depths of groundwater flow paths and their discharge
points to the river (Figure 7). Groundwater flow paths that
discharge to the Santa Fe River exhibit variable lengths.
The lengths of these flow paths depend on the location of
groundwater highs and the proximity of these highs to
rivers. We discuss flow path lengths and their potential
influence on river chemistry below by referring to a few
select flow paths (Figure 7) that represent some of the
longest paths that groundwater might follow in different
regions of the Santa Fe River basin. Flow paths with the
longest lengths occur in the eastern portion of the basin.
As an example, flow path A in Figure 7 is 74 km long.
Flow path A extends from a groundwater high near the
center of the platform and ends near the River Rise. The
termination of flow path A, and other long flow paths
within the region that would upwell at a similar location,
also coincides with water which has the highest measured
SpC values of ~510 mScm
1
in the Santa Fe River
(Figure 4B). This high SpC corresponds to elevated Mg
2+
and SO
4
2
concentrations in the groundwater from
gypsum dissolution and dedolomitization (Moore et al.,
2009). Similarly, elevated Mg
2+
and SO
4
2
concentrations
occur near the end of flow paths of similar length in
Central Florida because of dedolomitization reactions
(Back and Hanshaw, 1970; Figure 1). This water
chemistry suggests long horizontal flow paths extend
deep into the aquifer before upwelling at rivers in
unconfined portions of partially confined platforms, such
as the Santa Fe River basin, or along the coast, as in the
fully confined central Florida Platform.
Flow paths to portions of the lower Santa Fe River
downstream of the River Rise are shorter than flow paths
terminating at the River Rise (Figure 7). Flow paths C
(north of the river) and B and D (south of the river), are
40 km, 17 km and 7 km long, respectively. Water flowing
along these short flow paths would have shorter residence
times in the subsurface than water flowing along flow
path A, limiting the time for dissolution reactions.
Additionally, the depth of short flow paths would be
82°0'0"W
82°0'0"W
83°0'0"W
83°0'0"W
30°0'0"N 30°0'0"N
010205km
Gulf of
Mexico
0-5 m
5-10 m
10-15 m
15-20 m
20-25 m
25-30 m
30-35 m
35-40 m
40-45 m
45-76 m
Suwannee
River 312 15
6
12 12
15
18
21
12
15
12
9
6
3
9
12
18
21
24
A
B
C
D
Figure 7. Potentiometric (contour lines) and topographic (shaded inter-
vals) surface map of the upper Floridan aquifer using water table elevation
data from May 2000. Potentiometric contours have units of meters.
Flowpaths discussed within the text are shown as arrows, each of which
have been ascribed a letter. Key river gauging stations and sampling
locations are indicated by numbered squares here. Note, due to the
proximity of some sites and the scale of the figure, more than one site is
occasionally covered by one of the numbered boxes. All locations are
depicted in greater detail in Fig 2. In a downstream direction: 1 –New
River gauging station; 2 –Worthington Springs gauging station; 3 –
sampling points at the Santa Fe River Sink–Rise systems (Oleno State
Park), including the River Sink, Sweetwater Lake and Wells 2 and 4; 4 –
Fort White gauging station, including the Devil’s Cave System; 5 –
Ichetucknee River Gauging station, including Cedar Spring; 6 –Hildreth
gauging station
712 J. D. GULLEY ET AL.
Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. 28, 705–717 (2014)
shallower than longer flow paths (Toth, 1999), and this
shallow flow would not react with the gypsum that is
found deeper in the aquifer (c.f. Moore et al., 2009).
Limited mineralization is shown by decreasing SpC
values downstream from ~512 mScm
1
at Sweetwater
Lake to ~400 mScm
1
at Fort White and 360 mScm
1
at
Hildreth as less mineralized water discharges to the river.
Decreasing concentrations of Mg
2+
and SO
4
2
contribute
to the decrease in SpC values and indicate water
following these short flow paths have less interactions
with gypsum and dolomite in the lower Floridan Aquifer.
This downstream, coastward decrease in SpC, Mg
2+
and
SO
4
2
concentrations contrasts with the increase in SpC,
Mg
2+
and SO
4
2
that occurs along single, long flow paths
that extend towards the coast in fully confined or fully
unconfined carbonate platforms (i.e. Figure 1; Back and
Hanshaw, 1970).
