Investigation of Submarine Groundwater 

Discharge along the Tidal Reach of the 

Caloosahatchee River, Southwest Florida

By Christopher D. Reich

Prepared in cooperation with the

South Florida Water Management District

Open-File Report 2009–1273

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2009

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Reich, C.D., 2010, Investigation of Submarine Groundwater Discharge along the Tidal Reach of the Caloosahatchee 

River, Southwest Florida: U.S. Geological Survey Open-File Report 2009-1273, 20 p., plus apps., p. A1-A52.
iii

Contents

Introduction.....................................................................................................................................................1

Description of Study Area ...................................................................................................................2

Objective of Study.................................................................................................................................5

Methods of Investigation..............................................................................................................................5

Continuous Surface-Water Mapping ................................................................................................5

Time-Series Radon Sites......................................................................................................................6

Results and Interpretation............................................................................................................................7

Continuous Radon Mapping................................................................................................................7

Time-Series Radon and Groundwater Discharge Estimates.......................................................11

Whiskey Creek Site ...................................................................................................................11

Lock and Dam Site (S-79) .........................................................................................................12

Comparison with other Florida Sites ......................................................................................15

Continuous Resistivity Profiling Surveys ........................................................................................15

Summary........................................................................................................................................................18

Suggestions for Further Study...................................................................................................................18

Acknowledgments.......................................................................................................................................18

References Cited..........................................................................................................................................19

Appendix — Continuous Resistivity Profile image segments collected in the 

Caloosahatchee River.......................................................................................................................A1–A52

Figures

1. Map of Caloosahatchee River study area in Lee County, Florida ...................................................2

2. Map of Lee County, Florida, showing location of the Caloosahatchee River 

 relative to the Gulf of Mexico, time-series radon locations, and W.P. Franklin 

 Lock and Dam (S-79)...............................................................................................................................3

3. Graph showing rainfall and W.P. Franklin Lock and Dam (S-79) water releases 

 relative to study period of March 15 to 20, 2009................................................................................4

4. Map showing the surficial geology of the Lee County region.........................................................4

5. Photo of the R/V Halimeda and the equipment used for continuous radon 

 and resistivity profiling...........................................................................................................................5

6. Conceptual box model of radon inflows and losses to a coastal water body 

 where inventory is measured over time, and total flux is calculated by adjusting 

 for atmospheric and mixing losses......................................................................................................6

7. Continuous 222Rn map showing an increase in radon activity from near the mouth 

 of the river up to the S-79 structure.....................................................................................................8

8. Graph with table showing normalized radon per volume of river for the three river 

 areas that were divided on the basis of surface-water 222Rn activity in figure 7.........................9

9. Graph showing tidal signal collected by the NOAA gaging station located beneath 

 the US-41 (Edison) bridge on the Caloosahatchee River.................................................................9

10. Map showing continuous surface-water salinity values collected during radon 

 and resistivity surveys .........................................................................................................................10

11. Map showing continuous surface-water temperature values collected during 

 radon and resistivity surveys..............................................................................................................10
iv

12. Photo showing time-series radon equipment at the Whiskey Creek site ...................................11

13. Graphs of 222Rn in water, 222Rn net flux, and advective flux at the Whiskey Creek 

 time-series site, March 15-19, 2009 ...................................................................................................12

14. Photo showing the S-79 time-series site shows the R/V Halimeda used as the 

 platform for collecting radon and surface-water data, and the piezometer and 

 pump used to collect porewater and radon samples.....................................................................13

15. Graphs of 222Rn in water, 222Rn net flux, and advective flux at the S-79 time-series 

 site on March 19, 2009 .........................................................................................................................13

16. Graph showing vertical profile of surface-water and porewater salinity and 222Rn 

 collected at the S-79 time-series site................................................................................................15

17. Map showing individual segments of continuous resistivity profiles collected in 

 the Caloosahatchee River...................................................................................................................17

Tables

1. Table of porewater 222Rn, salinity, and temperature collected at two sites during the 

 time-series experiment near lock and dam structure S-79...........................................................14

2. Variability in advective fluxes and methods by which fluxes are derived for different 

 sites within Florida................................................................................................................................16
v

Conversion Factors

Multiply inch/pound units by To obtain SI units

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

mile, nautical (nmi) 1.852 kilometer (km)

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3) 

cubic inch (in3) 16.39 cubic centimeter (cm3) 

cubic inch (in3) 0.01639 liter (L)

cubic foot (ft3) 0.02832 cubic meter (m3) 

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

mile per hour (mi/h) 1.609 kilometer per hour (km/h) 

Acronyms and Abbreviations

SGD submarine groundwater discharge ohm-m ohm-meter

CRP continuous resistivity profiling cm centimeter

kph kilometers per hour m meter

ppt parts per thousand (salinity unit) km kilometer

dpm/L disintegrations per minute per liter °C degrees Celsius

dpm/m3 disintegrations per minute per 

cubic meter

min minute

dpm/m2 disintegrations per minute per 

square meter

hr hour

cm/d centimeters per day s second

USGS U.S. Geological Survey n number of observations

SFWMD South Florida Water Management 

District

~ approximately

YSI Yellow Springs, Inc. > greater than

GPS Global Positioning System < less than

WAAS Wide Area Augmentation System R/V research vessel

Investigation of Submarine Groundwater Discharge 

along the Tidal Reach of the Caloosahatchee River, 

Southwest Florida

By Christopher D. Reich

Introduction

The tidal reach of the Caloosahatchee River is an a major role (SFWMD, 2009). The pathway for many of 

estuarine habitat that supports a diverse assemblage of biota these nutrients is by land runoff and direct discharge from 

including aquatic vegetation, shellfish, and finfish. The stormwater drains. An often overlooked source of nutrients 

system has been highly modified by anthropogenic activity and other chemical constituents is from submarine groundwater discharge (SGD). SGD can be either a diffuse or point over the last 150 years (South Florida Water Management source (for example, submarine springs) of nutrients and District (SFWMD), 2009). For example, the river was chan- other chemical constituents to coastal waters (Valiela and nelized and connected to Lake Okeechobee in 1881 (via canal others, 1990; Swarzenski and others, 2001; 2006; 2007; C-43). Subsequently, three control structures (spillway and 2008). SGD can be composed of either fresh or marine water locks) were installed for flood protection (S-77 and S-78 in or various mixed ratios of fresh and marine water (Martin the 1930s) and for saltwater-intrusion prevention (S-79, W.P. and others, 2007). In coastal areas where water-table eleva- Franklin Lock and Dam in 1966). The emplacement of these tions (hydraulic gradients) are steep, such as in Hood Canal, structures and their impact to natural water flow have been Washington (Swarzenski and others, 2007; Simonds and blamed for water-quality problems downstream within the others, 2008), groundwater entering the coastal marine waters estuary (Flaig and Capece, 1998; SFWMD, 2009). Doering can be fresh (~1-4 parts per thousand, ppt). SGD in coastal and Chamberlain (1999) found that the operation of these locations that have low relief (low hydraulic gradients) such control structures caused large and often rapid variations in as the study area or other locations in Florida are typically salinity during various times of the year. Variable salinities driven by tidal pumping (Reich and others, 2002; 2008; could have deleterious impacts on the health of organisms in Swarzenski and others, 2008), and water advecting into 

the Caloosahatchee River estuary. surface water is composed of recirculated marine water mixed 