The measured decreases in SpC values downstream can
be used to estimate average SpC values of groundwater in
the lower Santa Fe River basin, and, by comparing
estimated groundwater SpC values to SpC values of
known water sources, we can make inferences about their
relative contributions to flow. These calculations assume
that SpC is conservative so that changes in the SpC of
river water result from simple mixing between the
groundwater sources and the river water. We consider
this assumption to be reasonable during baseflow
conditions because groundwater inflow to the lower
Santa Fe is at or near equilibrium with respect to calcite
and thus should not change its composition (or SpC
value) through mineral dissolution or precipitation
reactions (Figure 5). During the drought prior to Tropical
Storm Fay, baseflow discharge was 3.4 m
3
s
1
at the
River Rise, increasing to 16.4 m
3
s
1
at the Fort White
gauging station due to discharge of multiple springs along
this reach of the river, and 28.5 m
3
s
1
at Hildreth, which
includes a contribution of ~7 m
3
s
1
of groundwater from
the Ichetucknee River (Figure 4A). To reduce the SpC
from 510 to 400 mScm
1
between the River Rise
(3.4 m
3
s
1
) and Ft White (16.4 m
3
s
1
), groundwater
would need to have a SpC value of 371 mScm
1
and to
account for the dilution from 400 to 360 mScm
1
between Ft White (16.4 m
3
s
1
) and Hildreth
(28.5 m
3
s
1
), groundwater would need to have an
average SpC of 305 mScm
1
.
The estimated SpC of groundwater inputs to the Santa
Fe River downstream from the River Rise are similar to
the SpC of groundwater measured in the phreatic Devil’s
Cave System, which has conduits that extend beneath the
river, bringing groundwater from the north as well as the
south side of the river into the Santa Fe (Gulley et al., in
press). These conduits discharge to the Santa Fe River
via a series of springs a few km upstream of the Fort
White gauging station (Kincaid, 1998; Gulley et al., in
press). The northern conduits have average SpC values
of ~413 mScm
1
, while the southern conduits have
average SpC values of 345mScm
1
. The difference in
SpC values in water derived from north and south of the
river reflect flow path length, with longer flow paths
(similar in length to C) discharging water with higher SpC
values than water from shorter flow (similar in length to
flow paths B and D; Figure 7). SpC values in the Devil’s
Cave conduits only fluctuate by <10 mScm
1
between
drought and flood conditions and bracket the SpC value
of 371 mScm
1
value estimated to cause the value
measured at the Ft White gauging station. Assuming these
conduits represent SpC values of the groundwater inputs,
38% of inflow to the Santa Fe River would be derived
from north of the river, and 62% of inflow is derived from
south of the river. Between Fort White and Hildreth,
decreases in SpC and much of the increase in flow can be
explained by input from the Ichetucknee River, which has
a SpC of 307 mScm
1
, similar to the predicted SpC of
305 mScm
1
.
Although the flow path to the Ichetucknee River (flow
path C) is 40 km long, the SpC of water in the
Ichetucknee River is lower than the other sampling points
upstream with shorter flow paths (e.g. flow path D,
Figure 7). This discrepancy may arise because the actual
matrix flow path lengths of groundwater flowing into the
Ichetucknee River are shorter than we have predicted on
the basis of a potentiometric surface map that has a coarse
resolution (Figure 7). The Ichetucknee River is fed by
multiple springs that are known to have different
contributing areas (Martin and Gordon, 2000). Separate
spring basins are not readily identifiable from Figure 7,
but since approximately half of flow path C occurs where
the Floridan Aquifer is unconfined, some water dischar-
ging to the Ichetucknee Springs may be recent diffuse
recharge of dilute precipitation. Flow path C is therefore
likely to be an oversimplification of the recharge
dynamics. Nonetheless, the general trend is correct that
long regional flow paths that originate from beneath the
confining layer discharge highly mineralized water to the
Santa Fe River at the Cody Scarp and this mineralized
water is diluted downstream by less mineralized water
derived from shorter, shallower flow paths.
Impact of Cody Scarp on surface and groundwater mixing
The interaction of surface water and groundwater at the
Cody Scarp plays an important role in the quantity of
water in the lower Santa Fe River as well as its chemical
composition (e.g. Upchurch and Lawrence, 1984). Water
in the upper Santa Fe River is undersaturated with respect
to calcite regardless of discharge (Figure 5) because of
limited contact with carbonate minerals and because of
elevated CO
2
concentrations generated during organic
matter decomposition. As this undersaturated water flows
into the subsurface at the Cody Scarp, it reacts with the
carbonate aquifer rocks and mixes with groundwater that
has equilibrated with carbonate minerals, with different
sources of water discharging from the River Rise at different
ratios depending on magnitudes of flow contributions from
the headwaters of the Santa Fe River on the confining unit
(Martin and Dean, 2001; Moore et al., 2009; Moore et al.,
2010). During flood events, water from the upper Santa Fe
River becomes an increasingly large fraction of water
713INFLUENCE OF PARTIAL CONFINEMENT AND RIVER FORMATION ON GROUNDWATER
Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. 28, 705–717 (2014)
discharging from the River Rise, so water in the lower Santa
Fe becomes increasingly undersaturated with respect to
calcite (e.g. Figure 5). This undersaturated flood water thus
has the capacity to dissolve carbonate minerals in the lower
Santa Fe River basin.