Flow restriction along the Caloosahatchee has also been with either fresh or brackish groundwaters.

linked to surface-water eutrophication problems (Doering and The importance of SGD in the delivery of nutrients and 

Chamberlain, 1999; SFWMD, 2009) and bottom-sediment trace elements to coastal environments has been shown to be 

contamination (Fernandez and others, 1999). Sources of both beneficial and deleterious to ecosystem health (Valiela 

nutrients (nitrogen and phosphorous) that cause eutrophica- and others, 1990). The logical step in studying SGD is to 

tion are primarily from residential sources and agriculture, map areas where SGD occurs. Methods such as continuous 

though wastewater-treatment-plant discharges can also play surface-water radon-222 (222Rn) mapping and electrical 
2 Investigation of Submarine Groundwater Discharge along the Tidal Reach of the Caloosahatchee River....Florida

resistivity (continuous resistivity profiles, CRP) have been 

developed and used to identify potential SGD sites (Dulaiova 

and others, 2005; Swarzenski and others 2004; 2006; 2007; 

2008; Reich and others, 2008). CRP data record subsurface, 

bulk-resistivity measurements to depths up to 25 meters 

(m). The bulk resistivity can be representative of changes in 

porewater salinity or in lithology (Reich and others, 2008; 

Swarzenski and others, 2008). Radon-222 (half-life = 3.28 

days) is a natural tracer of groundwater, because sediments 

and rocks, containing uranium-bearing materials such as 

limestone and phosphatic material, continually produce 222Rn. 

Rn-222 (also referred to simply as radon) is an ideal tracer, 

because there is a constant source. Since radon is a gas, 

222Rn does not build up in the surface water but rather evades 

directly to the atmosphere (Burnett and Dulaiova, 2003; 

Burnett and others, 2003; Dulaiova and Burnett, 2006). 

Description of Study Area

The tidal reach of the Caloosahatchee River investigated 

during this study is located in Lee County, Florida (fig. 1). 

The tidal portion of the Caloosahatchee River runs from 

San Carlos Bay (mouth of the river) to the W.P. Franklin 

Lock and Dam structure (S-79), an approximate length of 

42 kilometers (km) (fig. 2; SFWMD, 2009). The densely 

populated communities of Cape Coral and Fort Myers occur 

along the lower portion of the river, while the upper section, 

located east of Interstate 75 (I-75) is mostly agricultural. The 

population and population density of Lee County, located in 

southwest Florida along the Gulf of Mexico, are 593,136 and 

211 persons per square kilometer, respectively (U.S. Census 

Bureau, 2009). The majority of the population is located in 

the cities of Cape Coral and Fort Myers.

80°0'0"

80°0'0"

85°0'0"

85°0'0"

30°0'0"30°0'0"

25°0'0"

25°0'0"N

Lee County

Lake

Okeechobee

0 40 80 160 240 320

Kilometers

G u l f o f M e x i c o

A t l a n t i c O c e a n

Caloosahatchee River

Figure 1. Caloosahatchee River study area in Lee County, Florida
Introduction 3

The Caloosahatchee River is located in a subtropical 

climate with an average annual rainfall of 134 centimeters 

(cm) (53 inches, in.) (SFWMD, 2009). Rainfall average for 

January thru April 2009 was 3.8 cm (1.52 in.), which falls 

well below the average total (1976-2005) for the 4 months 

(24 cm; 9 in.) (Lee County Government, http://lee-county.

com). Figure 3 shows rainfall events and water releases from 

S-79 from June 1, 2008, to April 1, 2009, as well as a marker 

identifying the study period (March 15-20, 2009). 

The surficial geology of the study area is composed of 

an undifferentiated quartz-sand unit (Pleistocene-Holocene; 

Qu), shelly units mixed with quartz sand and marly limestone (Tertiary-Quaternary shelly units; TQsu), and mixed 

siliciclastics and carbonate lithologies consisting of sandy 

limestone, quartz sand, and clayey sands (Tt; Tamiami 

Formation) (fig. 4; Missimer, 1978, 1999; Boggess and 

others, 1981; Scott, 2001;). The Qu unit acts as the primary 

surficial unconfined aquifer system throughout the study 

area. Water-table elevation maps (potentiometric maps) of 

Lee County show that water elevations can be as much as 

7.6 m (25 ft) in the southeast area of the county (Krulikas 

and Giese, 1995). The high water-table elevation is due 

to an increase in land-surface elevation in the same area. 

Water-table elevation closer to the Caloosahatchee River 

is <1.5 m (5 ft), indicating a very low hydraulic gradient 

pushing groundwater into the river. However, east of I-75, the 

contours of water-table elevation along the northern shore of 

the river are more closely spaced and run parallel to the river. 

The more closely spaced contours indicate a greater hydraulic 

gradient, which could potentially result in greater baseflow or 

discharge into the river via SGD. The upper Floridan aquifer 

could be another potential source of groundwater to the 

Figure 2. Lee County, Florida, showing location of the Caloosahatchee River relative to the Gulf of Mexico, time-series radon locations, 

and W.P. Franklin Lock and Dam (S-79).
4 Investigation of Submarine Groundwater Discharge along the Tidal Reach of the Caloosahatchee River....Florida

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

2000

0

4000

6000

8000

10000

12000

14000

16000

18000

20000

6/1/2008 7/21/2008 9/9/2008 10/29/2008 12/18/2008 2/6/2009 3/28/2009

Rainfall in inches

Discharge in cubic feet per second

Rainfall at S-79

Study period

Last release at S-79

prior to study (March 3)

S-79 Structure Flow

Figure 3. Rainfall (red line) and W.P. Franklin Lock and Dam (S-79) water releases (blue squares) relative to study 

period of March 15 to 20, 2009 (green bar). Data were obtained from gauging station located at the S-79 structure.