The Sink–Rise system acts as a capacitor for the
delivery of allogenic runoff to the lower Santa Fe River. It
stores water in the aquifer matrix and attenuates discharge
peaks as flood pulses propagate from Worthington
Springs to the River Rise (Figure 3), shown clearly
during flooding resulting from Tropical Storm Fay
(Figure 4). The decrease in peak discharge and extended
hydrograph recession curves indicate that floodwater was
stored as rapid increases in conduit heads drove water
from the conduit into the aquifer (Moore et al., 2010;
Bailly-Comte et al., 2011; Langston et al., 2012). Long
tails on the recession hydrographs demonstrate this water
was later released from storage as conduit heads
decreased after the flood (Moore et al., 2010). This
attenuation and storage limits the magnitude of flooding
that would have occurred if runoff flowed directly from
the confining layer into the lower Santa Fe River without
being routed through a system of conduits connecting the
Sink and Rise.
Attenuation of floods at the Santa Fe Sink–Rise system
creates different flood responses relative to the Suwannee
River, where flood water flow directly onto the
unconfined Floridan aquifer without being attenuated in
a Sink–Rise system. As a result, stage increases in the
lower Suwannee River faster than groundwater heads in
unconfined regions of the aquifer, hydraulically damming
inputs of groundwater to the river and allowing the SI
CAL
of water to decrease to values as low as 5 along the
entire length of the Suwannee River (e.g. Gulley et al.,
2011; Gulley et al., in review). In contrast, flood waters in
the lower Santa Fe River only reach a minimum SI
CAL
of
2, which is two orders of magnitude closer to saturation
than water on the confining layer (Figure 5). This finding
suggests that hydraulic damming of springs in unconfined
portions of the upper Floridan aquifer is not as prominent
in the lower Santa Fe River as it is on the Suwannee
River. Water in the lower Santa Fe River may be closer to
equilibrium because of dissolution within the Sink–Rise
system, or because small inputs of groundwater that is
saturated with respect to calcite continue to flow into
Santa Fe River between the River Rise and gauging
stations downstream. The most likely cause of lack of
undersaturation on the Lower Santa Fe River is from
continuous discharge of spring water at equilibrium with
carbonate minerals because the composition of water
changes little between the River Rise and the River Sink
during flooding (Martin and Dean, 2001; Moore et al., 2009;
Moore et al., 2010). Our hypothesis that groundwater
discharging to the river limits the downstream decrease in
SI
CAL
is supported by the increased peak discharge between
the River Rise and downstream gauging stations during
Tropical Storm Fay (Figure 4A). Such groundwater inflow
is only possible where groundwater heads remain elevated
above river stage. Even though the effect of undersaturated
runoff is muted in the lower Santa Fe River relative to the
Suwannee River, the Santa Fe River water remains
undersaturated at Hildreth, suggesting dissolution, and
hence river incision, can occur in the river channel during
large allogenic runoff events.
The effects of dilute floodwaters downstream from the
Cody Scarp are also shown clearly as decreases in SpC
values by about 250 to 300 mScm
1
following Tropical
Storm Fay, reflecting the arrival of low SpC water from
the upper Santa Fe River basin (Figure 4B). Although the
SpC at individual gauging stations generally decreases
downstream of the Rise during baseflow, the minimum
SpC value of the flood waters increases downstream
(Figure 4B), reflecting a combination of inputs of
groundwater to the stream and dissolution. Both of these
processes increase the calcite saturation state of the flood
water downstream (Figure 5).
Rapid changes in discharge and geochemical composi-
tions of water that are characteristic of the main stem of
the Santa Fe River contrast with minimal changes in the
same parameters in the Ichetucknee River. Unlike the
River Sink–Rise system, only small intermittent streams
occur at the Cody Scarp upstream of the Ichetucknee
River. If any of this water resurges in the Ichetucknee
River, its volume is insufficient to change the SpC of
water in the river during Tropical Storm Fay (Figure 4B)
or to result in undersaturation with respect to calcite. In
addition, any allogenic runoff that recharged to the aquifer
had sufficiently long residence times to equilibrate with
calcite. The supersaturation that is frequently observed in
the Ichetucknee River results in part from changes in the
CO
2
concentrations in the stream that are driven by the
metabolic processes of subaquatic vegetation (de Montety
et al., 2011). The lack of undersaturation with respect to
calcite in the Ichetucknee River indicates it will not incise
the river channel through dissolution. Incision of river
channels by dissolution and rapid generation of topographic
relief is most likely restricted to rivers that receive allogenic
runoff from the confining layer.