Figure 4. Surficial geology of the Lee County 

region (modified from Scott, 2001). [Geologic 

units (Scott, 2001) are as follows: Thp=MiocenePliocene, Hawthorn Group, Peace River Formation; 

Tt=Pliocene, Tamiami Formation; TQsu=Pliocene, 

Tertiary-Quaternary shell units; Qu=PleistoceneHolocene, undifferentiated sediments; and 

Qh=Holocene quartz and carbonate sands]

S-79

Cape Coral

Fort Myers

Sanibel Island

Gulf of Mexico

TQsu

Tt

TQsu

Qh

Tt

TQsu

Thp

Qu

Qu

Qh

82°0'0"

82°0'0"

27°0'0"27°0'0"

26°30'0"26°30'0"

0 5 10 20 30

Kilometers
Methods of Investigation 5

Caloosahatchee River basin. Although Lee County has few 

groundwater wells to be used in constructing upper Floridan 

aquifer potentiometric maps, Ortiz (2008) has demonstrated 

that significant head potential exists in the region. Water 

in a completely cased well to the upper Floridan aquifer 

would rise between 12 and 15 m (40 and 50 ft) above land 

surface. This significant hydraulic head could result in 

leakage into overlying surface water through fractures/

faults in the confining units. However, leakage likely does 

not occur because the intermediate and Tamiami confining 

units are impermeable and provide isolation between the 

upper Floridan aquifer and surficial aquifer (Missimer, 1999; 

Cunningham and others, 2001).

Objective of Study

The objectives of this study were to identify locations of 

potential groundwater discharge into the estuary/river system 

and to quantify advective groundwater influxes. To accomplish these objectives, we collected continuous surface-water 

radon (222Rn) and continuous resistivity profiles (CRP) along 

the tidal reaches of the Caloosahatchee River (figs. 1 and 2). 

Two stationary time-series radon surveys were also conducted 

to localize groundwater discharge inflow (advective fluxes) to 

the Caloosahatchee River.

Methods of Investigation

Continuous Surface-Water Mapping

Surface-water mapping of radon (222Rn) and electrical 

resistivity (CRP) took place from the U.S. Geological Survey 

7.5-m-long R/V Halimeda (fig. 5). The radon equipment 

consisted of a pole-mounted bilge pump set approximately 

0.5 m below the water surface that continuously fed water to 

an air-water exchanger. The water was forced through a small 

spray nozzle and misted, releasing radon gas. Approximately 

130-line-km of radon and resistivity data were collected. 

Continuous surface-water 222Rn mapping is designed to 

obtain an image (qualitative investigation) of coastal areas 

where groundwater, observed as an increase in 222Rn, 

discharges into surface water. Due to instrument limitations, 

this method integrates over 10 minutes (min), which equates 

to a distance of ~700 m at a boat speed of ~ 4 kilometers per 

hour (kph).

The internal air pumps of the radon-monitoring device 

(RAD7; Durridge Company Inc.) pulled air from the air-water 

exchanger through small-diameter tubing connected to the 

top of the air-water exchanger into a box where the air passed 

through a column containing a drying agent, reducing the 

humidity to approximately 6 percent. The dry air was then 

Radon monitors

Pole-mounted bilge

GPS for Radon pump and YSI probe

GPS for resistivity

Water-air exchanger

Resistivity Supersting

Resistivity cable

Figure 5. The R/V Halimeda and the equipment used for continuous radon and resistivity profiling. 

[YSI, Yellow Springs, Inc.]
6 Investigation of Submarine Groundwater Discharge along the Tidal Reach of the Caloosahatchee River....Florida

fed into a series of RAD7s (4 connected in series; fig. 5) 

where the radon was counted by a solid-state (silicon) alpha 

detector. As alpha particles are emitted, the RAD7 counts 

them and measures the 222Rn activity (Burnett and others, 

2001; Dulaiova and others, 2005). Surface-water data were 

also collected with a YSI 556 MPS mulit-probe system. This 

unit was affixed to the same pole as the bilge pump. Radon 

and YSI data were collected at 10-minute and 10-second (s) 

intervals, respectively. 

Electrical resistivity (continuous resistivity profiles; 

CRP) was run in conjunction with surface-water radon 

mapping where possible. A Supersting R8/IP unit (Advanced 

Geosciences, Inc.) and a 100-m-long cable with a 10-m 

electrode spacing was used to collect an electrical-resistivity 

image to a depth of approximately 25 m. The Supersting 

R8/IP injects a current of up to 2 amps and power of 200 

watts. In CRP mode, the Supersting unit can simultaneously 

measure eight channels while injecting a current at a rate of 

1 to 3 s. The 11-electrode cable was set up in a streaming 

dipole-dipole array. The first two electrodes produced the 

current (current electrodes) and the remaining nine measured 

the resulting voltage potential (potential electrodes). The boat 

towed the cable, suspended by pool floats, along the water 

surface, and apparent resistivity with eight simultaneous 

measurement points, each at a different apparent depth (0 to 

25 m), was collected. Data were geo-referenced by streaming 

time and position from a Wide Area Augmentation System 

(WAAS)-enabled Lowrance LMS-480 Global Positioning 

System (GPS) device directly into the AGI Supersting unit. 

An additional WAAS-enable Garmin GPS was used to collect 

position for the radon and YSI data.

Time-Series Radon Sites

To obtain an estimate of groundwater discharge 

(advection) rates at a single location, two time-series radon 

sites were established. The time-series radon system consisted 

of a RAD7 unit measuring surface-water radon and a RAD7 

unit measuring atmospheric radon. The setup was identical 

to that of the surface-water mapping efforts except that, since 

only one RAD7 was used, the sampling interval was set at 

30 min. Surface-water elevation, salinity, and temperature 

were either measured in situ at the site or downloaded from 

nearby meteorological/oceanographic stations. Advective 

fluxes were inferred from radon time-series data using a 222Rn 

water-column mass-balance model (Corbett, and others 2000; 

Dulaiova and others, 2005; Cable, and others 2006; Burnett 

and others, 2007). The model balances the radon watercolumn inventory with known sources (advective porewater 

influx, radioactive production) and sinks (atmospheric 

evasion, tidal mixing, and radioactive decay) of 222Rn, as 

defined by equation 1 and as depicted in figure 6 (modified 

from Burnett and others, 2007). 

Jbenthic – Fatm ± Fmix = λz(ARn – ARa) = λIxs , (1)

where

Jbenthic is benthic flux of 222Rn (dpm/m2/day),

λ is the decay coefficient (0.181 d-1),

ARa and ARn are the water-column activity of radium-226

 (226Ra) and 222Rn (dpm/m3), respectively, 

z is water depth (m), 

Fatm is the atmospheric flux of radon (dpm/m2/day), 

Fmix is the flux associated with tidal mixing 

 and/or unidirectional surface-water 

 flow(dpm/m2/day), and 

Ixs is excess-radon inventory (dpm/m2). 

The following text highlights how the mass-balance model is 

used to convert time-series radon measurements into advective-fluid fluxes (Dulaiova and others, 2005). The first step 

involves computing the excess 222Rn water-column inventory. 