Holocene sea level rise and implications for fresh
water resources
Although local and regional groundwater flow paths
currently discharge to the Santa Fe River, during the early
Holocene, the lower Santa Fe River is unlikely to have
existed (Watts and Stuiver, 1980; Watts and Hansen, 1988;
Watts et al., 1992; Gulley et al., in press). Low water tables
and high permeability of the aquifer rocks would have
precluded the formation of rivers in the region (Figure 8A,
B). All effective precipitation would have rapidly infiltrated
into the aquifer in regions of the watershed where the Upper
Floridan Aquifer was unconfined and a drier climate would
have resulted in less runoff from the confining layer (Cohen
et al., 1984). In the absence of rivers, the only location for
groundwater to discharge would have been the Atlantic
Ocean or the Gulf of Mexico, and this discharge could have
been as submarine groundwater discharge or diffuse seeps
along the coast. As a result, flowpaths in north-central
714 J. D. GULLEY ET AL.
Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. 28, 705–717 (2014)
Florida could only have extended outward from a central
high under the confining unit to the Atlantic and Gulf coasts,
a distance of about 125 km. Holocene sea level rise, coupled
with a transition to a wetter climate, increased the water
table to elevations where it exceeded lows in the surface
topography to form surface water (Figure 8C,D; Alvarez
Zarikian et al., 2005). Once the water table was sufficiently
elevated to allow runoff from the confining layer to stay at or
close to the land surface to form the modern Suwannee and
Santa Fe Rivers, flooding would drive incision through
dissolution. This incision created topographic relief and
perennial rivers where the water table locally exceeded the
elevation of surface topography. Simultaneously, ground-
water flowpaths would have been reoriented to discharge to
rivers downstream of the Cody Scarp but far inland from the
coast of the Gulf of Mexico. Consequently, we expect that
once river drainage developed on the unconfined portion of
the Floridan aquifer, that water draining from the confining
unit would predominately discharge as surface water rather
than groundwater to the Gulf of Mexico. Discharge of
surface water from the Floridan Aquifer differs considerably
from discharge from either the Yucatan Peninsula or from
the Central Florida high, which is all submarine (e.g. Back
and Hanshaw, 1970; Swarzenski et al., 2001).
Groundwater flow path reorganization following the
formation of the Suwannee and Santa Fe rivers would
have changed the location of discharge of highly
mineralized water from diffuse seepage faces at shallow
depths in the Gulf of Mexico to portions of rivers near the
Cody Scarp (e.g. Figure 7, flow path A). Mixing of this
deep water with groundwater that followed short flow
paths (e.g. Figure 7, Flow path D) in the Suwannee River
channel would have resulted in large volumes of less
mineralized flowing into the Gulf of Mexico at a discrete
point, rather than all groundwater from the platform
upwelling as highly mineralized water at seepage faces in
a diffuse manner.
Shortening of groundwater flow paths resulting from
surface water formation also has important implications
for volumes of water resources. Formation of surface
water results in more rapid depletion of freshwater
resources because groundwater flow paths upwelling in
rivers allows ground water to be discharged to the ocean
more rapidly than before rivers formed. Consequently, for
aquifers with similar hydraulic properties and climates,
average residence times of rainfall in aquifers are reduced
in basins that have rivers versus basins that lack rivers.
While results reported here emphasize the impacts of
river formation on groundwater flow path reorganization
and depletion of freshwater resources, similar reductions
in groundwater flow path lengths and volumes of
freshwater resources occurred on modern carbonate
platforms as a result of lake formation due to Holocene
sea level rise (Martin and Gulley, 2010). In the Bahamas
Islands, and other low lying carbonate platforms,
freshwater resources occur as thin lenses that float
buoyantly on underlying salt water (Vacher, 1988). On
some islands, Holocene sea level rise elevated fresh water
Water Table
Gulf of Mexico
Gulf of Mexico
Water Table
Surface Surface
A
B
C
D
Figure 8. Conceptual model for how variations in sea level and water table position affect flow paths in partially confined carbonate platforms. A) Cross-
section view of unconfined portions of a partially confined carbonate platform during lower sea levels. Elevation of the water table is below the land
surface with little or no surface water. Flow direction is out of the page. B) Plan view of the potentiometric surface (dashed lines) and flow paths (thick
arrows –deep flow paths; thin arrows –shallow flow paths) within the carbonate aquifer at lower sea levels. In the absence of surface water bodies,
groundwater can only discharge at the coast. C) Cross-section view of unconfined portions of a partially confined carbonate platform during higher sea
levels. Rivers form where the water table exceeds lows in the surface topography and incision into carbonates is promoted by runoff from the siliclastic
confining layer. Flowpaths within the carbonate aquifer that begin at topographic and potentiometric highs are redirected towards the river. D) Plan view
of the potentiometric surface (dashed lines) and flow paths (thick arrows –deep flow paths; thin arrows –shallow flow paths). A wetter climate during
interglacials results in surface drainage on the confining layer, but these rivers do not affect potentiometric contours in the underlying carbonate aquifer
because the rivers are isolated from the carbonate aquifer by the confining unit. Increased water table elevations allowed rivers to form where the
carbonate aquifer is unconfined. Deep regional groundwater flowpaths upwell in the river at the edge of the scarp and are diluted by shallow groundwater
flowpaths downstream
715INFLUENCE OF PARTIAL CONFINEMENT AND RIVER FORMATION ON GROUNDWATER
Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. 28, 705–717 (2014)
tables above the elevation of closed inland depressions to
form inland lakes. Evaporation of lake water reorganized
groundwater flow paths so that some water flowed to
discharge through the lakes via evaporation. Reduction in
groundwater flow path lengths and rapid discharge of
fresh water via evaporation resulted in upconing of saline
water to the lakes. This upconing causes islands with
inland lakes to lose more fresh water as a consequence of
rising sea level than islands with similar areas and
climates that lack lakes (Martin and Gulley, 2010).
Studies of low-lying coastal or island aquifer systems
should consider how changes in sea level, either past or
future, can affect groundwater flow path organization,
locations and magnitudes of fresh water discharge to
oceans or volumes freshwater resources through the
formation of surface water. Modern unconfined coastal or
island aquifers that are not perched on low permeability
strata and have water tables that are within ~1 m of the
land surface should be particularly vulnerable to sea level
rise. Because sea level rise of as much as 0.8 m by 2100 is
plausible (Pfeffer et al., 2008), these aquifers would be
particularly vulnerable to surface water formation.
CONCLUSIONS
Rivers affect the hydrology of north-central Florida in
numerous ways as they flow off of confining units and
across the unconfined Floridan aquifer to discharge into the
Gulf of Mexico. River incision and the generation of
topographic relief in north Florida has dissected the
potentiometric surface of the Floridan aquifer and created
systems of local and regional groundwater flow paths that
are directed towards rivers where the Florida aquifer system
is unconfined. Before Holocene sea level rise allowed the
lower Suwannee and Santa Fe rivers to form, these flow
paths must have been part of an integrated flow network
radiating towards the coast and discharging as diffuse seeps
at the coast or submarine. Deep flow paths from beneath the
confining unit now discharge water at springs at the Cody
Scarp, most notably the River Rise. River incision also
dissected the topography to create local flow systems that
recharge small springsheds with short flow paths down-
stream from the scarp. These short flowpaths reduce
groundwater residence time in the subsurface, depths of
groundwater flowpaths and mineralization of the water.
Inflow of less mineralized water to rivers dilutes the more
mineralized water as it flows downstream. As a result, the
most highly mineralized waters in the Santa Fe River Basin
occur in the center of the platform, and the least mineralized
waters occur closer to the coast, opposite of what occurs in
either fully confined or fully unconfined platforms.
Formation of surface water and the creation of short,
topographically controlled groundwater flow paths reduced
the length of time that recharged water is stored in carbonate
platforms relative to platforms that lack rivers. Potential
impacts of the shortening of groundwater flow paths that can
result from surface water formation should be considered
when considering the effects of sea level rise on the volume
freshwater resources of low-lying coastal aquifers.
ACKNOWLEDGEMENTS
We acknowledge funding from NSF grants EAR 0853956
and 09107941. Research was conducted under Florida
Department of Environmental Protection permit 04230712.
We thank the SRWMD and FGS for access to data. Portions
of this work were conducted while J Gulley was supported
by an NSF Graduate Research Fellowship. The National
Speleological Society Cave Diving Section and cave divers
Jeff Hancock, Bill Huth, Wayne Kinard and Jim Wyatt are
thanked for assistance with fieldwork in the Devil’sCave
system and providing key logistical support.
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