The Rad7 measures 222Rn activity (dpm/m3

air) as radon in air; 

this activity is multiplied by the Ostwald temperature concentration factor (Ko; unitless) (eq. 2) to get radon in water:

Ko = 0.105 + 0.405-0.0502*Tw (2)

where Tw is water temperature (oC).

Inventory

Ixs (dpm/m2

)

Fatm

Sea Land

Fmix

SGD

cm/day

Total flux

Jbenthic (dpm/m2

/day)

diffusion

other

processes

Figure 6. Conceptual box model of radon inflows and losses to a 

coastal water body where inventory (Ixs) is measured over time, and 

total flux (Jbenthic) is calculated by adjusting for atmospheric (Fatm) 

and mixing (Fmix) losses (modified from Burnett and others, 2007). 

[SGD, submarine groundwater discharge; dpm/m3, disintegrations per 

minute per cubic meter; cm, centimeter]
Results and Interpretation 7

The radon-in-water value is then corrected for in situ

production via the decay of 226Ra (ARa) to obtain excess 

222Rn. This study assumed a 226Ra value of 200 dpm/m3

(Swarzenski and others, 2008). The excess 222Rn is multiplied 

by the water-column thickness (water depth) to obtain the 

excess radon inventory (dpm/m2 ). Radon flux (dpm/m2/hr) is 

calculated by taking the change in excess-radon inventories 

over time and then multiplying by 222Rn-decay constant. 

Next, the different source and sink terms are computed 

from measured parameters. For example, the atmospheric 

flux (Fatm; dpm/m2/hr) is calculated by gradient in radon 

across the water-air interface interface:

Fatm = [Rnwater – (Ko * Rnair)] * K(600), (3)

where 

Rnwater is radon activity in water (dpm/m3),

Rnair is radon activity in air (dpm/m3),

Ko is the Ostwalt temperature-concentration 

 coefficient (unitless), and 

K(600) is the vertical diffusive coefficient (m/hr). 

The vertical diffusive coefficient is computed from the 

empirical relation defined by MacIntyre and others (1995):

 

€ 

K(600) = 0.45u10

1.6

(Sc/ 600)

−2 / 3

, (4)

where

u10 is wind speed measured 10 m above land surface 

 (m/hr) and 

Sc is the Schmidt number (ratio of kinematic viscosity and

 molecular diffusion; m/hr). 

Flood/ebb tidal corrections (Fmix) were calculated by 

subtracting adjacent water-level measurements and noting if 

the difference was positive (flood tide) or negative (ebb tide). 

Net radon flux (dpm/m2/hr) was calculated by adding radon 

flux and atmospheric flux and then subtracting losses to the 

system from flood/ebb tides. Mixing losses were calculated 

when the net radon fluxes were negative; otherwise, these 

values were zero because it was assumed that radon was 

being added to the system and not lost due to atmospheric 

evasion or mixed with lower radon flood/ebb tidal waters. 

Total benthic radon flux (Jbenthic; dpm/m2/hr) was then 

calculated by subtracting mixing losses from net radon flux. 

To obtain specific discharge (q; cm/d) from time-series radon 

measurements, the total radon flux (dpm/m2/hr) was divided 

by the mean porewater-radon activity (dpm/m3):

q = (Jbenthic *100*24) / Rnpw, (5)

where Rnpw is porewater-radon activity (dpm/m3).

Results and Interpretation

Continuous Radon Mapping

Radon-222 was mapped throughout the Caloosahatchee 

River from near the mouth (Little Shell Island) upstream to 

structure S-79. Integrated surface-water activities ranged 

from 1-2 dpm/L (disintegrations per minute per liter) near 

the mouth of the river to 12.7 dpm/L along the narrow river 

section east of I-75 (fig. 7). These data indicate that the tidal 

reach of the river can be broken into three segments: (1) from 

the mouth of the river up to the US-41 (Tamiami) bridges 

(area 1), (2) from the US-41 bridges to the I-75 overpass 

(area 2), and (3) from the I-75 overpass to structure S-79 

(area 3). Area 1 has low radon activity (1-5 dpm/L) and is 

indicative of a well-flushed coastal water mass (Fanning and 

others, 1982). The upper, narrow region of the river (area 3) 

has an increase in radon activity (8-13 dpm/L) that most 

likely results from groundwater discharge since the only 

source of 222Rn is from the subsurface. Area 2 is the transition 

zone between the well-flushed area 1 and the groundwaterinfluenced area 3, as indicated by radon activities (5-8 

dpm/L). The slightly elevated radon activities in area 2 

could originate from groundwater discharge or from simple 

mixing between areas 1 and 2. Without adequate knowledge 

of mixing rates or residence time of the surface water, no 

conclusion can be made based on the data presented as to 

whether area 2 has active groundwater discharge.

To assess the validity that the upper reach (area 3) of 

the river is influenced by greater groundwater inflows than 

the lower reach (area 1), an activity per volume of river 

was calculated. The average of all 222Rn activity (dpm/L) 

along with the area (km2) of the river in each of the three 

sections was computed, and a uniform average water depth 

of 5 m for the tidal reach of the Caloosahatchee was used 

in the volume calculation. The results (fig. 8) indicate that 

there is a significant increase in activity per volume in area 

3 (0.13 dpm/L/km3) over area 2 (0.0047 dpm/L/km3) and 

area 1 (0.00031 dpm/L/km3). These results indicate a greater 

groundwater component in the upper reach compared with the 

lower sections of the river.

Radon was collected during similar tidal (incoming) 

conditions (fig. 9) allowing for direct comparisons from day 

to day. Slight tidal-height amplitude changes over the week 

were minor (<12 cm) and should not impact the magnitude of 

radon observed. In addition to the radon, surface salinity and 

temperature were measured also. Salinity shows a gradual 

freshening from the mouth of the river to structure S-79 

(fig. 10). The salinity was most likely higher than normal for 

the river because there were no releases from structure S-79 

during the study period (previous release date was March 3, 

2009; fig. 3) and very low rainfall had occurred during 2009 

prior to data collection (total 2009 rainfall of 1.97 inches at 

S-79; fig. 3). A significant portion of the low-salinity water in 
8 Investigation of Submarine Groundwater Discharge along the Tidal Reach of the Caloosahatchee River....Florida

the upper section of the river likely originates from groundwater baseflow. Surface-water temperature was on average 

~26 °C. However, near the Florida Power & Light Company 

electric powerplant (near I-75), temperature was observed to 

increase to near 33 °C (fig. 11). 

Water-table contours for the surficial aquifer (fig. 7; 

Krulikas and Giese, 1995) generally support the zonation 

indicated by the surface-water radon maps. Data on the 

water-table map represent a 10-year average from 1982 to 

1991, so some caution is warranted in direct comparison 

between radon and water-table data. The closely spaced 

contours along the shore of the river east of I-75 indicate 

a steep hydraulic gradient capable of driving groundwater 

discharge into the river. Within area 2, the hydraulic gradient 

shallows, subsequently decreasing the possibility of groundwater discharge. As mentioned, the radon data do not support 

or refute discharge within area 2. One inconsistency between 

the water-table potentiometric map and radon map occurs 

within area 3. Low-radon activity in area 1 is inconsistent 

with the inferred positions of the 5-, 10-, and 15-ft contours 

along the southern shoreline within this area. Assuming that 

the contours are representative of water-table elevations, 

then one would expect much higher radon activity along 

the Fort Myers shoreline transect relative to the Cape Coral 

shoreline and midchannel transects. Since this is not the case, 

the inferred placement of the 5-, 10-, and 15-ft contours is 

apparently incorrect, and the contour data should be refined 

by additional observations. 

Figure 7. Continuous 222Rn map showing an increase in radon activity from near the mouth of the river up to the S-79 structure. 

Radon-222 activities (in disintegrations per minute per liter, dpm/L) are at 10-min intervals. Segments of the river are divided into areas 

1, 2, and 3 based on 222Rn activities in surface water. Water-table elevations (in feet) are shown on map (modified from Krulikas and 

Giese, 1995).
Results and Interpretation 9

0.000307

0.004681

0.130557

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

1 2 3

Area number

Average 222Radon (dpm/L/km3

)

Area Average Radon

(dpm/L)

Radon

stand. dev.

Area (km2) Average Radon

(dpm/L)/km2

Normalized Radon

(dpm/L/km3)

1 3.84 1.57 50 0.0768 0.000307

2 6.76 1.78 17 0.3979 0.004681

3 10.44 0.87 4 2.6111 0.130557

approx. avg. water depth = 5 m

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

3/15/093/16/09

3/17/09

3/18/09

3/19/09

3/20/09

3/21/093/22/09

Tide, in meters

DAY 1 DAY 2 DAY 3 DAY 4

NOAA gauging Station : DCP# 8725520

26o

 38.8’ N x 81o 52.3’ W

Figure 8. Normalized radon per volume of river (disintegrations per minute per liter per cubic 

kilometer, dpm/L/km3) for the three river areas that were divided on the basis of surface-water 222Rn activity in figure 7.

Figure 9. Tidal signal collected by the NOAA gaging station located beneath the 

US-41 (Edison) bridge on the Caloosahatchee River.
10 Investigation of Submarine Groundwater Discharge along the Tidal Reach of the Caloosahatchee River....Florida

Figure 10. Continuous surface-water salinity values collected during radon and resistivity surveys.

Figure 11. Continuous surface-water temperature values collected during radon and resistivity 

surveys. Note the increase in temperature near the Florida Power & Light Co. powerplant, located 

east of I-75.
Results and Interpretation 11

Time-Series Radon and Groundwater Discharge 

Estimates

Two time-series radon-monitoring sites were established 

to provide initial estimates of dry-season groundwater 

discharge into the Caloosahatchee River system (fig. 2). 

The first site was located at Whiskey Creek, a tributary of 

the Caloosahatchee River, and was monitored from March 

15 to 19, 2009. The second site was located near structure 

S-79 and was monitored for 8 hr on March 19, 2009. The 

S-79 site was located on the undeveloped portion of the 

southern shore <1 km west of S-79. Four RAD7s (same setup 

as for the mapping surveys) were used to measure radon at 

10-min intervals over the day-long experiment. In addition, 

a piezometer was used along the shore to measure porewater 

radon and salinity at various depths to obtain a porewater 

profile as well as to measure radon and salinity at a specified 

depth over the span of the experiment. Advective fluxes were 

calculated based on formulas presented in the Methods of 

Investigation section.

Whiskey Creek Site

The Whiskey Creek site was established on a private 

dock and the instruments run for 4 consecutive days 

(March 15-19, 2009; figs. 12 and 13). A single RAD7 was 

used at the dock to measure surface-water radon and a second 

RAD7 was used to measure radon in air. The air-monitor data 

were used for both the S-79 and dock calculations. Tidal data 

were collected from a NOAA tidal station (DCP# 8725520). 

Radon activities at the dock were slightly elevated when 

compared to just offshore from the Whiskey Creek entrance. 

Values in the river were approximately 4 dpm/L (fig. 7), and 

values during the week at the time-series site ranged from 8 

to 22 dpm/L, after omitting the radon spike at 0800 on March 

16 (fig. 13A). Radon concentration was inversely correlated 

with tide, indicating that groundwater was being added to 

the system when the hydraulic gradient was the greatest (low 

tide). The net radon-flux and advective-flux estimates are not 

strongly correlated to tide (fig. 13B, C), however there is a 

subtle tidal response during the lowest low tides on March 18 

RAD7 and exchanger

bilge pump

R/V Halimeda

water-supply hose

Figure 12. Time-series radon equipment at the Whiskey Creek site. Note black box 

(foreground) on dock containing RAD7 radon monitors and air-water exchanger. Also 

note modified shoreline at Whiskey Creek site compared to S-79 time-series site in 

figure 14.
12 Investigation of Submarine Groundwater Discharge along the Tidal Reach of the Caloosahatchee River....Florida

and 19, 2009. Radon concentration and advective flux do 

not correspond similarly to tide. This is primarily due to the 

fact that conservative mixing estimates used in the calculations smooth out the values for advection. The corrections 

for atmospheric and tidal mixing (tides and reflux of surface 

water into the subsurface) can result in the smoothing of the 

calculated advective fluxes. The average advective flux calculated for the dock site was 5.7 ± 6.4 cm/d using an average 

porewater 222Rn concentration of 3,000 dpm/L.

Lock and Dam Site (S-79)

The 222Rn time-series results from the S-79 site (fig. 14) 

show that groundwater discharge is occurring in the upper 

reach of Caloosahatchee River and is modulated by tides 

(fig. 15A). Rn-222 activity increased steadily until 1100 

(maximum activity 88,000 dpm m-3) and declined exponentially to an activity of 17,900 dpm m-3 toward the end of the 

survey. The peak in 222Rn activity occurred within 80 min 

3/15/09

3/16/09

3/17/093/18/09

3/19/09

3/20/09

3/21/09

Advection rate, incentimeters per day

10

-5

0

5

15

20

25

-6000

-4000

-2000

0

2000

4000

6000

8000

0

5000

10000

15000

20000

25000

30000

Net Rn flux, in disintegrationsper minute per square meter per hourMixing losses, in disintegrationsper minute per square meter per hour

222Rn concentration, in disintegrations

per minute per cubic meter

Water level, in meters Water level, in meters Water level, in meters

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Figure 13. Graphs of (A) 222Rn in water, (B) 222Rn net flux, and (C) advective flux at the Whiskey Creek 

time-series site, March 15-19, 2009. Changing water level indicates a semi-diurnal tidal cycle during the 

4-day-long experiment. Gray line in B is the zero reference line to help identify flux as net positive or net 

negative.
Results and Interpretation 13

R/V Halimeda

piezometer

radon time-series platform

peristaltic pump

Figure 14. The S-79 time-series site shows the R/V Halimeda used as the platform for collecting 

radon and surface-water data, and the piezometer and pump used to collect porewater and 

radon samples (foreground).

Figure 15. Graphs of (A) 222Rn 

in water, (B) 222Rn net flux, and 

(C) advective flux at the S-79 

time-series site on March 19, 

2009. Changing water level 

indicates a rising (flood) tide. 

Gray line is the zero reference 

line to help identify flux as net 

positive or net negative.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Water level, in meters

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

March 19, 2009

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Water level, in meters

-80000

-70000

-60000

-50000

-40000

-30000

-20000

-10000

0

10000

20000

30000

-60

-40

-20

0

20

40

60

80

100

120

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Advection rate, incentimeters per day

Water level, in meters

Net Rn flux, in disintegrationsper minute per square meter per hourMixing losses, in disintegrationsper minute per square meter per hour

222Rn concentration, in disintegrations

per minute per cubic meter
14 Investigation of Submarine Groundwater Discharge along the Tidal Reach of the Caloosahatchee River....Florida

of the survey, which is beyond the typical equilibration 

time (~ 30 min) for 222Rn measured using the RAD7. This 

indicates that the high radon activity is not an artifact of 

the equilibration process and is the result of groundwater 

discharge. The subsequent decline in radon following the 

peak indicates that groundwater become less important as the 

survey progressed. Figure 15B shows both net radon flux 

and mixing losses. The maximum radon activity and inferred 

groundwater discharge occur during the later stages of the 

low tide (onset of flood tide), which is consistent with tidally 

modulated groundwater discharge. Ataie-Ashtiani and others 

(2001) showed that groundwater discharge was out of phase 

with tides (high discharge during low-tide). In addition to the 

tidal forcing, the enhanced discharge may also have resulted 

from a short but intense storm that produced 0.5 in. of rain 

during the early hours of March 19, 2009 (fig. 3). Percolation 

of recent precipitation through the unsaturated zone and 

subsequent mounding of the water table may have enhanced 

the hydraulic gradient between the surficial aquifer and the 

river, allowing enhanced groundwater discharge. 

Additional support for the tide-influenced groundwater 

discharge comes from porewater time-series sampling 

conducted simultaneously with the surface-water survey at 

the S-79 site (table 1; fig. 14). The porewater salinity and 

radon profiles (fig. 16) show a well-defined gradient with 

depth. Salinity in the surface water at the time-series site 

averaged 8 parts per thousand (ppt) and decreased to 2.8 ppt 

at 55 cm below the sediment-water interface. Radon activity 

increased with depth from an average of 38.5 ± 22 dpm/L 

in surface water to 9,590 ± 608 dpm/L at 55 cm. The timeseries radon data collected from a depth of 55 cm over 6 hr 

slowly decreased from 9,590 dpm/L to 7,570 dpm/L. Salinity 

showed an inverse pattern by increasing over the 6 hr from 

2.7 ppt to 3.75 ppt. The increase in salinity indicates that 

incoming tidal waters were being pushed into the surficial 

sediments, displacing or pushing back the higher radon and 

lower salinity porewaters, resulting in the cessation of the 

advective-flow component shown in figure 15A.

Under the assumption that these data reflect tidally modulated groundwater discharge, the advective component can be 

separated into their respective tidal stages (low-tide discharge 

and high-tide discharge). Based on the 10-min time-averaged 

radon flux of 32,798 dpm/m2/hr and the porewater radon 

activity of 6,085 dpm/L, the low-tide discharge estimate averages 129.5 ± 205.2 cm/d. Since a complete tidal cycle was not 

monitored, the upper end of groundwater discharge was used. 

During high tide, groundwater discharge averages 14.5 ± 25.9 

cm/d, assuming an average radon flux of 3,680 dpm/m2/hr and 

porewater radon activity of 6,085 dpm/L. In comparison, the 

average flux for all data was 66.7 ± 151 cm/d (n=44). 

Table 1. Porewater 222Rn, salinity, and temperature collected at two sites during the time-series experiment near lock and dam 

structure S-79.

[cm, centimeter; °C, celsius; ppt, parts per thousand; dpm/L, disintegrations per minute per liter

Site Date/Time North 

latitude

West 

longitude

Porewater 

depth (cm)

Temperature 

(°C)

Salinity 

(ppt)

Radon 

(pCi/L) Error Radon (dpm/L) Error

Individual porewater samples along Caloosahatchee River

PW-1-Cal 3/18/2009 15:23 26.7241 -81.7067 30 23.61 4.59 1,020.00 70.40 2,264.40 156.29

PW-2-Cal 3/18/2009 16:07 26.7157 -81.7510 30 23.97 6.22 167.00 12.40 370.74 27.53

Time series at Lock and Dam site: incoming tide

PW-1-S79 3/19/2009 10:30 26.7241 -81.7067 15 21.06 7.05 1,150.00 119.00 2,553.00 264.18

PW-2-S79 3/19/2009 10:05 26.7241 -81.7067 25 21.15 6.04 1,060.00 104.00 2,353.20 230.88

PW-3-S79 3/19/2009 10:16 26.7241 -81.7067 35 21.56 5.00 2,010.00 175.00 4,462.20 388.50

PW-4-S79 3/19/2009 10:38 26.7241 -81.7067 55 21.78 2.70 4,320.00 274.00 9,590.40 608.28

PW-5-S79 3/19/2009 12:38 26.7241 -81.7067 55 21.65 3.57 3,760.00 124.00 8,347.20 275.28

PW-6-S79 3/19/2009 14:40 26.7241 -81.7067 55 21.85 3.75 3,480.00 234.00 7,725.60 519.48

PW-7-S79 3/19/2009 16:28 26.7241 -81.7067* 55 22.01 3.75 3,410.00 389.00 7,570.20 863.58

Average values (at S-79 site) 21.58 4.55 2,741.43 202.71 6,085.97 450.03
Results and Interpretation 15

Comparison with other Florida Sites

The mass-balance estimates of advective fluxes for both 

the S-79 and Whiskey Creek sites correspond well to other 

data from elsewhere in Florida (table 2). Table 2 compares 

advective fluxes at various locations that were derived from 

three methods: direct measurement by seepage meters, 

radon mass-balance, and radium mass-balance derivations. 

The flux estimates for groundwater entering coastal areas in 

Florida show that low hydraulic-gradient regions produce 

groundwater flows in the range of 0 to 49 with an average of 

approximately 13 cm/d.

Continuous Resistivity Profiling Surveys

Electrical resistivity is a useful tool for mapping 

potential sources and pathways for groundwater (fresh or 

saline) flow but is not a quantitative method for directly 

measuring groundwater discharge. CRP measures an 

“apparent” resistivity that includes both the pore-fluid and 

lithologic resistivity. A priori knowledge of either the geology 

or groundwater-fluid conductivity is beneficial in the interpretation of the CRP profiles. Interpretation of resistivity in 

this study was based on generalized geologic and hydrologic 

maps because sediment/rock cores or monitoring wells do not 

exist for use in ground-truthing. 

Approximately 100 line-km of electrical resistivity 

were collected during the study period. The CRP lines are 

divided into 52 line segments (fig. 17 and appendix). CRP 

tracklines were run simultaneously with radon surveys and 

covered both the southern and northern shorelines from near 

the mouth of the river (Little Shell Island) to the lock and 

dam (S-79; fig. 17). A CRP track along the river axis was also 

run; however, weather conditions had deteriorated on March 

18 such that wind caused a moderate chop on the river and 

resulted in errors within the resistivity data. Noticeable errors 

in CRP lines CAL09 and CAL11 (appendix) are marked 

with “poor data” labels and error-prone zones are grayed out. 

Weather conditions during the first 2 survey days were near 

perfect for resistivity and radon data collection.

0 1 2 3 4 5 6 7 8 9

0 2000 4000 6000 8000 10000 12000

-6

-13

-20

-27

-34

-41

-48

-55

222Rn, in disintegrations per minute per liter

Salinity in, parts per thousand

Depth below beach surface, in centimeters

surface water

sediment

PW-1-S79

Surface

PW-2-S79

PW-3-S79

PW-4-S79

Figure 16. Vertical profile of surface-water and 

porewater salinity and 222Rn collected at the S-79 

time-series site. Blue line shows salinity decreasing 

with depth and red line shows increasing 222Rn 

activity with depth.
16 Investigation of Submarine Groundwater Discharge along the Tidal Reach of the Caloosahatchee River....Florida

The resistivity profiles are shown in the appendix. 

The resistivity survey was divided into two sections: lower 

section, west of I-75, and upper section, east of I-75. These 

two sections have two different log-linear scales. Normalizing 

the profiles to a common-scale range allows direct comparison between profiles. The lower river section resistivity 

values were lower due to seawater influence (seawater 

resistivity = 0.20 ohm-meters, ohm-m). The upper river 

was influenced by both shallow and deep, fresh to brackish 

water. Resistivity along the upper tidal reach of the river 

ranged from 0.20 to 50 ohm-m, whereas the middle and lower 

sections of the river had resistivity values that ranged from 

0.20 to 15 ohm-m. The highest resistivity for the area south of 

the US-41 bridges was typically <10 ohm-m. 

The geology of the Caloosahatchee River valley is 

complex, and lithologies vary over the 45-km stretch of river 

surveyed in this study, making interpretation of resistivity 

challenging (fig. 4). Resistivity profiles (appendix) exhibit 

a variety of possible interpretations that range from artifacts 

to geological variability. Buried cables or pipelines or bridge 

pilings can be recorded by the CRP system. CAL04_6000-

8000 (5660 m) and CAL06_8000-10000 (7420 m) contain 

examples of bridge-piling artifacts. CAL08_200-2000 

(~800 m) contains an example of a buried pipeline or cable 

or, at least, is coincidental with a buried cable as marked on a 

USGS topographic map. 

Table 2. Variability in advective fluxes and methods by which fluxes are derived 

for different sites within Florida.

[cm/d, centimeter per day

Florida location Reference Advective flux, average 

or range (cm/d)

Florida Bay1 Shinn and others, 2002 0.1 to 6

Florida Bay3 Corbett and others, 2000 0.3 to 2.5

Florida Bay3 Chanton and others, 2003 0 to 15

Florida Bay3 Swarzenski and others, 2008 12.5 ± 11.8

Biscayne Bay2 Reich (unpublished data, 2004) 23.2 ± 19.6

Hendry County (C-3 canal)1 Cunningham and others, 2001 <1 to 49

Panhandle (FSUML)2 Burnett and Dulaiova, 2003 6 to 13

Panhandle (FSUML)3 Burnett and Dulaiova, 2003 13 ± 9

Panhandle (FSUML)4 Burnett and Dulaiova, 2003 11.2 to 21

Loxahatchee River4 Swarzenski and others, 2006 2 to 7

Loxahatchee River2 Swarzenski and others, 2006 1.7

Indian River Lagoon1 Cable and others, 2006 1 to 12

Indian River Lagoon3 Martin and others, 2007 16 ± 43

Indian River Lagoon1 Martin and others, 2007 1.6 to 18.4

Indian River Lagoon3 Smith and others, 2008 2.5 to 24

Caloosahatchee River3 this study (upper river reach) 12.3 ± 21.9

Caloosahatchee River3 this study (lower river reach) 5.7 ± 6.4

1 Seepage meter (Lee type, bag)

2 Seepage meter (autonomous) 

3 Radon mass balance

4 Radium mass balance
Results and Interpretation 17

Uniform geology, whereby marine water penetrates 

uniform quartz-sand units of the TQsu, is present in CRP 

profiles CAL01_2000-4000 and CAL06_200-2000. These 

two examples are error free and are consistent across 

the profile. There are, however, resistivity profiles that 

demonstrate geologic processes that may represent paleokarst activity, such as subsidence and sinkhole activity. 

An example of a possible sinkhole feature can be seen in 

line CAL06_2000-4000 (~1,800-2,200 m). CAL06_2000-

4000 shows high-resistivity material (~6 ohm-m) flanking 

a moderately resistive material (1.5 ohm-m) in what could 

be considered the “throat” of a sinkhole. This example has 

been seen in CRP images collected in freshwater lakes near 

Orlando, FL, that are known to have formed from sinkhole 

activity (Reich and others, 2009). Other examples of possible 

solution/sinkhole activity can be seen in lines CAL06_4000-

6000 (~3,330 m), CAL04_10000-11400 (9,550-9,700 m), and 

CAL05_8000-10000 (~8,745 m).

Shore-normal CRP tracklines were conducted on 

March 17 in the area between the US-41 and I-75 bridges. 

In addition to possible sinkhole or subsidence features, sags 

or swales in resistivity profiles running shore-normal may 

represent buried paleochannels. Three profiles contain areas 

where moderately resistive material (~1.5 to 2 ohm-m) 

depresses into higher-resistivity material (~7 ohm-m). CRP 

lines CAL08_3441-3992 (~3,500 m), CAL08_6913-8744 

(~6,450 m), and CAL08_10057-11816 (~9,200 m and 

~9,360 m) exhibit a possible buried paleoriver-channel shape. 

High-resolution seismic-reflection profiles along similar transects would aid in the correct identification of the paleoriver 

channels since there would be a distinctive erosional and 

infilling pattern. 

There is also a response in the CRP data that potentially 

could be a result of fresh groundwater. This response is 

limited to the narrow section of the Caloosahatchee River 

from I-75 to site S-79. The higher resistivity in the CRP data 

Figure 17. Individual segments of continuous resistivity profiles (CRP) collected in the Caloosahatchee River. Each CRP 

segment corresponds to its respective resistivity profile shown in the appendix
18 Investigation of Submarine Groundwater Discharge along the Tidal Reach of the Caloosahatchee River....Florida

above the I-75 overpass is indicative of increased occurrence 

of freshwater in the subsurface, because freshwater has a 

much higher resistivity than seawater. The presence of fresher 

groundwater is the reason the span in the log-linear scale 

on the resistivity is much greater (0.2-50 ohm-m) than the 

section of the river west of I-75 (0.2-15 ohm-m). CRP line 

CAL03_8000-10600, the line closest to site S-79, shows 

the highest-resistivity value (~48 ohm-m) of any CRP data 

collected during this survey. It is known from this study 

that shallow-porewater-salinity values at the time-series site 

were ~4 ppt (fig. 16 and table 1). The CAL03_8000-10600 

profile shows a higher shallow subsurface resistivity along 

the undeveloped and slightly higher elevation land near the 

end of the profile (9,220 to 9,929 m). This could be indicative 

of fresh groundwater underlying the river. It would be ideal 

to establish a shore-normal resistivity transect and porewater 

sampling effort along this stretch of the river to confirm the 

presence of the fresher, subsurface groundwater. 

Summary

Combined radon and electrical-resistivity data from this 

study demonstrate the variability in the geologic controls 

on groundwater discharge to the Caloosahatchee River. The 

increase in surface-water radon upstream from I-75 indicates 

that groundwater is discharging into the narrow section of the 

river at a higher rate than is observed in the wider section of 

the river south of I-75. An increase in resistivity values in the 

subsurface confirms that fresher groundwater underlies the 

upper part of the Caloosahatchee River and is coincident with 

increased 222Rn activities. 

Groundwater-discharge rates were estimated using 

a mass-balance approach on data from time-series radonmonitoring stations at two locations along the river. The 

monitor at Whiskey Creek (southern region of the river) was 

run for 4 consecutive days. The average estimated groundwater advective flux for this site was 5.7 ± 6.4 cm/d. The 

second monitoring time-series station was established near 

the S-79 structure, but was only run for 8 hr. Surface-radon 

concentrations were extremely high at the beginning of the 

experiment (~88 dpm/L) and were attributed to enhanced 

groundwater discharge caused by a storm that produced 

~0.5 in. of rain recorded at the S-79 structure. The radon 

activity decreased at an exponential rate during the 8-hr 

study period and ended near background values observed 

3 days prior to the measurements. The advective flux 

estimated for the upstream site was 66.7 ± 151 cm/d. This 

value is most likely an overestimate due to the high radon 

values at the beginning of the study and the short duration 

of the study. A more conservative estimate would be to 

average the smaller peaks that ranged from 0 to 93 cm/d 

(average = 12.3 ± 21.9 cm/d). 

Suggestions for Further Study

This study was intended to provide the SFWMD with 

a map of potential groundwater-discharge sites and an 

initial estimate of groundwater advective fluxes. To fully 

grasp the SGD contribution of water to the tidal reach of 

the Caloosahatchee River, several (~4) time-series radon 

monitors could be installed within the three areas of the river 

identified in figure 7. This study took place during the dry 

season, and therefore the values derived here are at the lower 

estimates of SGD to the river. Advective fluxes of ~13 cm/d 

(upper) and 5 cm/d (lower) are good estimates and are values 

that are acceptable and relevant, considering the hydrologicregime differences between the two sites. Future work could 

include establishing two time-series monitor sites in area 1, 

one in area 2, and one in area 3. The 8-hr radon time-series 

near the S-79 site gave an important first-order approximation 

of groundwater advective fluxes, but a longer time-series 

(4-5 days) effort would result in a better estimate over tidal 

cycles and lock and dam releases. Additional porewater-radon 

measurements at all time-series sites would be critical to 

advective-flux calculations. Land-based (stationary) resistivity measurements would also benefit the understanding 

of advective fluxes. Though resistivity is not quantitative, as 

shown in this report, it can lend credence to understanding 

the hydrogeology that drives groundwater flow. Land-based 

resistivity can be collected hourly and can image the impact 

of tidal or meteorological events on the movement of groundwater (Swarzenski and others, 2007; Simonds and others, 

2008). Although time-consuming, collecting cores in the river 

and porewater chemistry (salinity) would help ground-truth 

the resistivity data.

Acknowledgments

The author thanks Kathy Haunert and Detong Sun of 

the South Florida Water Management District for funding 

this study (project agreement number 4500031361), and to 

Eduardo Patino of the U.S. Geological Survey for initiating 

this investigation. The author thanks Peter Swarzenski for 

access to electrical-resistivity and radon equipment. Special 

gratitude is extended to Patricia Smiley for granting the 

USGS use of her boat dock. This report was greatly enhanced 

by the diligent reviews of Michael Byrne, Christopher Smith, 

Barbara Lidz, and John Watson.
References Cited 19

References Cited

Ataie-Ashtiani, B.A., Volker, R.E., Lockington, D.A., 2001, 

Tidal effects on groundwater dynamics in unconfined 

aquifers: Hydrological Processes, v. 15, no. 4, p. 655-669. 

Boggess, D.H., Missimer, T.M., and O’Donnell, T.H., 1981, 

Hydrogeologic sections through Lee County and adjacent areas of Hendry and Collier Counties, Florida: U.S. 

Geological Survey Open-File Report 81-638, 1 p.

Burnett, W.C., Bokuniewicz, H., Huettel, M., Moore, W.S., 

and Taniguchi, M., 2003, Groundwater and pore water 

inputs to the coastal zone: Biogeochemistry, v. 66, p. 3-33. 

Burnett, W.C., and Dulaiova, H., 2003, Estimating the dynamics 

of groundwater input into the coast zone via continuous 

radon-222 measurements: Environmental Radioactivity, 

v. 69, p. 21-35.

Burnett W.C., Kim, G., and Lane-Smith, D., 2001, A continuous 

radon monitor for use in coastal ocean waters: Journal of 

Radioanalytical and Nuclear Chemistry, v. 249, p. 167–172.

Burnett, W.C., Santos, I.R., Weinstein, Y., Swarzenski, P.W., 

and Herut, B., 2007, Remaining uncertainties in the use 

of Rn-222 as a quantitative tracer of submarine groundwater discharge: Proceedings of Symposium HS101 at 

IUGG2007, Perugia, Italy, July 2007, 10 p.

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